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Mary H. Meyer, extension horticulturist and professor, University of Minnesota

Mary Meyer

Table 1: BEST Crabapples for Minnesota

References and Further Reading:

Beckerman, J., J. Chatfield, and E. Draper. 2009. A 33-year Evaluation of Resistance and Pathogenicity in the Apple Scab-crabapples Pathosystem. HortSci. 44(3):599-608.

Chatfield, J. A. E. A. Draper, and B. Cubberley. 2010. Why Plant Evaluations Matter. American Nurseryman 210(9):10-15.

Draper, E. K., J. A. Chatfield, and K. D. Cochran. 2005. Marvelous Malus--Ten Crabapples Worthy to Know, Show, and Grow. Brooklyn Botanic Garden. Accessed October 6, 2014.

Green, T. L. 1995. Results of the national crabapple evaluation program. Accessed online October 3, 2014.

Green, T.L. 1996. Crabapples--When you're choosing one of those apple cousins, make flowers your last consideration. Amer. Horticult. 75:18-23.

Guthery, D.E. and E.R. Hasselkus. 1992. Jewels of the landscape. Amer. Nurseryman 175(1):28-41.
Iles, J. 2009. Crabapples..... With No Apologies. Arnoldia. Accessed online October 10, 2014.

Koetter, R. and M. Grabowski. 2014. Managing apple scab on ornamental trees and shrubs. Accessed online October 10, 2014.

Romer, J., J. Iles, and C. Haynes. 2003. Selection Preferences for Crabapple Cultivars and Species. HortTechnology 13:522-526.

Schmidt, J. Frank and Sons. 2014. Crabapple Information Chart. Accessed online October 8, 2014.

Mary H. Meyer, extension horticulturist and professor, University of Minnesota

Mary Meyer

Photo 1: Adirondack close-up

Mary Meyer

Photo 2: Adirondack - whole tree

Mary Meyer

Photo 3: Beverly close-up

Mary Meyerd

Photo 4: Beverly - Whole tree

Mary Meyer

Photo 5: Bob White close-up

Mary Meyer

Photo 6: Bob White - whole tree

Mary Meyer

Photo 7: Donald Wyman - whole tree

Mary Meyer

Photo 8: Firebird close-up

Mary Meyer

Photo 9: Firebird - whole tree

Mary Meyer

Photo 10: Louisa close-up

Mary Meyer

Photo 11: Louisa - whole tree

Mary Meyer

Photo 12: Pink Spires close-up

Mary Meyer

Photo 13: Pink Spires - whole tree

Which one would I plant in my yard? Anyone from this list, but something in the name 'Professor Sprenger' does resonate with me! It is a lovely tree that greets visitors on the Snyder Terrace at the Arboretum. Others you can see easily at the Arboretum are: 'Donald Wyman' planted in mass in the first parking lot bay across from the Oswald Visitor Center; 'Adirondack' marks the entrance to the espalier in the Cloister Herb Garden; 'PrairiFire' makes the double allée at the Sensory Garden, 'Pink Spires' flanks the entry to the new Green Play Yard at the Andrus Learning Center, and two 'Prairie Maid' trees fill an island in the staff parking lot.

Mary H. Meyer, extension horticulturist and professor, University of Minnesota

Mary Meyer

Photo 1: Prairie Maid close-up

Mary Meyer

Photo 2: Prairie Maid - whole tree

Mary Meyer

Photo 3: PrairiFire close-up

Mary Meyerd

Photo 4: PrairiFire - Whole tree (left) Sargentii espalier (right)

Mary Meyer

Photo 5: Professor Sprenger close-up

Mary Meyer

Photo 6: Professor Sprenger - whole tree

Mary Meyer

Photo 7: Red Jewel close-up

Mary Meyer

Photo 8: Red Jewel - whole tree

Mary Meyer

Photo 9: Royal Raindrops close-up

Mary Meyer

Photo 10: Royal Raindrops - whole tree

Mary Meyer

Photo 11: Sargentii close-up

Mary Meyer

Photo 12: Sugar Tyme close-up

Mary Meyer

Photo 13: Sugar Tyme - whole tree

This summer I was asked so many times "What is wrong with my crabapple?" that I started LOOKING anew at crabapples. 2014 was a banner year for apple scab, discoloring the foliage and causing premature leaf and even fruit drop. Affected plants looked dormant, or as many homeowners feared, dead. Apple scab can weaken trees, but rarely is fatal. Scab may allow secondary organisms to attack the tree and can decrease its winter hardiness, so it is best to purchase a scab-resistant crabapple. WHICH crabapples are resistant to scab, is complicated as the newest study (Beckerman et al, 2010) shows a new strain of this disease may now infect previously resistant cultivars. Additionally, we tend to think only about the FLOWERS on crabapples, and especially LOVE the showy pink or red flowers that are unfortunately often more susceptible to scab.

I recommend the first criteria for selecting a crabapple should be the ultimate SIZE, height and shape of the plant, followed by scab resistance, fruit, and finally the flowers. It is a misconception that crabapple fruit is messy: the small colorful fruit (5/8 inch or less) is a valuable food source sought by birds throughout the winter, and adds color and interest for many months.

Late fall is an ideal time to walk the crabapple collection at the Minnesota Landscape Arboretum and pick your favorite, because you can easily select scab resistant plants with clean, healthy leaves. Additionally, you can evaluate the fruit which varies widely in color and can last for MONTHS, easily six or even eight months. Remember that crabapple flowers last only for DAYS, perhaps a week. Why not select a crabapple for healthy leaves, and attractive fruit and just let the flowers be an added bonus?

From endless lists of hundreds of crabapples, the short list below was developed based on fall appearance with clean foliage at the Arboretum. Additionally, these plants are also top recommendations from long-term research trials conducted at the Ohio State University (their Crablandia field plots); Morton Arboretum, Illinois; Purdue University, Indiana; and the multi-state National Crabapple Trials.

I propose these 13 crabapples as "the best" for Minnesota. If your favorite is not here, let me know! All of the plants listed show good to excellent resistance to apple scab with an asterisk * for those showing some susceptibility to the new apple scab strain now in the Midwest. 'Red Splendor' is a showy red-pink prolific flowering crabapple that originated in Minnesota, however, it is susceptible to scab, needs plenty of space due to its large size and can be defoliated and defruited in mid-summer due to scab.

Which one would I plant in my yard? Anyone from this list, but something in the name 'Professor Sprenger' does resonate with me! It is a lovely tree that greets visitors on the Snyder Terrace at the Arboretum. Others you can see easily at the Arboretum are: 'Donald Wyman' planted in mass in the first parking lot bay across from the Oswald Visitor Center; 'Adirondack' marks the entrance to the espalier in the Cloister Herb Garden; 'PrairiFire' makes the double allée at the Sensory Garden, 'Pink Spires' flanks the entry to the new Green Play Yard at the Andrus Learning Center, and two 'Prairie Maid' trees fill an island in the staff parking lot.

There is an amazing variation that exists in these tough plants. A crabapple that grows well in Ohio, may not show the same disease resistance to apple scab here in Minnesota. The weather and climate makes a difference. Touring the famous crabapple collection at the Arboretum and the newer plantings in the display gardens can give you a first-hand look at how the plants grow in our climate. Ideally, we would annually rate crabapples three times: for foliage and fruit in September and October; for winter interest and fruit (bird food) in January; and flowering in May. Look for yourself at a garden center or the Arboretum, so you can decide which form and fruit is best for your garden and landscape.

Identifying Low-Maintenance Hydrangeas

If you drove or walked the three-mile drive at the Minnesota Landscape Arboretum during late summer or fall in 2013, chances are you stopped for a visit at the Earth-Kind® hydrangea trial. Planted in the fall of 2010, this planting exploded with growth and bloom last year and it was hard to resist stopping for a walk through the beds.

K Zuzek, UMN Extension

Photo 1: Locations of the 5 Earth-Kind Hydrangea trials

The hydrangea planting at the arboretum is one of five Earth-Kind® plantings in the Upper Midwest (Photo 1) that are being used to provide information on the performance of 24 hydrangeas being grown under low input maintenance conditions. The Earth-Kind® program was started in the early 1990's by Dr. Steven George at Texas Agrilife Extension Service to promote environmentally responsible landscape management practices that address diminishing water resources, the overuse and misuse of fertilizers and pesticides, and poor soil health that can diminish plant health. Earth-Kind® landscape practices include minimizing irrigation, providing fertility and improving soil health through the use of compost and organic mulches rather than fertilizers, the reduced use of pesticides, and the identification of genetically strong cultivars and species that will perform well under these low input conditions. The hydrangea trial sites are being used both to identify these strong cultivars and as outdoor classrooms to educate the public on environmentally friendly landscape management.

K Zuzek, UMN Extension

Photo 2: Plot preparation at the MLA

Cultivar trial establishment involves creating 4 blocks (planting beds) at each trial site. Each of the 24 cultivars being trialed is planted once in each of the 4 blocks in a randomized design. This means that 4 plants of each cultivar are planted at each site and their location is different within each of the 4 blocks. Four plants at each of the 5 sites provide us with 20 plants of each cultivar to focus our evaluation efforts on. This replication and randomization gives us statistically strong evaluation data to draw conclusions from. Plot establishment includes eliminating native vegetation (this is the one of the few times an herbicide is used) (Photo 2), incorporating 3" of compost into the native soil, planting, and applying 3" of organic mulch (usually wood chips) (Photo 3). Plants receive consistent irrigation as they establish during year 1.

K Zuzek, UMN Extension

Photo 3: Wood chipping during plot establishment

During years 2-4, irrigation is minimized and watering occurs only if plants wilt during periods of severe drought. Throughout the 4 year study, herbicides are applied only to control invasive weeds such as Canadian thistle if they appear in the plots. The 3" mulch layer minimizes weed establishment and weeds that do appear in the plots are removed by hand weeding. No fertilizers are applied. Organic mulch is reapplied as needed to maintain a 3" depth.

K Zuzek, UMN Extension

Photo 4: MLA plots in year 2 (July 2012)

During years 2-4, evaluation data is collected monthly on the 96 plants at each site. Data is collected on floral and foliar quality, plant size and habit, tolerance to environmental stresses (cold hardiness, drought tolerance, high soil pH, etc.), disease and insect tolerance, and the ability to perform well across a wide variety of soil conditions. Superior hydrangea cultivars that perform well across years and trial sites will be designated as Earth-Kind plants for their region so that gardeners and horticultural professionals know that these cultivars perform well with basic plant care.

K Zuzek, UMN Extension

Photo 5: MLA plots in year 3 (July 2013)

Twenty-four hydrangea cultivars were planted in the trial. Fourteen of the cultivars are panicle hydrangeas (Hydrangea paniculata) and include First Editions® Great Star, First Editions® Tickled Pink®, First Editions® Vanilla Strawberry™, First Editions® White Diamonds®, 'Grandiflora' (also known as PeeGee), 'Limelight', 'Little Lamb', Little Lime™, 'PeeGee Compact', 'Pink Diamond', Pinky Winky™, Quick Fire®, 'Tardiva', and 'Unique'. Seven of the cultivars are smooth hydrangeas (H. arborescens) and include 'Anabelle', 'Bounty', Endless Summer® Bella Anna®, 'Hayes Starburst', Incrediball®, Invincebelle® Spirit and White Dome®. The remaining three cultivars are bigleaf hydrangeas (H. macrophylla) and include Endless Summer® Blushing Bride, Endless Summer® The Original, and Endless Summer® Twist-n-Shout®.

The Benefits of Earth-Kind®
Low input gardening and the identification of plant cultivars that thrive with minimal maintenance benefit gardeners and the environment. The use of genetically strong and well-adapted plants makes it much easier and more enjoyable for gardeners and landscapers to create and maintain beautiful landscapes. There is also a large reduction in labor and the cost of maintenance. Few plants need replacing if adapted and pest-tolerant cultivars are selected for use in a landscape. The use of these tolerant cultivars minimizes the amount of irrigation needed and the use of pesticides. Replenishing organic mulches can be labor intensive but the benefits to gardeners and plants far outweigh the added labor. Mulch provides weed control, reduces the need for irrigation by decreasing evaporation of water from soil, and buffers soil temperature to protect roots during the intense cold of winter and the heat of the growing season. As it decomposes mulch improves soil structure and creates healthier root environments for garden plants: nutrient- and water-holding capacity increase in sandy soils and soil porosity, water infiltration and drainage, oxygen levels, and root penetration improve in heavier clay soils. The health and appearance of plants improves as soil quality improves. Improved soil texture also goes a long way towards making the job of hand weeding a much easier task for gardeners.

As individuals, the impact of our landscape management practices on the environment may be very small but collectively we have an enormous and sometimes a negative impact. As we change our gardening practices, we can reduce or eliminate these negative impacts. Fertilizers and pesticides have the potential to decrease water quality if they move over impervious surfaces such as driveways and sidewalks and into our streams, rivers and lakes through storm sewer systems. As the use of these chemicals is reduced, so is the potential for them to reach the water bodies that we treasure so much in Minnesota. The use of water-wise practices such as drip irrigation and the selection of drought-tolerant plants help to conserve water resources in a time when climate change is creating longer and more frequent periods of drought that put additional demands on these diminishing water resources. The use of organic mulches improves soil quality and reduces the amount of yard waste entering landfills.

The benefits of low input landscape management practices have been documented in Texas where the Earth-Kind® program has been in existence for over 20 years. In gardens or communities where Earth-Kind® landscape management practices are practiced, there have been 50-70% water savings, a 98% reduction in the use of pesticides, and a 20% reduction of yard waste entering landfills. In Addison, TX where the parks & recreation department uses Earth-Kind® management, there was a 50% reduction in labor costs due to the reduced need for irrigation, weeding, fertilizers and pesticides, and replanting. The department saw a 70% reduction in water usage and lost the dubious honor of being the town's largest water consumer.

The effectiveness of the Earth-Kind® plant evaluation effort can also be seen in Texas. To date, 23 roses have been designated as Earth-Kind roses for the southern United States. These plants have high tolerances to pests and perform beautifully under harsh summer temperatures and drought conditions such as those seen in 2011 when Dallas set records for the most 100 degree days, highest daytime and night temperatures, and drought (3.6" of rainfall from March to August instead of the average 17").

Blossom blight blasts crabapple blossoms

Michelle Grabowski, UMN Extension Educator

M. Grabowski, UMN Extension

Photo 1: Brown blossoms and leaves infected with fire blight clinging to a crabapple tree

This June, many gardeners were surprised to see the blossoms on their crabapple trees turn brown and shrivel along with the leaves growing alongside them. In many cases blossoms and leaves became discolored all along several branches (Photo 1). In other cases individual clusters were affected. A month later, these brown blossoms and leaves remain attached to the tree and are likely to stay through the growing season. These unattractive brown clumps of foliage not only affect the beauty of the tree but have a significant impact on the progression of the disease.

These unusual symptoms are caused by the disease fire blight. Fire blight is caused by the bacteria Erwinia amylovora, and it affects trees and shrubs in the Rosaceae family. Fire blight can often be found on crabapple, apple and mountain-ash in Minnesota. Serviceberry, raspberry, cotoneaster, and hawthorn are susceptible but less commonly infected.

Although blossom blight is a rare occurrence in Minnesota, fire blight is not. The fire blight bacteria can infect young green shoots and leaves in addition to blossoms. In order to start an infection, the fire blight bacteria need warm wet weather (110 degree days above 65F to be exact) and a wound or natural opening. In a typical Minnesota spring, temperatures are too cool during crabapple blossom to allow the fire blight bacteria to multiply and start an infection. When weather warms up later in the season, fire blight often infects young shoots that have been injured by insect feeding, wind whipping or other factors. This year early warm wet conditions allowed the bacteria to blight blossoms in addition to shoots.

Regardless of how the bacteria first infect the tree, once an infection has begun, the fire blight bacteria can move through the infected blossoms or shoots and into the adjoining branches. With time the bacteria can move into the main trunk and even into the roots of the tree. Infection of the trunk or roots is lethal but infection of blossoms, shoots and branches can be pruned away if caught in time.

M. Grabowski, UMN Extension

Photo 2: Purple red discoloration of bark where the fire blight infected blossoms attach indicates progression of the disease into the branch. Healthy brak of this cultivar is yellow green

Gardeners that have observed symptoms of fire blight in their crabapple trees this spring should monitor trees throughout the growing season. The extent of the progression of the disease through the tree will vary depending on the resistance level of the tree. In highly resistant trees, the infection may not progress beyond the blossoms. In less resistant trees the infection will continue into the adjoining branch causing a canker. In highly susceptible trees, the infection can progress rapidly through the branch. Home gardeners can monitor the progression of the infection by looking for purplish brown discoloration of the bark (Photo 2) and continuing death of leaves along the branch.

Infections within branches should be noted for removal during the dormant season. February and March are ideal times to prune out branches infected with fire blight because the bacteria are not active at that time. Pruning cuts should not be made during the growing season (due to risk of the pruning wound becoming infected) unless the infection is close to infecting the main trunk. This may happen if suckers or water sprouts became infected or if the tree is highly susceptible. In that case, the branch should be cut 8-10 inches below the discolored bark on a cool dry day. Pruning tools will need to be sterilized after every cut with rubbing alcohol, a 10% solution of household bleach or Lysol®. Trees infected with fire blight should not be fertilized this summer as this will promote young succulent growth that is highly susceptible to infection. More information about fire blight can be found on the UMN Extension Yard and Garden page.

The following list is updated with links to new and revised online Extension publications as they become available. 

June 17, 2014

General/Curiosity Insects (NEW!)
A new addition to the Extension Y&G diagnostic tool "What insect is this?" Find information to help identify and understand insects (1) with obvious wings (flies, wasps, moths, etc.) and (2) insects without obvious wings (beetles, bugs, ants etc.).

May 27, 2014

Carpenter ants (revised)
Bronze birch borer and twolined chestnut borer in Minnesota (revised)
Maple petiole borer
Nightcrawlers (revised) 

December 2, 2013

Pest management in the home strawberry patch (new)
Pest management for home blueberry plants (new)
Integrated pest management for home raspberry growers (new)
Integrated pest management for home stone fruit growers
Leafminers in home vegetable gardens

August 26, 2013

Masked hunters (revised)
Fourlined plant bug in home gardens (revised)

July 16, 2013

"Annuals" have been added to What's wrong with my plant? diagnostic tool.

June 18, 2013

Root maggots in home gardens (new)
Emerald ash borer in Minnesota (revised)
The Extension EAB web page has also been revised

May 23, 2013

Anthracnose (revised)
Powdery Mildew
Cedar Apple Rust and Other Gymnosporangium Rusts
Crown Gall (revised)

Managing Apple Scab on Ornamental Trees and Shrubs
Managing Impatiens Downy Mildew in the Landscape (new)

Managing Impatiens Downy Mildew in Greenhouses, Nurseries, and Garden Centers
Basil Downy Mildew

May 1, 2013

Aphids on Minnesota Trees and Shrubs (revised)
Woolly Aphids on Minnesota Trees and Shrubs (revised)
Spotted Wing Drosophila (new)
Two-spotted Spider Mites in the Home Garden and Landscape (new)

Phomopsis Spruce Decline Found in Minnesota

M. Grabowski, UMN Extension

M. Grabowski, UMN Extension

Photo 1: Needle loss from Phomopsis spruce decline

A new form of an old disease has been identified in Minnesota. The fungal pathogen Phomopsis sp. has long been known to cause shoot blight on spruce trees in nurseries and tree farms. The same pathogen has now been found causing cankers, needle loss, branch death and in severe cases tree death of mature spruce trees in the landscape. This disease has been named Phomopsis spruce decline. The first case of Phomopsis spruce decline in Minnesota was identified by the University of Minnesota Plant Diagnostic Clinic this spring (May 2014).

In nurseries and on tree farms, Phomopsis shoot blight causes young needles and shoots to turn brown and curl downward. In some cases dark resinous cankers formed on young stems but damage does not commonly extend beyond that years new growth. Although this disease causes damage in nurseries and on tree farms, it has not been a common problem in landscape trees.

In recent years, Dr. Fulbright of Michigan State University has been researching decline of mature spruce trees in landscapes and on tree farms. Declining trees had discolored needles that would fall off prematurely. The buds at the tip of the branch would remain alive for awhile but eventually the branch would die. These symptoms would often progress from the bottom of the tree up into the canopy. In severe cases the tree died.

M. Grabowski, UMN Extension

Photo 2: Dark discoloration of wood caused by a Phomopsis canker

Dr. Fulbright's research led to the discovery of a new type of disease caused by an old pathogen. When bark was peeled off branches of declining spruce trees dark brown staining of the wood could be seen. These infections, called cankers, were caused by Phomopsis sp. the same fungal pathogen capable of causing shoot blight in nursery spruce trees. Since then Dr. Fulbright's research group have found Phomopsis infections associated with decline of mature spruce trees in landscapes all throughout Michigan's lower peninsula.

Many questions remain to be answered about Phomopsis spruce decline. It is unknown how or why this pathogen began infecting mature landscape trees. It is known that in nurseries the disease is significantly more severe on stressed trees. It is therefore possible that stress from environmental conditions, other pests or pathogens or other factors have played a role in the development of Phomopsis spruce decline. Several other fungal pathogens infect spruce trees in both Michigan and Minnesota. These pathogens cause needle discoloration and drop, as well as branch cankers. How all of these known pathogens interact with Phomopsis unclear. At this time there are no management recommendations for trees suffering from Phomopsis spruce decline.

While research on the biology and management of this disease continues, gardeners should do their best to reduce stress on spruce trees as well as all landscape trees. Water trees during times of drought. Mulch the area around the tree with 2-4 inches of wood chips or other organic mulch. Protect trees from damage from weed whips, lawn mowers and other equipment.

For diagnosis of spruce diseases, contact the UMN Plant Diagnostic Clinic at or 612-625-1275.

Gooey Orange Fungi on Junipers

M.Grabowski, UMN Extension

Photo 1: Cedar apple rust galls on juniper

M. Grabowski, UMN Extension Educator

This time of year, several species of fungi from the genera Gymnosporangium are producing bright orange gelatinous spore producing structures on the branches of Junipers. The brilliant orange color of these fungi gives them their common name: Rust. Cedar apple rust is perhaps the most well known of the Gymnosporangium rust fungi, but here in Minnesota four different species can be found infecting junipers. Hawthorn rust, quince rust and juniper broom rust all produce similar gelatinous bright orange spore producing structures on junipers. Although these fungi can be startling to see, they rarely cause significant damage to the junipers. Spores produced in these orange gelatinous structures will not reinfect the juniper. Instead spores are carried by wind and rain to infect nearby trees and shrubs in the Rosaceae family. Each of the four Gymnosporangium rust fungi infect different members of the Rosaceae family including crabapple, serviceberry, and hawthorn. To learn more read Cedar Apple Rust and other Gymnosporangium Rusts by UMN Extension.

M. Grabowski, UMN Extension

Photo 1: Pagoda dogwood only partially leafed out. Dead branches were killed by golden canker.

M. Grabowski, UMN Extension Educator

Golden Canker, caused by the fungus Cryptodiaporthe corni, can easily be seen on the branches of Pagoda dogwood trees (Cornus alternifolia) this time of year. Infected branches and stems turn bright yellow with raised orange blister like spots. This diseased plant tissue clearly contrasts with the purplish green bark of a healthy Pagoda dogwood. Unfortunately branches infected with golden canker are unlikely to leaf out. The disease can continue to spread through infected branches and even into main stems. It can kill all above ground parts of the tree but will not kill the roots.

M. Grabowski, UMN Extension

Photo 2: Orange bark killed by golden canker contrasts sharply to healthy reddish purple bark.

The best time to prune out branches infected with golden canker is in March or February when fungal spores are less likely to be present to infect pruning cuts. If cold weather and deep snow prevented this from happening on time, however, branches can be pruned out on dry day. Make the pruning cut at least two buds below the visible discoloration of the bark. Be aware that in some case a canker spreads irregularly and discoloration may extend several inches longer on one side of the branch than on the other. Remove infected branches from the area. They can be burned, buried or taken to a municipal compost facility.

Karl Foord, Extension Educator - Horticulture

Karl Foord

Photo 1: Buds of Austrian Pine (Pinus nigra)

Karl Foord

Exhibit 2: Healthy buds of Swiss Stone Pine (Pinus cembra)

Karl Foord

Exhibit 3: Dead buds of Swiss Stone Pine (Pinus cembra)

Karl Foord

Exhibit 4: Buds of Korean Fir (Abies koreana)

Kathy in her article on winter burn states the following:

Wait until spring before deciding how to care for your winter burned plants. If leaves are dead but buds and stem tissue near dead foliage are still alive, new plant foliage will regrow to replace winter burned foliage.

I had significant winter burn damage on the following: all hemlocks (Tsuga canadensis), Austrian pine (Pinus nigra), and Scots pine (Pinus sylvestris), Swiss stone pine (Pinus cembra), and Korean fir (Abies koreana). I had little or no damage on the following: Mugo pine (Pinus mugo), Japanese white pine (Pinus parviflora, Glauca group), Limber pine (Pinus flexilis), Taylor's Sunburst (Pinus contorta), and Uncle Fogy pine (Pinus banksiana).

Following Kathy's lead I looked at the buds of some of these plants.

The buds on the Austrian and Scots Pines looked healthy (Photo 1 of Austrian pine buds). There were healthy and dead buds on the Swiss stone pine (Photos 2 & 3) as there were on the Korean fir (Photo 4). The bud sizes are quite different with the one on the right looking more robust.

As Kathy said we will just have to wait and see, but having a close look at the buds gives you a sense of what to expect. Frankly I hope your evergreens overwintered better than mine.

Yes, You Can Grow Oranges in Minnesota!

Mary H. Meyer, Extension Horticulturist

Mary Meyer

Photo 1: Calamondin orange trees need maximum light, such as the south facing window, shown here. Photo taken January 4, 2014

"Oh my! How do they expect me to grow an orange tree in Pennsylvania?" my Grandmother Rena Anderson exclaimed as she unwrapped her Plant-of-the-Month gift on a summer day in the 1960's as we sat in her screened porch amid her many plants. She laughed and potted the tiny plant. Today I enjoy this same orange tree in an even colder Minnesota climate and yes, it produces oranges!

The calamondin orange ( Citrofortunella mitis) is a tough houseplant IF you have enough sunlight and can keep it watered in well drained soil. My plant spends May-October in an unheated porch with large south facing windows and the rest of the year in a corner of my living room with south and west facing windows (Photo 1).

In other words: the brightest light we can supply in Minnesota. Over the nearly 50 years since my grandmother received it, I have moved it through 5 states, 12 homes and many repottings. In October 1986, the orange tree was the last thing I put on the moving van in Philadelphia and the first thing I took off when the van doors opened in Plymouth, Minnesota a few days later.

I am attached to the orange plant as it was my grandmother's, and she was one of my plant mentors. But this orange can fill my porch or living room with a sweet orange blossom fragrance and it sets fruit well enough, that most years, I can make marmalade.

Here is a brief history of some recent orange harvests:

Calamondin Oranges ------------- Harvest Date

112-------------------------------- January 9, 2011
52-------------------------------- November 25, 2012
25-------------------------------- September 21, 2013
27--------------------------------January 5, 2014

Mary Meyer

Photo 2: 112 oranges were harvested from the tree in January of 2011.

Here is what I have learned from, yes, only 1 orange plant, over the years:

Bright light for several hours every day is necessary for citrus to do well in Minnesota. Moving the plants outdoors in the summer really helps. Gradual exposure to direct sunlight in the summer is important, as leaves that develop indoors are not able to grow outdoors unless they are acclimated; they easily get sunburned.

Adequate water is also essential. Citrus leaves are thin and easily wilt. Regular watering is essential for good growth. Just as important is good drainage, water should never stand at the bottom of the container. For many years, I used a plastic container, so I did not have to water as often, however today the plant is in a clay pot and it prefers the better air exchange for the roots.

Citus requires a lot of nitrogen and iron to grow well, and iron is often unavailable in high pH soils, which tends to happen over time with the alkaline water we use on indoor plants. Yellow foliage is a common sign of iron deficiency in citrus and means you need to add a fertilizer that has available or water soluble iron. Throughout the summer, I use a readily available, water soluble fertilizer, such as 20-20-20 once or twice a month. In the winter, I rarely use fertilizer. And about once a year, I use an iron supplement to keep the foliage a healthy green color.

After a few years of treating mealy bugs, I gave up on insecticidal soaps, which will control most insects and used a stronger systemic control that eliminated the mealy bugs. However, I did not eat the citrus for two years. Careful, regular inspection is necessary to prevent insects from becoming a problem, especially if plants are exposed to other plants or outside conditions.

Meyer lemons are also fun to grow and will produce a few lemons in Minnesota. I bought two plants a few years ago and they have produced about 5 lemons in total. These lemons take a long time to ripen (months) and are a pale orange instead of yellow when ripe. These semi-sweet lemons are a cross between a lemon and an orange, so they are much milder than regular lemons.

The tree was brought to the United States from Beijing, China in 1908 by Frank Meyer, a plant explorer of the U.S. Department of Agriculture, (no relation to me :(). The plants can be gangly and leggy plants; you should prune them after harvesting the fruit, to keep the plants in a manageable size and shape. Growing conditions are similar to calamondin oranges, however be sure to purchase plants from a reputable garden center so you do get the true Meyer, or Improved Meyer lemon.

A fun fact about citrus plants is they can have evergreen foliage, flowers, immature and mature fruit all at the same time. For Minnesota, it is fun to have fragrant flowers, and developing attractive fruits over the months when we often see too little green.

I have two daughters, and I may have to propagate my orange from a softwood cutting in the spring so they each have a plant to enjoy in their homes.

But for now, I plan on harvesting Grandma Rena Anderson's calamondin oranges for many years to come.


Growing Citrus Indoors in Minnesota


Four Winds Growers in California is one supplier for indoor citrus plants:

Take a Survey and Win an iPad Mini

K. Zuzek, UMN Extension

Photo 1: Rose frost

University of Minnesota Extension is still looking for feedback from gardeners, horticultural professionals, and other members of the public to help direct future tree and shrub educational programming. Don't delay. Tell your friends. By taking a short 10-15 minute survey, you will be entered into a drawing to win an iPad mini. Find the survey here. Thanks you for your help in planning future Extension educational programs! If you have questions regarding this survey, please contact: Kathy Zuzek, Extension Educator - Woody Ornamentals, University of Minnesota Extension at (952) 237-0229.

Minnesota's Native Holly

K. Zuzek, UMN Extension

Photo 1: Winterberry, November 11, 2013

Mention holly during the month of December and we all think of cut holly branches adorning homes during Christmas season. But our native holly, called winterberry or Ilex verticillata, is just as ornamental outdoors in our early winter landscapes because of its colorful and abundant fruit (Photo 1).

Winterberry is native throughout the eastern United State (Photo 2) and in Minnesota it is usually found growing in forested wetlands in the eastern half of the state along with larch, willows, and speckled alder. You may also see it growing along lakeshores and ponds or in acidic sandy soils with high water tables.

USDA Natural Resources Conservation Service Plants Database

Photo 2: Native range of Ilex verticillata

Although there are over 400 species of holly (Ilex spp.) worldwide, less than a dozen species are commonly used as landscape plants. Winterberry is one of these species. Approximately 35 cultivars have been selected within the species for fruit color (red, orange, and gold), fruit size and abundance, and compact plant size.

Winterberry cultivars range in size from 4-10 feet in the landscape. Small, inconspicuous flowers are produced from mid-June to early July. Winterberry's dark green foliage provides a beautiful backdrop to the brightly colored fruit that become showy in September. After leaves drop, the fruit will continue to light up a winter garden until birds find and eat it.

Winterberry is an easy plant to grow in light or heavy soils. Because it is native to swampy areas, it does well in wet conditions. It does prefer acidic soils with a pH between 4.5 and 6.5; chlorosis will develop in high pH soils. The other important fact to remember is that winterberry is dioecious, meaning male and female flowers are produced on separate plants. Besides planting a female cultivar that will produce the showy fruit, you must also plant a male plant whose pollen will be produced at the appropriate time to pollinate the flowers on your female cultivar that will later develop into the fruit. Two male cultivars are available at garden centers: the early blooming 'Jim Dandy' and the later blooming 'Southern Gentleman'. Plant labels and garden center staff can help you select the appropriate male cultivar for your fruiting female cultivar.

Fall Color up Close

Karl Foord - Extension Educator, Horticulture

The fall leaf color spectacular is alive and well this year, due to a warm fall and clear sunny skys.

I was curious about the variation in colors coming from one tree. The tree in question is Autumn Blaze Maple (Acer x freemanii) 'Jeffersred'. The variation in color is due to where the leaf was located on the tree and how exposed the leaf was to light. Bright light permits the leaf to produce significant amounts of sugars which are needed for anthocyanin (red pigment) production. Thus more light more red color. Another contributor to the color palate are carotenoid pigments providing orange and yellow. The various combinations give the leaves their variation in color.

Consider the following photos and see how many different colors exist in these fall leaves.

Karl Foord

Photo 1: Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 2: Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 3: Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 4: Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 5: Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 6:Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

The next surprise was the beauty of the underside of the leaves not only in terms of color and color contrast, but also because of the architecture of the leaf veins. Consider these photos and the colors we don't see on the upper surface of the leaves.

Enjoy the color of this fall. Because the weather has such a strong effect on leaf color we cannot expect to see the same colors next year.

Karl Foord

Photo 7:Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 8:Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 9:Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 10:Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 11:Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

Karl Foord

Photo 12:Autumn Blaze Maple (Acer x freemanii 'Jeffersred')

M. Grabowski, UMN Extension

Photo 1: Elm tree killed by Dutch elm disease

Michelle Grabowski, UMN Extension Educator

Dutch elm disease (DED) is a fungal disease that results in yellowing, wilting and browning of leaves, death of branches and eventually death of the entire tree. Today, the disease can be found in every county in Minnesota yet it is estimated that 1 million elms still remain within communities. Several management strategies have been developed that allow elms to survive if properly cared for. Elm trees can be protected from DED through fungicide injections, infections can be removed and treated if caught early, and new cultivars of elm offer resistance or tolerance to DED. To learn more on how to identify and manage the disease read the new UMN Extension publication on Dutch elm disease.

Bright orange slimy fungus in junipers

M. Grabowski, UMN Extension

Photo 1: Cedar Apple Rust Gall on Juniper

M. Grabowski, UMN Extension Educator

There is nothing that catches the eye so much as a bright orange slimy tentacle-like fungus hanging from a landscape juniper tree. What is that thing? you might ask. The answer is cedar apple rust, one of several Gymnosporangium rust fungi that infect junipers in the state of Minnesota. Gymnosporangium rust fungi are unique in that they spend part of their life infecting juniper trees, causing unusual brightly colored galls, and the other part of their life infecting leaves, fruits, and green stems of trees or shrubs in the Rosaceae family. The Rosaceae family includes crabapples, hawthorns and serviceberry. Different Gymnosporangium rusts infect different trees and shrubs.

On junipers, the cedar apple rust results in brown woody galls that can grow to over an inch in diameter. With wet weather, the galls produce bright orange tentacle like spore producing structures. These release spores that are carried to nearby crabapple or apple trees, later to become bright orange and red leaf spots. Cedar apple rust galls on juniper can dry down and rehydrate multiple times in one spring. With repeat rain events in Minnesota this year, the cedar apple rusts have been putting on a colorful show. The good news is the galls will dry up and die when warm dry weather arrives. Juniper trees are rarely harmed by cedar apple rust galls although gardeners might notice some dieback on branch tips girdled by large galls.

More information about cedar apple rust and other Gymnosporangium rusts can be found on the UMN Extension webpage, including information on how to identify and manage the different diseases on different host plants.

Anthracnose of shade trees thrives in cool wet spring

M. Grabowski, UMN Extension

Photo 1: Anthracnose on Maple

M. Grabowski, UMN Extension Educator

As cool wet weather persists in Minnesota this year, a common leaf blight disease of shade trees known as anthracnose thrives and spreads. Anthracnose is a fungal disease that causes leaf blotches, leaf distortion, shoot blight, and leaf drop. This disease is caused by several related fungi that thrive in cool wet conditions. Ash, maple and oak trees are all commonly infected with anthracnose and symptoms of the disease have been seen on all of these trees this spring. Despite blackening of leaves and shoots, anthracnose actually only results in a minor stress on the health of the tree. Only young growing leaves and shoots are susceptible to infection. Mature leaves are relatively resistant to the disease. Once warm dry weather arrives in Minnesota, leaves will mature and trees will replace lost leaves with a new flush of growth. As long as cool wet weather persists, however, expect this fungal disease to spread throughout the trees canopy. More information can be found about this disease at the UMN Extension publication on anthracnose in shade trees.

Karl Foord - Extension Educator, Horticulture

This spring I noticed the black color on the branches of my Taylor's Sunburst Pine (Pinus contorta 'Taylor's Sunburst'). I remember seeing the same thing on my Uncle Fogy Jack Pine (Pinus banksiana) last year. The cause was the same in both cases Sooty Mold that has formed on the honeydew secretions of the Pine tortoise scale (Toumeyella parvicornis). To confirm this I went to the What's Wrong with My Plant Diagnostic Tool via the Extension Garden web page/Pest Management/Diagnose a Problem link. I tracked through "Evergreen Trees and Shrubs" to "Pine" to Black powdery coating on "needles" and "shoots". This confirmed the Sooty Mold. I also tracked through "Sticky substance coating needles" to confirm the Pine tortoise scale as the culprit.

Karl Foord

Photo 1: Uncle Fogy Pine (Pinus banksiana 'Uncle Fogy'

The scale organism put such stress on my Uncle Fogy Pine that the needles it produced were half the size of the normal needles. Upon removal of the scale the needles returned to normal size (photo 1). I had learned my lesson about the impact of Pine tortoise scale on my trees.

I knew that Imidacloprid had worked on Uncle Fogy so I grabbed the bottle and reread the label - not trusting my memory. I confirmed that yes this product is labeled for soft scales. I also reviewed the precautions about letting the chemical get into aquatic environments - no lakes, streams, or ponds nearby. I agreed with the label that whatever I used to measure the chemical should not be followed by using the same utensil for soup.

There was nothing on the label regarding protective equipment, however the label warned that this chemical can be harmful if absorbed through the skin so I followed my previous procedures of long sleeve shirt, long pants and plastic gloves for handling.

Because this was an evergreen ornamental tree I was not concerned about the potential effects of a systemic pesticide on pollinators because there would be no flowers to pollinate. Also because the product is used as a drench around the plant, I didn't have to worry about airborne spray drifting onto other plants, people, or pets. In addition there were no wet leaf surfaces so there was no issue of reentry to the area or drying time of the product.

The directions called for the tree trunk circumference in inches at a height of 4.5 ft. and that this length in inches should equal the number of ounces of product to be applied to the plant. But there were too many needles to measure the circumference so I estimated the diameter and multiplied by Pi. How often do you get to use the trigonometry you learned in high school? The amount came to @ 5 oz. I added this to a gallon of water as instructed and applied the solution evenly in a circle 2 ft. from the tree trunk. Put the container away and washed up.

I checked the tree every other day or so and several days later the scales were starting to look ill. In approximately two weeks all the scales were dead. My Taylor's Sunburst Pine is looking and growing nicely without having hundreds of Pine tortoise scale stylets sucking its "life's blood" plant sap.

Reading the label wasn't all that painful. Reading size 6 font text definitely required glasses for "mature" eyes. However, I did not skim the label or skip parts. I read carefully and made sure that I understood all parts and the risks involved. Yes following the label is the law. Following the label is also the responsible and intelligent thing to do. By proceeding in this manner I felt in command of the situation and understood that the action I was taking was going to solve the problem with minimal impact to the environment and all the other actors in our theater of life.

Rabbit Damage Revealed on Trees and Shrubs

M.Grabowski, UMN Extension

Photo 1: Rose bush with extensive rabbit damage

As gardeners inspect their landscapes this spring, many trees and shrubs have been found with extensive rabbit damage. Rabbits are one of the most commonly seen mammals in the urban environment. In Minnesota, our most common rabbit is the Eastern cottontail, Sylvilagus floridans.

Rabbits will spend much of their time eating grasses and other herbs but they will also chew on the bark of trees and shrubs and eat the buds of shrubs in the winter and spring. If left unprotected, rabbits will sometimes eat the bark from around the base of a tree or shrub. This is called "girdling" and can kill the plant.

Read Rabbits and Trees and Shrubs by wildlife expert Jennifer Menken of the Bell Museum to learn more about protecting landscape plants from rabbit damage.

What a difference a year makes

Karl Foord - Extension Educator, Horticulture

Karl Foord

Photo 1: Autumn Blaze Maple 3 29 2012

Minnesota definitely offers us weather variability. And although we may remember last year's early spring it is often difficult to compare a memory with a memory in the making. To this I offer the following two photographs of buds from the same 'Autumn Blaze' maple tree (Acer freemanii 'Jeffsred'). The picture to the left was taken last year on March 29th. As you can see the samaras with whirligig characteristics permitting the seeds to move away from the parent tree have already started to form.

Contrast this with the picture to the right taken this year on April 30th. The bud is just beginning to break and this picture was taken a month later than the previous. And snow is predicted for tomorrow in many parts of the state. Mercy.

Karl Foord

Photo 2: Autumn Blaze Maple 4 30 2013

Spirea Stunt Phytoplasma Found in Minnesota

Michelle Grabowski, UMN Extension Educator

M. Grabowski, UMN Extension

Photo 1: Spirea with a few witches brooms from spirea stunt phytoplasma

University of Minnesota researchers Dr. Ben Lockhart and Dimitre Mollov have identified a new disease of spirea in Minnesota. Spireas are common landscape shrubs grown for their delicate foliage and summer flowers. The new disease, known as spirea stunt, causes plants to produce abnormally small leaves, that may be discolored yellow to reddish purple. Small discolored leaves grow only on clumps of weak branches known as witches' brooms. As a result, infected plants have one to many pom-pom like clumps of leaves and branches. The whole plant may be stunted and many infected plants do not survive the winter.

Spirea stunt disease is caused by a phytoplasma, a tiny bacteria that lives within the vascular system of infected plants. Dr. Lockhart determined that the phytoplasma infecting spirea in Minnesota belongs to the X-disease group and is related to similar pathogens that cause witches' brooms and stunting in blueberry, prunus and other shrubs. Little is known about the spirea stunt phytoplasma at this point. It is likely that this pathogen is transferred from plant to plant by leafhoppers or through plant propagation.

Once a plant is infected with spirea stunt phytoplasma, the pathogen spreads throughout the entire plant. There is no way to cure infected plants. Gardeners should remove shrubs infected with the spirea stunt phytoplasma as soon as possible.

Summer 2012 Imprelis Update

Kathy Zuzek, UMN Extension Educator, Horticulture

In June, the Office of Indiana State Chemist published the 2012 Imprelis Soil and Vegetation Sampling and Analysis Follow Up Study showing 2011 and 2012 levels of aminocyclopyrachlor, the active ingredient of Imprelis, from 11 sites where Imprelis was applied. The level of aminocyclopyrachlor (ACP) was measured in both soil and vegetation. Soil samples were taken to a depth of 4 inches and were then divided into the top 2" and the bottom 2 inches. Vegetation sampled included dead spruce twigs, willow twigs, and honeylocust twigs and leaves on Imprelis-damaged trees. Levels were widely variable in both 2011 and 2012 across the 11 sites. But on average, ACP levels in the 2012 samples of were 10%, 6%, and 23% of the levels measure in 2011 for the top 2" of soil, the bottom 2" of soil, and the vegetation samples respectively.

What does this information tell us? 1) ACP levels still present in plant tissue are higher than levels in soil. 2) ACP levels are dissipating as expected according to the half-life (the time it takes for the chemical concentration to reduce by one-half) of Imprelis in turf environments. Unfortunately, there is no information available as to what the remaining ACP levels in soil and vegetation mean in terms of continuing damage or a timeframe for safe replanting in Imprelis-applied soils.

Here is what is happening to tree and shrub species on Imprelis-applied sites in MN since the last update :

Remember that the effects of Imprelis on non-target species (the injured pines, spruce, and other species growing near or in turf where Imprelis was applied as a weed killer) are the same as the effects seen on targeted weed species. ACP is a synthetic auxin or plant hormone. When applied to plants, ACP causes undifferentiated cell division and elongation in areas of new plant growth, primarily branch tips and root tips. The undifferentiated cell division and elongation results in downward bending of leaves or needles, stem thickening, severe necrosis (tissue death), stunted new growth, calloused stems and leaf veins, and crinkling, cupping and twisting of leaves, needles and stems.

K. Zuzek

Photo 1: Late bud break on Colorado spruce in July

K. Zuzek

Photo 2: Thickened shoot tips on eastern white pine

Some species like Colorado blue spruce and cottonwood had a delayed bud break and leaf out this year. Bud break on Colorado blue spruce on an Imprelis-applied site I have been observing occurred in July rather than April (Photo 1).

Among highly susceptible species like eastern white pine, Norway spruce, and white spruce that showed bud and shoot tip injury or mortality shortly after Imprelis applications in 2011, behavior is variable. Some trees continued to decline and died. Others broke bud but the new shoot tips are thickened and often have twisted needles or are completely lacking needles (Photo 2). Others are producing adventitious buds and shoots below dead stems in an effort to put on new growth. It will take time to see if these plants can be managed and restored to worthwhile landscape plants.

K. Zuzek

Photo 3: Cupped distorted foliage on honeylocust in 2012

K. Zuzek

Photo 4: Crown dieback in honeylocust

Among deciduous trees and shrubs, leaves produced in 2012 are sometimes still showing distortion, twisting, and cupping due to 2011 Imprelis applications. Honeylocust (Photo 3) and lilacs are good examples of this.

The most recent development on Imprelis-applied sites in Minnesota is on honeylocust. As has been seen in states east of Minnesota, honeylocust seems to be a particularly vulnerable deciduous tree species. Many honeylocust have died. Among those that survived, many have dieback in their crowns this year (Photo 4), a more open canopy due to reduced leaf formation, twisted and distorted foliage, and galls or tumors on the trunk (Photo 5) and throughout the tree canopy (Photo 6).

K. Zuzek

Photo 5: Honeylocust trunk galls

K. Zuzek

Photo 6: Galls in honeylocust canopy

Leafcurl Ash Aphids on Ash

Jeffrey Hahn, Asst. Extension Entomologist

This June has seen a lot of activity by leafcurl ash aphids, a type of woolly aphid, not only in the Twin Cities but also in a number of other areas in Greater Minnesota. Like other aphids, leafcurl ash aphids use piercing - sucking mouthparts to feed on the sap in the leaves. This feeding causes leaves to become tightly curled, puckered, and distorted. To verify leafcurl ash aphids, unroll the leaves. The aphids are a light green and no more than 1/8 inch long. They produce a conspicuous white waxy material that covers the aphids as well as the leaves.

Kim Sullivan

Photo 1: Leafcurl ash aphid. Note the curled leaf and the white waxy material

These aphids also produce a lot of honeydew. Honeydew is a sticky waste material because the aphids are not able to digest all of the sugars in the sap. Any objects under a leafcurl ash aphid infested tree can get coated with this substance. Later you might find sooty mold, a black fungus developing on the honeydew. Fortunately, light infestations of sooty mold causes little damage to plants.

These aphids were even reported as nuisances when they would fall down on people in their yards (which is not conducive for graduation parties and other outdoor activities).

Leafcurl aphids feed on the new growth that expanded this spring. Colonies last until mid-summer. Winged forms are produced which migrate to the roots of ash where they remain for the rest of the year. Leafcurl ash aphids have always been in Minnesota but have been infrequently noticed or reported over the last 5 - 10 years.

Although the damaged leaves are conspicuous, when you look closely, just a small number of leaves within a tree are actually affected by leafcurl ash aphids. Even the leaves that are distorted can still photosynthesize so there is very little risk to the health of ash.

While insecticides, such as imidacloprid and dinotefuran are effective against these aphids, they are rarely warranted to protect trees and are not suggested. Even if you kill the aphids, the distorted leaves will remain for the rest of the season. While this can affect the trees' appearance that is of small consequence compared to other problems, especially the risk of emerald ash borer

Look out for disease problems on new plants

Michelle Grabowski, University of Minnesota Extension

I always recommend that gardeners thoroughly inspect every plant for disease problems before they purchase it. Examine both the upper and lower surfaces of the leaves, especially those leaves closest to the soil. Look at the stems and even the roots. Any spots, streaks, dark discolored or soft, mushy tissue is a warning sign that your new plant may come with a disease problem. Once a new pathogen is introduced into a garden, it may cause problems for years to come. In addition, nursery plants may move across the country before they make it to a local garden center. As a result infected plants can bring new plant diseases to Minnesota.

Boxwood Blight

Virginia Tech, Plant Disease Clinic

Photo 1: Leaf spots and blighting due to boxwood blight

One disease of concern is boxwood blight, a fungal disease caused by Cylindrocladium buxicola (syn C. pseudonaviculatum). Boxwood blight was first identified in the eastern United States in 2011. It has since been found in 8 eastern states as well as in Oregon. To date boxwood blight is known to infect all species of boxwood (Buxus spp.) and Pachysandra sp.

Boxwoods infected with the boxwood blight fungus first develop tan leaf spots with a dark brown border. Chalky white spores form on the lower surface of the leaf just below leaf spots, under humid conditions. Leaf spots grow large enough to blight the entire leaf. Infected leaves fall off, resulting in bare branches throughout the shrub. The fungus also infects branches anywhere from the soil line to the tip of the branch. These infections start out as dark brown to black short lines, and grow to completely encircle and girdle the stem. Roots of the boxwood shrub are not infected by the fungus and remain healthy.

Virginia Tech Plant Disease Clinic

Photo 2: Stem lesions caused by boxwood blight

Boxwood blight has not yet been identified in Minnesota. If you suspect your plant is infected with boxwood blight first use the University of Minnesota Extension's online diagnostic tool for Boxwood to determine if the problem might be a more common look alike. If Boxwood blight appears to be the most likely suspect, please contact the Minnesota Department of Agriculture by sending an email with photos of the plant to or leave a voice message at 1- 888-545-6884. A MDA employee will respond in 1 to 2 days.

Downy Mildew
A second invasive disease to be on the look out for this year is downy mildew of basil. This disease is caused by the fungus-like organism Peronospora belbahrii. Downy mildew on basil was first found in the United States in Florida in 2007. It was seen throughout the eastern states in 2008 and was identified in Wisconsin in 2010. Downy mildew of basil has not been officially identified in Minnesota.

M. McGrath, Cornell University,

Photo 1: Downy mildew symptoms on the upper leaf surface of a sweet basil plant

Only basil plants (Ocimum sp.) are susceptible to this species of downy mildew, although other species of downy mildew can infect other common garden plants. The commonly grown sweet basil, Ocimum basilicum, is highly susceptible and growers have reported 100% crop loss due to the pathogen. Infected plants first display yellowing on the lower leaves. As the disease progresses these leaves develop dark brown to black spots. If the lower surface of infected leaves is examined, gardeners will notice that the leaves look dirty, or are covered with a thin layer of gray to brown fungal growth. This dirty looking fungal growth includes spores and spore producing structures of the downy mildew pathogen.

The downy mildew pathogen can move into a garden on infected seed, transplants or on wind currents. Unlike other downy mildew pathogens that prefer cool weather, basil downy mildew will tolerate cool weather but thrives in warm conditions. This means that the pathogen is actively growing and spreading for most of Minnesota's growing season.

At the end of April, University of Wisconsin plant pathologist Amanda Gevens reported identification of basil transplants infected with downy mildew, available for sale at a local store in Wisconsin. This early arrival of the pathogen in the Midwest could mean that more severe disease is found in the region this year as the pathogen will have time to grow and spread.

S. Jensen, Cornell University,

Photo 2: Dark growth on the lower leaf surface of a plant infected with basil downy mildew

Avoid downy mildew of basil by purchasing only healthy transplants and spacing plants to provide good air movement and quick drying of leaves after rain or dew. The basil varieties 'Spice' 'Blue Spice' and 'Blue Spice Fil' have been reported to have good resistance to downy mildew. 'Genovese', 'Martina', 'Italian Large Leaf' and 'Superbo' are all highly susceptible to the disease. 'Red Leaf' 'Red Rubin' 'Lemon' 'Lemon Mrs. Burns' 'Lime' and 'Sweet Dani Lemon Basil' were susceptible to downy mildew but had fewer symptoms than the most susceptible varieties.

If you suspect your basil plants are infected with downy mildew, please contact the Minnesota Department of Agriculture by sending an email with photos of the plant to or leave a voice message at 1- 888-545-6884. A MDA employee will respond in 1 to 2 days.

Michelle Grabowski, UMN Extension

JAR Y&G.jpg

N. Gregory, UDEL,

Photo 1: Sporulating gall of Japanese apple rust on juniper

Cool wet spring weather stimulates galls of several rust fungi (Gymnosporangium spp.) to produce bright orange gelatinous spore producing structures that readily catch a gardeners eye. Several species of Gymnosporangium rust fungi are native to Minnesota and infect eastern red cedar trees, Juniperus virginiana during part of their lifecycle and trees and shrubs from the Rosaceae family during a different part of their lifecycle. Native Gymnosporangium rusts include cedar apple rust, quince rust, hawthorn rust and juniper broom rust. Photos and descriptions of these plant diseases can be seen at the UMN Extension online plant diagnostic tool What's wrong with my plant?

A new Gymnosporangium rust has recently been found in the United States but not yet in Minnesota. Japanese apple rust is caused by Gymnosporangium yamadae. This fungus does not infect Minnesota's native red cedar trees but does infect Juniperus chinensis and J. squamata. Both of these species of juniper are sold as ornamental trees or shrubs here in Minnesota. Although Japanese apple rust is unlikely to cause any serious damage to junipers, this fungus also infects apple trees. At this time, it is unknown how the apple varieties grown in Minnesota will respond to Japanese apple rust.

To identify Japanese apple rust in juniper, look for round woody galls that are 1/2 to 1 inch in diameter. When wet, these galls become covered in a 1/4 inch thick layer of orange gelatinous goo. As this spore filled goo dries, it will become apparent that they arise from short orange projections, like small shelves sticking out from the side of the gall. In contrast, the native cedar apple rust galls produce orange gelatinous tentacles that swell to 1-2 inches long when wet.

If you suspect a juniper in your area has Japanese apple rust, please contact the Minnesota Department of Agriculture by sending an email to or leave a voice message at 888-545-6884. The MDA will contact you with further information in 1-2 days.

Forest Tent Caterpillars Are Out Now

Jeffrey Hahn, Asst. Extension Entomologist

Did you have a problem with forest tent caterpillars (FTC) last year? If you did, expect to see them again soon as they have started to hatch during late April. You can recognize these caterpillars from their blue and black body and white footprint or keyhole shaped spots on their back. Despite their name, FTC do not construct conspicuous webs. If you find a large tent in a tree this spring, that is from eastern tent caterpillars.

Jeffrey Hahn

Photo 1: Forest tent caterpillar and damage

FTC are primarily a problem because they feed on the leaves of trees and shrubs, especially aspen, birch, oak, and linden/basswood. If they are abundant and their normal food is in short supply they will crawl down trees and also feed on fruits, vegetables, and flowers. They can also become nuisances when they wander around looking for sites to pupate (which has earned them the nickname 'armyworms'). This can lead them to crawl onto nearby buildings and other structures.

Populations of FTC are cyclical, with periods of few and increasing numbers of FTC lasting about 8 - 13 years. Eventually these increasing numbers hit outbreak numbers which lasts about three to four years. FTC populations in the Twin Cities, though, appear to be less cyclical.

Fortunately, healthy, mature trees can tolerate severe defoliation, even in several consecutive years. Young and unhealthy trees are more susceptible to injury and should be monitored closely for the potential need to treat. There are a variety of residual insecticides that you can use if you want to protect your plants. Consider using products that have a low impact on the environment, such as Bacillus thuringiensis, spinosad, and insecticidal soap. Bacillus thuringiensis is a particularly good product if the tree is flowering since it will not harm visiting honey bees.

There are also insecticides available to protect garden plants, including food crops. Be sure to check the label to be sure the particular product you want to use is cleared to treat the plants you wish to protect.

An Imprelis Update

Kathy Zuzek, UMN Extension Educator, Horticulture

The History of Imprelis
In October of 2010, a new broadleaf weed killer by the name of Imprelis became available to turf professionals. Key to DuPont's release of this herbicide was its effectiveness at very low concentrations, its low toxicity to humans and other mammals, and its effectiveness on difficult-to-control turf weeds such as creeping Charlie, wild violets, clover and Canada thistle.

During the spring of 2011, damage to ornamental plants in landscapes where Imprelis had been applied began to appear in the eastern half of the United States including Minnesota. Damage to new growth of plants became visible within a matter of weeks after an Imprelis application and included twisting and/or browning of shoot tips, leaves, and needles (Photo 1).

K. Zuzek

Photo 1: Brown, twisted, & drooping shoot tips from Imprelis applications

As the summer progressed, impacted shoots and their associated leaves, needles, and buds often died on the most susceptible species. In some cases, entire trees or shrubs died. Broadleaf and conifer species of ornamental plants were impacted but conifer species were impacted much more severely. The most seriously impacted species from states east of Minnesota were Norway spruce (Picea abies) and eastern white pine (Pinus strobus) trees. The most seriously impacted species in Minnesota were white spruce (Picea glauca) and eastern white pine, but noticeable damage to Colorado blue spruce (Picea pungens), Norway spruce, Austrian pine (Pinus nigra), red pine (Pinus resinosa), and arborvitae (Thjuja occidentalis) were also common. A wide range of damage was seen among all of these species. Some plants showed little injury throughout the 2011 growing season, others died quickly, and the symptoms on others became more severe as the growing season progressed. The number of plants damaged, the level of damage, and plant mortality was highest among white spruce and eastern white pine in Minnesota.

In August of 2011, sales of Imprelis were stopped but damage to trees and shrubs in landscapes continues to be an issue. DuPont initiated a claims resolution process to compensate customers with damaged or killed plants. The deadline for claims submission was February 1, 2012 and DuPont is now processing claims submissions. Lawsuits have also been filed against DuPont.

How Imprelis Works
Aminocyclopyrachlor is the active ingredient in Imprelis and is classified as a synthetic auxin or growth regulator type of herbicide. Imprelis and other growth regulator herbicides are absorbed by roots and leaves and then move via the xylem & phloem to two of the meristematic regions responsible for new plant growth: shoot tips and root tips. In susceptible plants, growth regulator herbicides produce characteristic twisting and curling of the shoot tips and their foliage and plant mortality often follows. Herbicides are meant to kill and eliminate targeted plants such as weeds. When they impact non-targeted plants such as conifers in the case of Imprelis, problems arise.

Imprelis does not bind well to soil particles and is a very water-soluble compound. It is also a very stable compound in soil. Because it is active over a long period of time in soils and because its low adsorption and high solubility allow it to move downward in soil once applied to turf, it appears that Imprelis was able to reach the root systems of ornamental plants. Absorption of the herbicide occurred through the roots of these plants, followed by movement through the plant to new growth areas, and injury occurred. It is still not known why particular spruce and pine species are so susceptible to Imprelis.

K. Zuzek

Photo 2: Branch with active bud-break


K. Zuzek

Photo 5: Branch with no bud-break

Will Trees Damaged by Imprelis Recover? The answer to this question depends on the severity of damage. Trees that showed minimal damage (such as a small amount of injury at shoot tips in the upper branches of a tree) during the spring of 2011 and had little additional decline during the growing season will probably survive.These trees have active bud-break and are putting on new growth throughout most of their crowns this spring (Photo 2). Most broadleaf trees and shrubs and some of the conifer species that were impacted by Imprelis are in this group. Retaining these trees and shrubs in the landscape will depend on how badly damage impacts their appearance. Care of these plants during the recovery period should include irrigating the plants during dry periods to minimize moisture stress, avoiding over-watering that can cause additional stress, and avoiding fertilization for a minimum of one growing season to avoid excess growth that can compound herbicide injury.

Trees that showed damage last spring (Photo 3), further decline (dieback of shoots, dead needles and buds, yellowing of the tree canopy) during the 2011 growing season and/or the winter of 2011-2012 (Photo 4), and limited or no bud-break throughout the tree crown this spring (Photo 5) will probably continue to decline and then die. Even if these trees do not die, they will be of little or no aesthetic value in the landscape. In Minnesota, the majority of trees in this group are white spruce and eastern white pine.

K. Zuzek

Photo 3: Initial symptoms on white spruce in July, 2011

K. Zuzek

Photo 4: Symptoms on the same white spruce in April, 2012

DuPont's Compensation Plan If you are a property owner with Imprelis-damaged plant materials and you and your lawn care professional are part of the claims resolution process that DuPont enacted, you can access information on the claims process here. Compensation will cover removal and disposal of impacted trees, replacement costs or direct payment for removed trees, care of replacement trees , two-year warranties for all replacement trees and for any other trees impacted by Imprelis over the next two years, maintenance of impacted trees as they recover, and additional compensation for inconveniences associated with Imprelis.

Removal and Disposal of Plant Materials and Soil containing Imprelis
Listed below are some of Dupont's recommendations for the disposal of plant materials and soils containing Imprelis. These recommendations are for individuals who had Imprellis damage to plant materials but are not part of the claims process against DuPont. You can access the full list of recommendations here .

    Disposal Recommendations
  • No tree debris should be left on site.
  • DuPont recommends disposal of tree and excavated soil materials in solid waste landfills that will accept such waste.
  • The Imprelis label prohibits the use of grass clippings for mulch or compost.
  • Under no circumstances should tree material be used for mulch or compost or disposed of in facilities that would turn it into compost or mulch (e.g., recycling).
  • Trees that are cut down may be used for lumber, firewood, or to fuel various wood burning processes if such usage is otherwise consistent with state, regional and local regulations. Open burning is also an alternative if approved under local regulations.
  • If not landfilled, excavated soil should be disposed of in locations where it will not impact any other plantings through direct application or runoff. With the property owner's consent, excavated soil may be used on site as long as it is applied well away from desirable plant root.
Recommendation for planting replacement plants:
  • To address potential effects of Imprelis remaining in the soil, activated charcoal (that serves to deactivate any residual herbicide) should be applied to the backfill soil in accordance with manufacturer's instructions:

  • For liquid applications, apply at a rate of 1 pound of activated charcoal in each gallon of water uniformly to sides and bottom of hole dug for tree planting as well as to the complete root ball of the tree to be planted. Also, spray the burlap covered root ball if burlap is left in place. Thoroughly coat to the point of run-off the surfaces of the hole and the root ball.

    For dry applications, during tree planting, apply activated charcoal at a rate of 7 to 14 lbs/1000 cubic feet of soil and thoroughly mix with the clean soil. Use this soil as the new backfill soil when planting the tree.

  • Excavated soil should not be used as backfill. New soil, of a similar nature as the existing soil, should be used to backfill around the root ball. No fertilizer should be added to the backfilling mixture.

  • The replacement plant should be watered according to nursery recommendations.

  • The property owner should ensure adherence to best management practices consistent with the geographic area in which it is performing this work, taking into account any unique environmental and climate conditions, and any state, regional or local ordinances. Information on selecting, planting, and care of trees in Minnesota can be found here.

Based on the degradation rate of Imprelis in soils and on recommended planting times for Minnesota, a good additional recommendation would be to not plant susceptible species into Imprelis-impacted landscapes until September of 2012.

Karl Foord - Extension Educator, Horticulture

Click on the link to see the video with host Dr. Mary Meyer, Professor of Horticulture

Sungold and Moongold Apricots (Prunus armeniaca mandshurica 'Sungold' & 'Moongold'

Karl Foord

Photo 1: Apricot Flower (Prunus armeniaca)

Karl Foord

Photo 2: Young Apricot Fruit (Prunus armeniaca)

Regent Serviceberry (Amelanchier alnifolia 'Regent'

Karl Foord

Photo 3: Regent Serviceberry (Amelanchier alnifolia 'Regent'

Northern Strain Redbud (Cersis canadensis 'Northern Strain')


Karl Foord

Photo 4: Northern Strain Redbud (Cersis canadensis 'Northern Strain')

Emerald Triumph Viburnum (Viburnum 'Emerald Triumph')

Karl Foord - Extension Educator, Horticulture

Click on the link to see the video with host Dr. Mary Meyer, Professor of Horticulture

Acrocona Norway Spruce (Picea abies 'Acrocona')

Karl Foord

Photo 1: Acrocona Norway Spruce (Picea abies 'Acrocona')

Clove Currant (Ribes odoratum)

Karl Foord

Photo 2: Clove Currant (Ribes odoratum) flower

Flowering Crabapple (Malus hybrida 'Indian Summer')

Karl Foord - Extension Educator, Horticulture

While exploring the Minnesota Landscape Arboretum I encountered Dr. Stan Hokanson the woody plant breeder in the Department of Horticulture Science. He introduced me to the Clove Currant (Ribes odoratum). The odoratum nomenclature is well deserved. Clove Currant give off a very pleasant spicy fragrance hinting of clove and alspice (Photos 1 and 2). The plant has yellow flowers and a corolla long enough to restrict its pollinators to those with long tongues like bumblebees (Photos 3 and 4).

Another tree with fragrant flowers is the Korean crabapple (Malus bacatta jackii) one of the earlier flowering crabapples. The flowers are being visited by native bees (Photos 5 and 6).

Karl Foord

Photo 1: Clove Currant (Ribes odoratum) flower close-up

Karl Foord

Photo 2: Clove Currant (Ribes odoratum) flower

Karl Foord

Photo 3: Bumblebee showing long tongue required to reach nectaries on Clove Currant

Karl Foord

Photo 4: Bumblebee pollinating Clove Currant

Karl Foord

Photo 5: Native bee (Andrena ssp.) on Korean crabapple (Malus baccata jackii)

Karl Foord

Photo 5: Native bee on Korean crabapple (Malus baccata jackii)

Frost Damage to Apple Flowers

Karl Foord, Extension Educator, Horticulture

Temperatures in the mid to low 20's were encountered last Tuesday April 10th in many parts of central and southern Minnesota. Apple fruit flowers are damaged at temperatures lower than 28 degrees F. depending on the stage of the flower bud and the length of time at the low temperature. The more open the flower bud the more susceptible the bud is to low temperature damage.

The following picture gallery shows the types of frost damage experience by apple flowers in Chaska, Minnesota. The gallery begins with heathy flowers with green stigmas and cream anthers, shows partially damaged flowers with dead stigmas and soom dead anthers, and finally a flower will all parts killed.

Karl Foord

Photo 1: Healthy Apple Flower

Karl Foord

Photo 2: Healthy or partically damaged apple flower

Karl Foord

Photo 3: Totally damaged flower top left and partially damaged flower bottom right

Karl Foord

Photo 4: Frost killed apple flower with brown stigmas and anthers

Karl Foord

Photo 5: Frost killed apple flower with brown stigmas and anthers

Karl Foord, Extension Educator - Horticulture

Click any image below to open a slideshow.

Many tree flowers are described as inconspicuous. This may be more a matter of size and location. Tree flowers tend to be shhort lived, inaccessible if higher up in the tree, and smaller than many of our showy garden flowers. On closer iinspection you may find them to be rather intreguing. Consider the first photo of Autumn blaze Maple.

A Look at the New USDA Hardiness Zone Map

A hardiness zone map (HZM) provides information that gardeners and professional horticulturalists use in determining which herbaceous and woody perennial plants will survive cold temperature in a particular geographic area. Last week the United States Department of Agriculture released a new hardiness zone map to replace the older 1990 version.

As with past maps, the new map:
  • is a visual representation of average annual minimum temperatures across the United States. Data points used to create the map were the lowest daily minimum temperatures recorded at thousands of temperature data stations during each of the years sampled.
  • divides the U.S. into multiple hardiness zones with 10o F differences in average annual minimum temperatures. Zone 1 is the coldest zone (-50o F to -60o F).
  • divides each hardiness zone into "a" & "b" with "a" being the colder half of any zone and "b" the warmer half.

There are changes in the new map and the process that was used to develop it:


    Photo 1: 1990 USDA Hardiness Zone Map

  • The 1990 HZM was based on data from a 12-year period (1976-1990) while the new HZM is based on data from a 30-year period (1976-2005).
  • The data used to create the new map was more complete, and a complex algorithm was used to interpolate between recording stations. Temperature data from more than 8000 temperature data stations belonging to the National Weather Service, the USDA Natural Resources Conservation Service, the USDA Forest Service, the Bureau of Reclamation, and the Bureau of Land Management was used. Average minimum temperatures were then calculated for ½ mile square computer grids for the entire country to create the new hardiness zone map. This was followed by a review process that included climatologists, agricultural meteorologists, and horticultural experts who checked for errors, looked for the source of errors, and corrected errors.


    Photo 2: 2012 USDA Hardiness Zone Map

  • The new map is Geographic Information System-based. This means that the map is more accurate, is interactive (by zip code) to improve user experiences, and has higher resolution that can show smaller areas of zone delineations than before. While the 1990 map was a static image and was not designed for web use, the new map allows users to zoom in to a local area to see the higher temperatures of cities that are heat sinks, the lower temperatures on mountain tops, and the buffering effects of large bodies of water on temperature.

What does the new hardiness zone map show?
In general, the new map shows what we have all been experiencing in recent history: warmer low temperatures during winter. A shift of ½ of a zone was common for much of the country. Closer to home, here is what happened to Minnesota's hardiness zone map:


    Photo 3: 1990 USDA MN Hardiness Zone Map


    Photo 4: 2012 MN Hardiness Zone Map

  • There was a ½ zone shift for much of the state because Minnesota, like the rest of the U.S., has been experiencing warmer annual minimum temperatures during the time period used to create the new HZM.
  • Zone 5a (with average minimum temperatures of -15o to -20o) crept up into the south central portion and the far southeastern corner of Minnesota.
  • Much of the southern ½ of Minnesota that was formerly divided into zones 4a & 4b is now zone 4b (with average minimum temperatures of -20o to -25o).
  • The four pockets of Zone 2b (with average minimum temperatures of -40oto -45o) disappeared from northern Minnesota.
  • The amount of Minnesota that is zone 3a (with average minimum temperatures of -35o to -40o) shrank significantly due to an increasing area of zone 3b (with average minimum temperatures of -30o to -35o).
  • Parts of the far northern shore of Lake Superior that were formerly zones 4b and 4a are now designated as 4a and 3b, meaning they are colder.

What kind of impact should the new hardiness map have on Minnesota gardeners?

  • We can all rest easy knowing that the warmer minimum annual temperatures we have been enjoying over the past years really did happen!
  • Remember that a HZM is created based on average annual minimum temperatures and should only be used as a general guide. By the very definition of average, we know that temperatures lower than the average minimum temperature of the zone you live in will occur. Pick your plants accordingly. Maybe we can broaden the palette of plants we choose to grow in Minnesota a bit, but be cautious and wise in your weighing of risk vs. gain as you trial new plants. Losing an herbaceous perennial or quickly-maturing shrub to winter injury may be of little concern in terms of the time it takes to establish a replacement plant. Losing a slow-growing shrub or a tree that takes decades to grow to mature size creates more pain.
  • Hardiness zone maps are of no help in predicting plant damage or mortality during acclimation and deacclimation. Remember that hardiness is not just about the lowest temperature a plant must survive during a winter. Every year, starting in late summer, perennial plants goes through a multi-month process called acclimation that prepares them for winter survival. In spring dormant plants go through a reverse process called deacclimation that restores their ability to actively grow during the growing season. Plants can be winter-injured or killed by abnormally low temperatures during the months of acclimation and deacclimation too. This is especially true of marginally hardy plants from warmer parts of the country or world that we may try to grow in Minnesota.
  • Hardiness zone maps provide gardeners with one category of plant performance information: winter survival. Good plant performance is not just about winter survival. If the new HZM persuades you to plant cultivars and species new to you, remember that there are other selection categories to consider as you match a plant to your planting site: soil texture, soil moisture, soil pH, light exposure, precipitation, etc.

What to Do about Pot-Bound Plants

em>By Jeff Gillman, Associate Professor - Department of Horticultural Science

So you've just bought a pot-bound plant and you don't know what to do? Gary Johnson, Chad Giblin and I have been testing various techniques to get trees out of their pot-bound states for the last 8 years or so, and here are some of the things that we've found.

The number one problem with planting a pot-bound tree is that they are usually planted too deeply. Trees in containers often have three inches between their uppermost roots and the soil line -- or even more! When a tree like this is transplanted into a landscape without having its planting height adjusted, the roots circling near the top of the container will eventually press up against the stem of the tree and strangle it to death (photo 1).

Jeff Gillman

Photo 1: Pot bound plant's root system 5 years after planting; tree was planted too deep based on media level in pot

When planting a container grown tree make sure that the first large root connecting to the stem (usually about ¼ inch in diameter) is visible after you fill in the planting hole. This will ensure that as the tree gets older and roots and stems expand there will be no compression of the stem.

Roots do not continue to grow in a circle after a tree is planted, even in severely pot-bound plants. Roots which were already circling in a pot-bound plant when the plant was transplanted will not straighten out after planting, but as the roots grow they will grow outwards, not in a circle (photo 1). Right now, with the research we currently have, it is not clear how circling roots will affect a tree if they are only present below the stem. Yes, they look ugly, but looking ugly doesn't mean they're not doing their job.

Most of the common techniques that the extension service recommends for pot-bound trees will not really do that much. Scoring the sides of pot-bound root balls with a razor knife or butterflying the root ball with a shovel just doesn't work that well. If you're really serious about not having any circling roots then you need to use a technique called a box cut. A box cut is performed by cutting the root ball into the shape of a box by using a pruning saw (photo 2).

Jeff Gillman

Photo 2: Box cut on potted arborvitae

Root balls treated using the box cut method generally had good looking root systems after 5 years in the ground (Photo 3). Circling roots were drastically reduced, but not entirely eliminated.

Jeff Gillman

Photo 3: Box cut plant 5 years after planting

In terms of what we're actually recommending -- Right now we are recommending that you check the planting depth of all container grown plants before planting. Only in rare cases do we think that you'll find them planted at the proper depth. After removing the media from the top of the root ball to correct planting depth look for circling roots. If you don't see any circling roots thicker than a pencil then it probably isn't worth your time to do anything besides planting the tree, being careful to make sure that the uppermost root is planted at the soil surface. However, if you see any circling roots with a diameter greater than a pencil, we recommend using a box cut on the root ball.

On a final note, none of the trees we purchase ever need to be pot-bound. There are many different containers out there that will all but eliminate circling roots, such as Smart Pots, Superoots, and Root Trappers (photo 4). If we demand that companies provide trees planted in these containers then, someday soon, we may never need to worry about circling roots again.

Jeff Gillman

Photo 4: Root Trapper Pot

Pruning Trees to Avoid "Disasters"

Karl Foord, UMN Extension Educator

USDA Forest Service

Figure 1: Crown thinning

Karl Foord, UMN Extension Educator

I have Autumn Blaze maple trees that produce great fall color and grow very quickly. In the tree trade they are known as "disasters" because they produce a very dense canopy that is subject to limb breakage in ice storms and uprooting in high winds. To avoid my trees becoming disasters I have pruned them to the point where I was pretty high up in the tree and getting in precarious positions. It was time to get an arborist. I took pictures of the trees before pruning and after pruning to see the difference and then looked on line for verification.

USDA Forest Service

Figure 2: Crown raising

The arborist pursued two strategies, crown thinning (Figure 1) and crown raising (Figure 2) both taken from the USDA Forest Service publication, "How to Prune Trees".


The before and after pictures show that some lower limbs were removed and how the center of the tree was opened up, reducing wind resistance and the number of limbs available for ice accumulation.


The trees in the picture were planted in 2000. I have another set of Autumn Blaze maples planting in 1997. What is the value of the trees? If they are damaged in a storm, how much time has been lost? I would rather hedge my bets and see that they are properly pruned. The last photo shows one of the trees planted in 1997.

Tree planted in 1997.

What's in a Cone?

Karl Foord

Photo 1: Korean Fir Abies koreana 'Horstmann's Silberlocke.' To view all photos, please open PDF.

Karl Foord, UMN Extension Educator

Cones are a big part of the ornamental appeal of evergreens. The cones provide spectacular colors, such as the iridescent cones of the Korean fir (Photo 1), and striking geometric patterns formed by the interlacing scales of red tamarack cones and light pink Acrocona spruce cones (Photos 2 and 3).

To view all photos in this article, please click here: Gymnosperms 7-1-11.pdf

Most but not all gymnosperms have male and female cones on the same tree (Photo 4). The male cones are short lived but the female cones persist for several years. The persistent female cones are the seed bearing structures of gymnosperm plants of which conifers are the most abundant. The word "gymnosperm" comes from the Greek meaning naked seed as opposed to other flowering plants termed angiosperms whose seeds are enclosed during pollination. Conifer seeds develop on the surface of the scales of the cone which open to receive the pollen to fertilize the egg cell and then close to protect the growing seed. Seed maturation can vary from 6 to 24 months depending on species. Most pines exhibit first and second year female cones (Photo 5). When the cone matures in the second year, the scales will separate to free the mature seed (Photo 6 and 7).

These seeds are the edible seeds we know as pine nuts. Worldwide some 20 pine species produce sufficiently large seeds for commercial purposes. The Stone Pine (Pinus pinea) has been the primary source of pine nuts In Europe and was domesticated some 6,000 years ago.

In North America three western pinyon pine species are the main source of pine nuts; Colorado pinyon (Pinus edulis), Single-leaf pinyon (Pinus monophylla), and Mexican Pinyon (Pinus cembroides).

Another fascinating thing about cones is the shape of the interior space between the scales of the cone. This space exhibits aerodynamic qualities enabling the cone to filter large amounts of pollen from the air and deposit them at the most advantageous position for pollination to occur.

I'll never look at a pine cone in the same way ever again.

Espalier - An Art Based on Science

Karl Foord, UMN Extension Educator



Espalier is a plant shaping/pruning method where plants are grown in a single plane limiting their height and width to a defined area. The area is usually defined by a permanent framework which stabilizes the plant. The espalier is developed and maintained by pruning techniques of which timing is a critical part, for a pruning cut made in early spring will likely have different results than one made in midsummer.


Role in the Landscape

Espalier takes on a number of high profile roles in the landscape. It can be the focal point of the landscape design or take on lesser roles as privacy screens or backdrops. It can function as a key plant softening the appearance of walls or act as an accent or specimen plant. An accent plant has year around interest like most evergreens. A specimen plant has seasons of interest such as flowering, fruiting or attractive bark. Because the espalier technique reduces the number of leaves on a plant, the stems, bark texture, leaf shapes, flower and fruit are more exposed and emphasized. Due to its spatially defined nature, espalier makes efficient use of space and permits a greater variety of plants than if full sized plants were used.

Espalier is a gardening technique of long standing. It has been practiced in gardens of Egyptian Pharaohs, middle age monks, and French kings. One of the more famous locations where espalier is on display is at Claude Monet's garden in Giverny, France (Exhibit 1 & 2).

Espalier techniques

Mastering espalier technique involves understanding how the plant responds to pruning cuts and shape manipulation. It involves choosing the buds one wants to form the branches that will create the desired shape. At least four techniques are essential to success.

The first involves cutting and bud orientation. When a terminal bud is cut, its hormonal inhibition of buds down the stem is released and the cut stem establishes many branch point s (Exhibit 3). The idea is to make the cut above the bud facing in the direction you want the stem to grow.


The second involves knowing how to bend a branch. Branches should be bent when they are young and most supple. The best way to proceed is to attach the branch to a splint prior to the bend (Exhibit 4) and bend the branch over a few weeks time, adjusting the angle of the branch 5-10° at a time (Exhibit 5). Care should be taken to not girdle any stem with plant ties and to move them annually if necessary.



The third involves maintaining the plant within the proscribed limits by precise pruning of the branch laterals and sub-laterals (Exhibits 6 & 7). This serves to limit the length of stem growth, and encourages the development of fruiting spurs.



Fourth involves eliminating unwanted buds through the technique of rubbing. Should a bud exist in a place where a stem is not desired, the bud is removing by rubbing it off the stem (Exhibit 8).


An example of an espaliered apple (Exhibit 9) as well as many other plants can be observed at the Landscape Arboretum.


Want to know more?

This article is by necessity a very basic introduction. A very helpful book on the technique is, Espalier: Essentials of the Candelabra Pattern by Katherine Aby. Espalier can be appreciated and understood from books but, espalier techniques are truly learned and developed by practice. There will be an opportunity to observe and practice next month. Katherine will be teaching a one day class on the basics of espalier on Wednesday, June 15 to interested parties. Location will be based on # of registrants being either in South Minneapolis or closer to the Arboretum. If you are interested please contact Katherine at:

Special thanks to Katherine Aby for the use of her illustrations. More illustrations and a more detailed discussion of techniques can be found in her book, which can be obtained on her website .

Spring Pruning Tips for Woody Landscape Plants

Kathy Zuzek, UMN Extension Educator, Horticulture

Photo 1-1.jpg

Photo 1:Rodent damage girdled these stems. K. Zuzek, UMN Extension.

Many shrubs are planted in landscapes for their wonderful floral display. One of the few shrubs that have bloomed to date during this slow-to-arrive spring is forsythia. With masses of yellow flowers, forsythias are wonderful plants in the spring landscape. But cultivars with appropriate flower bud hardiness for use in Minnesota are large shrubs with rapid growth rates. Their ability to quickly produce large quantities of upright and arching stems that tangle together to produce an unkempt-looking plant turns them into aesthetic liabilities in the landscape after blooming. Consistent pruning is the solution. Enjoy the floral display but remember that if you have reason to prune your forsythia, pruning should happen immediately after bloom.

Like many of our shrubs that bloom in early spring, forsythias produce flower buds on previous years' stems. To enjoy maximum bloom on these shrubs both this year and next year, it is important to prune these plants immediately after flowering stops this spring. This allows enough time for plants to produce stems during the 2011 growing season that will house next year's flower buds. A list of shrubs that bloom on previous years' wood can be found at

You may also need to prune shrubs this spring because of damage that occurred during winter thanks to rodents, rabbits, and low winter temperatures.

Mice and voles were busy under the snow line this winter stripping bark at the base of shrubs and young or thin-barked trees (Photo 1). Rabbits can create the same kind of injury by stripping bark above the snowline. If this feeding removed bark completely around stems and trunks, plants are effectively girdled and stems and trunks will die above the point of girdling.

Photo 2-1.jpg

Photo 2: Winter injury of rose canes. K. Zuzek, UMN Extension.

Most deciduous shrubs have the ability to produce new shoots from their crowns or from roots. Prune off the girdled stems this spring and new shoots will replace the damaged stems over the next few years. On the other hand, the best course of action for a completely girdled tree is removal and replacement with a new tree. Many trees sucker below girdling damage. Many trees are also produced through grafting or budding onto rootstocks. Suckers from below the graft union that are trained into a new tree will give you a different (and probably undesirable) tree than the one you started with.

Rabbits will often clip entire stems off of deciduous shrubs as they feed during winter. If this browsing injury left you with irregularly shaped shrubs and stems of widely different heights, you may want to prune these plants to recreate their natural plant habit. Entire stems can be removed at the base to allow growth of new replacement stems from the crown or roots. Or stems can be pruned back to ¼" above a bud that will direct growth in a desirable direction as the bud expands and grows. Make this pruning cut at a 45° angle with the high point of the cut directly above the bud.

Winter injury should also be removed from woody plants this spring. Shrub roses are a prime example of this. Many repeat flowering shrub roses experience extensive cane mortality due to low winter temperatures. The injury is very evident in spring. Dead canes will be brown or black and there will be no actively expanding buds. Healthy canes can be green, red, or brown (depending on cane age and cultivar) and you will see buds along these canes actively expanding and growing in spring (Photo 2). Often low winter Photo 3-1.jpg

Photo 3:Sap bleeding from pruning wound. K. Zuzek, UMN Extension.

temperatures kill canes protruding above snow while the bases of canes covered by snow are not injured. Find an outwardly facing bud in the live portion of canes and prune canes back to ¼" above this bud using the 45° angle cut mentioned above. If canes are killed to the base, remove them completely and new canes will develop from the crown and roots of the plant.

As you prune woody plants in spring, you may see sap oozing from pruning wounds (Photo 3). This may continue for several days and happens to varying degrees on many trees and shrubs. It is most noticeable on trees because pruning wounds are larger and are at or above eye level. This oozing or "bleeding" occurs when increasing temperatures in spring activate enzymes that convert the starches that store energy in plants into sugars that can be easily transported throughout the plant. As starches are converted into sugar, water movement is initiated upwards in stem and root xylem tissue due to differences in sugar concentrations throughout the plant. Pressure increases in the plant and this pressure Photo 4-1.jpg

Photo 4: : Oak wilt fungal spore mat. J. O'Brien, USDA FS ,

results in the "bleeding" from new pruning wounds. Bleeding can be unsightly but is not harmful to the plant. It actually signals active cell activity and a rapid pace of wound healing.

Remember that oak trees should not be pruned in April, May, or June. Sap feeding beetles of the Nitiduldae family are the insects commonly responsible for long-distance movement of fungal spores that lead to new infection centers of oak wilt. The beetles are attracted to both fresh wounds (like a pruning cut) on oak trees and to the fruity smell of the fungal spore mats produced in April, May, and June on oaks infected with oak wilt (Photo 4). As they visit the mats, they pick up spores that can be transferred to a fresh wound on a healthy tree. As these spores infect the tree, oak wilt occurs.

You buy a plant at a garden center. Where did it come from?

Karl Foord, UMN Extension Educator


Vanilla Strawberry™, Hydrangea paniculata 'Renhy'PP20,670

To answer this question I traveled to Newport, Minnesota to visit with Debbie Lonnee who is a Planning and Administration Manager at Bailey Nurseries. Don't be fooled by the title, Debbie knows hers plants intimately. Listening to her describe plant cultivars is like watching Monet paint.

Bailey Nurseries

Bailey Nurseries celebrated its 100 year anniversary in 2005 and is still managed by fourth generation members of the Bailey family. Bailey has production locations in Minnesota, Illinois, Iowa, Oregon, and Washington where it produces and distributes fruit and shade trees, ornamental shrubs and vines, roses, evergreens, fruits, perennials, and annuals. As a wholesale operation they sell to over 4500 retail garden establishments, landscapers, and growers located in 47 U.S. states as well as Canada, Europe, South Africa, Australia, and Japan. Try to imagine the effort involved in getting this wide variety of plants delivered bellaanna.jpg

Bella Anna™ Hydrangea arborescens 'PIIHA-1' PPAF

at the right time of year for each of these locations. It boggles the mind.


Endless Summer® Blushing Bride™ Hydrangea, Hydrangea macrophylla 'Blushing Bride' (PP17,169).

Breeding efforts and partnerships

The breeding efforts focus on improved habit and ornamental qualities, and disease resistance to name a few among a host of characteristics. The disease resistance objective is particularly noteworthy as its goal is to eliminate chemical treatments and to replace plants which have been shown to be particularly susceptible to diseases. Bailey has a breeding program but they do not restrict their efforts to their own program, as they solicit breeding partners from around the world. This greatly increases their reach and at the same time increases our exposure to newly produced plants. To get a sense of how this brings plant opportunities to you and me, I asked Debbie to trace the development of three new cultivars.

New Cultivars

One of the new cultivars is 'Vanilla Strawberry™' which is a Hydrangea paniculata with conical shaped flower. The interesting thing about this cultivar is that the flower starts out stark white and gradually changes from white to pink from the bottom up. This cultivar came from a partnership with SAPHO, an organization formed between French breeders and the Angers branch of the French Research Institute (INRA). Presently there are 20 breeder/stockholders including M. Renault, the breeder of 'VANILLE FRAISE ® 'Renhy'.


Twist-n-Shout® Big Leaf Hydrangea, Hydrangea macrophylla 'PIIHM-I' PP20,176

Another new cultivar is 'Bella Anna' which is a Hydrangea arborescens from the Endless Summer® series of reblooming Hydrangeas developed by Dr. Michael Dirr who spent most of his career at the University of Georgia. 'Bella Anna™' produces giant pink flowers that can measure up to 10 inches across. It is in essence a pink 'Annabelle' hydrangea.

It should be noted that H. arborescens does not change flower color in response to soil pH. The three other cultivars in the Endless Summer ® collection (Endless Summer™, Blushing Bride™, and Twist-n-Shout™) are all H. macrophylla species which does respond to soil pH. There are products called Color Me Pink™ and Color Me Blue™ which will change the color of your hydrangea flowers. The flowers become Pink in alkaline soil (pH greater than 7), and blue in acidic soil (pH less than 7).

A third cultivar is 'Little Devil™' Ninebark which is a Physocarpus opulifolius. This cultivar was bred by Dr. David Zlesak who was the previous editor of the Yard & Garden News and now a professor at the University of Wisconsin-River Falls (UWRF).
One of the advantages of this cultivar is its size. David was able to introduce a dwarfing gene creating a 4 foot tall dark leaved ninebark vs. a 10 foot tall 'Diablo' dark leaved littledevil.jpg

First Editions® Little Devil™ Ninebark Physocarpus opulifolius 'Donna May' PPAF

shrub. Much of today's breeding work focuses on creating small stature or dwarf varieties. This permits traditionally larger plants to fit into today's smaller landscapes as well as permitting a greater variety of textures, forms and colors for the landscape palette.

Last spring Twin Cities Live did a short segment on Bailey Nurseries, in which they showed a number of the production systems and facilities. This video is no longer available.

All photos courtesy of Bailey Nurseries.

Prunus x maackii: Amur Chokecherry Tree

Emily Dusek, UWRF

ed1.jpgWith a peeling, golden-hued bark reminiscent of the Betulaceae (birch) family, along with its dainty white blossoms dangling on two to three inch racemes from early to mid-May, it is clear the Gold Rush Amur Chokecherry Tree is a beautiful enhancement to any landscape!

The Amur Chokecherry (actually a member of the Rosaceae or rose family) calls the eastern side of the world home--specifically, Russia, Korea and Manchuria. It was brought to the United States in 1878 to the Arnold Arboretum as a gift from the Botanic Garden in St. Petersburg, Russia. Despite it's being native to the harsh taiga of Eurasia, this tree is still considered a desirable choice from zones 3a to 7. Though it's susceptibility to cherry pests (which include borers, scale, aphids, tent caterpillars, canker and leaf spot) increases in warmer climates, the Amur Chokecherry is recommended for cool northern climates where the majority of these problems are nonexistent for this species. It's a relatively trouble-free tree for the harsher climates of the world.ed2_Page_1.jpg

The Amur Chokecherry is tolerant of full/part sun and/or part shade in acidic, humus-rich, well-drained soils. While it has a tolerance for drought, it prefers to be kept moist (but not wet) at all times. On newly propagated trees, there is an occasional onion skin effect where the bark peels off on the young branches; exposing an array of colors that vary between yellow-brown, to orange-brown, to a copper color with a very glossy effect!

During the second or third year, the glabrous stems will start to develop lenticels. These branches will develop into a dense, round or broad-oval formation, spreading out to 25-35 feet in width. The Amur Chokecherry also has a tendency to grow with several trunks. Depending on what the owner desires, this tree may be grown in either clump form or as a single specimen (if trimmed to do so)! The maximum height ranges from 30' to 45.' The bark is similar to the mature branches in regards to how the colors vary from a bright, highly glossy copper to gold and amber. The lenticels are also prominent on the bark, forming long, horizontal, decorations across the thin smooth bark. As it ages, the bark will start to exfoliate in a manner similar to that of a river birch (Betula nigra).
With such ornamental bark, one might expect it to be the only positive feature of the Amur Chokecherry--but that is hardly the case! In early to mid-May, there is a lovely display of white flowers that consist of 5 petals, 3/8" in diameter. They occur in 2 to 3" long racemes that have about 20 to 30 flowers on them. The fruit then develops numerous small red drupes that are ¼" in diameter, and will then mature to black as they ripen. These fruits can be used to make jams, jellies, juice, or just left on the tree, luring birds near and far.

ed3.jpgThe dense, dark green summer foliage of the Amur Chokecherry contrasts nicely with the shiny bark. Come fall, the Amur Chokecherry is not the most exciting of specimens due to a lack of fall foliage interest, when the leaf color ranges from a drab yellow-green to an even drabber green. But when this uninteresting foliage is combined with such splendid bark and sweet little is an easily overlooked shortcoming.
The Amur Chokecherry is radiant in the winter season when its bark presents a stark contrast to the snow. Though this type of tree has multiple uses, it is most commonly used as an easily viewed specimen tree, small shade tree, or in a grouping because of its winter interest. Since this tree has such a cold tolerance, it may also be used as a container patio tree, or even as a replacement or complement to the dogwoods and evergreens so common in Christmas winter planters. Though the Amur Chokecherry is only known to be moderately tolerant of ozone pollution, it has been recommended by some sources for use as a bufferstrip around parking lots and even for median strip plantings along highways!

If cold tolerance and pest resistance were the Amur Chokecherry's only attributes, it would still be considered a valuable tree. But when combined with its superior golden peeling bark and flowers, this tree becomes an ideal ornamental tree that is as close to being a 'tree for all seasons' as is possible!

The author, Emily Dusek, graduated with an A.S in Horticulture from Century College in 2009. She is currently attending University of Wisconsin-River Falls to parlay her A.S. into a Bachelor's of Science. She has also received invaluable hands-on lessons working at Farrill's Sunrise Nursery in Hudson, Wisconsin. Emily first got into horticulture as just a baby; she has been told her first birthday present was a mini wheelbarrow and watering can!

What's New in Hardy Compact Shrubs?

Kathy Zuzek, UMN Extension Educator


Photo 1: Fall color of burning bush. Kathy Zuzek.

Plantsmen and plant breeders have been busy developing and selecting compact and small-stature colorful shrubs in the last few years. Some of those cultivars that will grow in our northern gardens are Fire Ball® burning bush, Little Lime™ hydrangea, Northern Accents® Sigrid rose, and Oso Happy™ Petit Pink rose. Photo2.jpg

Photo 2: Green flowers of Limelight & Little Lime. Bailey Nurseries.

Burning bush (Euonymus alatus) is a highlight in our Minnesota fall landscapes where it grows as a large shrub or small tree in landscapes and adds bright red to our fall landscapes when grown in full sun. In partial shade, burning bush provides wonderful combinations of pink and green in fall. 'Compactus', a commonly grown cultivar in Minnesota, grows to 6-8'. Although it is rates as hardy to Zone 4 (or hardy to -30° F), it often suffers some stem injury when winter temperatures drop to -25°F. Photo3.jpg

Photo 3: Little Devil ninebark. Bailey Nurseries.

Fire Ball® burning bush is a plant selected for its superior winter hardiness and tighter branching by Cole Nursery in Ohio. This is really not such a new cultivar as it was selected years ago. But it has been recently rebranded and is finally being widely marketed. Fire Ball's height and width are 5-6' and in Michigan where this plant has been tested, no winter injury was seen on stems of Fire Ball® during the harsh winters that did injure 'Compactus'.


Photo 4: Northern Accents Sigrid. Kathy Zuzek.

Hardy hydrangeas in Minnesota are the panicle hydrangea (Hydrangea paniculata) and the smooth hydrangea (H. arborescens), and many wonderful cultivars have been developed or selected among both species. Limelight® panicle hydrangea is a popular 8-foot tall cultivar whose new flowers emerge in tones of soft lime green and ivory and then mature to pink and burgundy in fall. A new introduction from Spring Meadow Nursery is Little Lime™ hydrangea with green and ivory flowers on a 3-5' plant and rated as Zone 3 hardy (hardy to -40°F.)Photo5.jpg

Photo 5: Oso Happy™ Petit Pink. David Zlesak.

Ninebarks (Physocarpus opulifolius) are large plants that can reach 8-10 feet in height and width. Several cultivars have been selected for their gold or red-purple foliage and some of these have more compact growth habits of 5-7 feet. The newest and smallest addition to the ninebark collection was bred here in Minnesota by Dr. David Zlesak and is named Little Devil™. Little Devil is a upright ninebark that grows to 3 or 4 feet in height and width with smaller burgundy leaves and white-pink flower clusters that pair beautifully with the small plant stature. Little Devil is listed as Zone 3 hardy.

Two new small statured roses are available this year for northern landscapes. The fourth cultivar of the University of Minnesota's Northern Accents® collection is Sigrid, with its double red flowers. Like its predecessors, Sigrid is a repeat-flowering polyantha rose with a 3' compact dense plant habit, large sprays of 1" flowers, and a high level of blackspot tolerance. Sigrid is crown-hardy in Zone 4 hardy.

Oso Happy™ Petit Pink rose is another Minnesota introduction from Dr. David Zlesak. Petit Pink is a Zone 4 hardy, repeat-flowering miniature rose whose pink and yellow petals add lots of warmth to a landscape. Petit Pink dies back in winter to the crown or within a few inches of the ground and then grows back into a 2.5'x 3' rounded dense plant.

Hand Pollination of Apple Trees?

Karl Foord, UMN Extension Educator


Business Week.

I recently attended a conference on native pollinators presented by Jennifer Hopwood of the Xerces Society. The Xerces Society (founded in 1971) has worked to conserve invertebrates and their habitat by focusing on conservation policy, advocacy, education, and research.

One rather alarming topic presented at the conference involved Maoxian County in the Chinese province of Sichuan (see map). In this region farmers have been forced to pollinate their apples and pears by hand because there are insufficient natural insect pollinators to ensure proper fruit set and thus a crop. These are high value crops that must be free of cosmetic defects to be marketable. To achieve this, the growers have resorting to spraying when there is the least hint of a problem. This has resulted in marketable material, but at the cost of having destroyed all the native pollinators in the region. There are beekeeping services but these individuals hesitate to locate their bees in the area because of the danger presented to their hives by the pesticide use strategies of the fruit producers. The result hand pollination by humans. This report comes from one county in a province that produces a little more than 1% of Chinese apples. Nonetheless, the province still produces some 409,000 metric tons of apples (in 2009). And this pollination problem is not an isolated case, but rather extends to other countries in the region such as Pakistan, India, and Nepal.


Hand pollination in China.

Such a situation does not translate well into American agriculture especially considering labor wages. If we were to do so it would look like this: It takes twenty Chinese workers working for 10 hours to pollinate a half acre (see pictures). Translated into an orchard in the United States where the workers were paid $9 per hour, it would cost the growers $3,600 in pollination services. This would probably double the cost of apples.

How can we best relate to this situation? A certain amount of the problem comes from an ignorance of the complexities involved in the relationships between insect pollinators and crop plants. However, we in the U.S. are not immune, even though the problems we face may be different. Our commercially managed honeybee populations are facing a number of challenges as noted by Dr. Marla Spivak in a previous article. We have also seen a dramatic decline in a number of native bumblebee species that were previously quite numerous. The exact causes for such a decline have not been definitively identified, however it is likely that there is no one cause but rather a series of causes. And again the complexity we face with biological systems rears its head.


Map of China's Provinces.

Hand pollination of fruit crops is about as unsustainable as a system can get. The fact that it exists anywhere sends me a strong signal to be extremely vigilant about land management, pollinator habitat, and pesticide use to avoid such an outcome in our fruit production systems.


Pollination problems in China

Status Review of Three Formerly Common Species of Bumble Bee in the Subgenus Bombus

What are your woody plants doing right now?

Karl Foord, UMN Extension Educator

Well, here we are in Minnesota where most parts of the state are sitting under several feet of snow. This led me to think about what was the state of my apple and maple trees.
The plants in our state are adapted to northern climates with harsh winters that are unfavorable to plant growth. One mechanism that these plants have adopted to survive involves a suspension of growth termed dormancy.

The bud that overwinters in an apple tree is a miniature shoot with apical meristem, leaf and perhaps flower primordial, and axillary buds enclosed by modified leaves termed bud scales. Bud scales protect the bud from mechanical injury, restrict gas exchange and prevent desiccation.

Preparation for winter and true dormancy
KFTbl.jpgThe buds are the photoperiod receptors and in preparation for winter undergo a series of physical and physiological changes triggered primarily by the shorter days of late summer. These short days (actually long nights) trigger the production of abscisic acid (ABA) which acts as a growth inhibitor. ABA has been found to build up to high levels in the fall. Although cool temperatures are not the primary trigger they facilitate dormancy of the buds. There is a point at which the bud cannot be induced to grow even given under optimum environmental conditions. At this point the bud is said to be in true dormancy. The only way the bud can be induced to grow is by experiencing a chilling period. Temperatures need to be below 45° F (7.2° C) and last for between 800 and 1,000 hours for northern adapted apples (Table 1). It may be the presence of ABA that inhibits growth and only after this inhibitor decays over time that the plant has the ability to respond to favorable environmental conditions. This removes the internal block to growth, but external factors such as low temperature can also inhibit growth in the early spring.

Dormancy can be distinguished from quiescence where the bud is in a resting state in response to adverse environmental conditions, but will resume when the environmental conditions become favorable again. Fascinatingly enough, roots overwinter in a in a quiescent state.

When the soil begins to warm, promoters of growth such as gibberellin and cytokinins build up, signaling the bud to resume growth.

Intracellular water management for plants in cold climates

Another aspect to surviving harsh winters other than the dormancy state is the management of cellular water either through deep supercooling or intracellular dehydration. Temperate woody plants utilize one of these two mechanisms.

Supercooled water is water below 32° F (0° C) that remains in a liquid state. Supercooled water can remain in the liquid state down to -36.6° F (- 38.1° C) and in the presence of dissolved solutes to -43.6° F (-42° C). This temperature is called the Homogeneous Nucleation Point. Without nucleating points no ice crystals will form above this temperature, and plants avoid cold damage by not allowing nucleating points. At temperatures below -43.6° F (-42° C) ice will form and the plant cells will be damaged or killed. Most temperate plants in North America utilize this mechanism.

Plants growing in parts of the world where temperatures fall below -43.6° F (-42° C) utilize a different mechanism. These plants avoid injury by preventing intracellular (within the cell) ice formation. Water freezes in the extracellular spaces which pulls liquid water out of the living cells leaving them dehydrated. These plants avoid damage by freezing but can be injured by dehydration. This mechanism permits plants to survive in areas where the temperatures drop below -43.6° F (-42° C).

The Uncertain Future of the Butternut Tree

Michelle Grabowski, UMN Extension Educator


Photo 1: Nuts from a butternut tree B.Cook Michigan State University,

Once commonly planted near farmstead houses for the nuts it produces, many Minnesotans would not recognize a butternut tree today. Butternut (Juglans cinerea), also known as white walnut, is a native tree in Minnesota and a close relative of black walnut (Juglans nigra). This nut bearing tree provides food for squirrels and other rodents and is used for wood carving and furniture building. Butternut is hardy to zone 3 and is therefore a valuable tree in northern Minnesota, where black walnut will not grow.

Although butternut is naturally found in small numbers in native forests of the United States, these populations have decreased due to a lethal disease known as butternut canker. Butternut canker is caused by the fungus Sirococcus clavigignenti-juglandacearum and is responsible for the near extinction of butternut in some eastern forests. Populations of butternut still grow in Minnesota, but many of these trees are already infected with butternut canker.


Photo 2: Butternut tree suffering from canker MNDNR

Infected trees can be recognized by dead branches in the trees canopy and elliptical cankers on trunks and branches. The fungus infects through wounds or natural openings like leaf scars, bark cracks or stomata. The first year of infection, dark sunken oval cankers can be seen on infected branches. These often ooze sticky dark liquid in the spring and may have a sooty black center with a white border later in the season. The fungus infects and kills bark and the sapwood beneath it. Black staining of the sapwood can be seen if the bark is peeled back.

Infections on small branches quickly girdle and kill the branch, resulting in dieback within the tree's canopy. On large branches or the main trunk of the tree, cankers do not grow quickly enough to girdle the tree in one season. Rather the bark over the canker cracks open and ridges of wound wood develop around the canker. With time, multiple cankers accumulate on the main trunk of the tree and it succumbs to the disease.


Photo 3: Sooty discoloration on bark from butternut canker J.OBrien, USDA FS,

The butternut canker fungus produces spores during wet weather throughout the growing season. These spores are washed down the trunk or splashed onto nearby trees, starting new infections where they find an entry point. It is unknown how the fungus moves long distance. Seed can carry the pathogen, and it is suspected that insects or birds may play a role. People can also move the pathogen by moving infected wood from place to place. The fungus survives in wood infected with butternut canker for two years after the tree has been killed.

Unfortunately there is no strategy to prevent infection with butternut canker. Infected butternut trees often survive many years however. During this time the trees provide shade and nuts. Large branches and trees that have been killed by the disease should be removed. Butternut is not as rot resistant as black walnut. Dead trees and limbs can be a hazard if located in areas near people or valuable property.


Photo 4: Black staining of infected sapwood J.OBrien, USDA Forest Service,

The US Department of Agriculture Forest Service has an active butternut breeding program, working to identify canker resistant butternut trees. If you know of a mature butternut tree that is thriving despite having a few cankers or a tree that is healthy despite many neighboring butternut trees succumbing to the disease, contact Dr. Ostry at the USDA North Central Forest Service ( 651-649-5113). These trees are a valuable natural resource that may provide disease resistance to butternut canker.

For more information on butternut canker visit the USDA Forest Service website.

Help Your Woody Ornamentals Survive the Coming Winter

Kathy Zuzek, UMN Extension Educator

Photo 1.JPG

Photo 1: Winter burn symptoms. Kathy Zuzek, UMN Extension.

The effects of winter sun, wind, temperature fluctuations, snow, and ice can all combine to make winter a high-hazard time for tree and shrub health. Animal browsing is an additional challenge in our winter landscapes. Here's a checklist for gardeners who want to minimize injury to woody ornamentals in the coming winter.

Apply tree wraps and tree guards to prevent sunscald. Sunscald occurs when winter sun heats up bark on the south or west side of a tree enough to stimulate cambial activity. When shading or sunset causes the air temperature to drop quickly, the activated cambium is killed resulting in dead bark on the south or west side of a tree in the form of sunken, dried, or cracked areas. To prevent sunscald, tree wraps and guards should be applied in fall. They should be light-colored to reflect sun and to keep bark at a lower temperature on sunny winter days. Pay special attention to trees that have bark newly exposed to sun from recent pruning to raise tree canopies, trees that have been recently transplanted from shady sites to sunny sites, and young trees and other thin-barked trees such as cherries, crabapples, plums, maples, mountain ash, basswood, and honey locust. You will need to protect newly planted and thin-barked trees for several years but remember to remove tree wraps each spring and reapply them in autumn.

Avoid over watering and late season fertilizing that can encourage late season succulent growth. Late season growth is vulnerable to winter injury.

Avoid under watering of your trees and shrubs. Remember to water trees and shrubs in your landscape until the ground freezes. As temperatures drop in autumn and plants are acclimating for winter, it is easy to forget that they still need water. Inadequate watering stresses plants. Avoid this stress by watering until the ground freezes. The average freeze date of soils in Minnesota ranges from mid-November through mid-December; the average date for Minneapolis and St. Paul is December 6.

Photo 2(2).JPG

Photo 2: A burlap barrier to prevent winter burn. Kathy Zuzek, UMN Extension.

Roots are less hardy than stems of woody plants and can be injured when soil temperatures reach 10-15 ° F. Thankfully, soil temperatures are much higher than air temperatures in winter. A moist soil holds more heat than dry soil and helps to prevent deep frost penetration.

Evergreen foliage that turns brown during winter due to desiccation and injury is called winter burn (Photo 1). One way this damage occurs is when winter sun and wind trigger transpiration in evergreen foliage. During transpiration stomata (the openings on foliage that allow gas and water exchange with the atmosphere) open and water is lost from the foliage. Because the soil is frozen, the plant cannot replace the lost water and foliage desiccates and turns brown. Damage often occurs on the south, southwest, or windward side of evergreens but in severe cases, an entire plant can be affected. Winter burn can affect all evergreens but yew, arborvitae, and hemlocks are particularly susceptible. Minimize winter burn by watering evergreens until the ground freezes in late autumn or early winter.

Construct a barrier around plants susceptible to winter burn to protect them from winter sun and wind. A second way that winter burn occurs is similar to sunscald damage on tree trunks. Cellular activity in evergreen foliage can be stimulated by sunny winter days that increase tissue temperature in evergreen foliage. When shading or sunset occurs, foliage temperature drops, resulting in foliage injury or mortality. Whether it is caused by transpiration and water loss or by sun's activation of foliage tissue, winter burn is an indication that evergreens were not sited properly in a landscape. A short term and aesthetically unappealing solution to this problem is to construct a barrier of burlap or cut evergreen branches around plants to protect them from winter sun and wind (Photo 2).

Don't apply antitranspirant sprays to prevent winter burn. Most research shows that antitranspirant sprays do not protect evergreens from winter burn.

Photo 3(2).jpg

Photo 3: Narrow branch angle. J. O'Brien, USDA Forest Service,

Apply several inches of mulch around your trees and shrubs to buffer soil temperatures. Mulch is especially important at preventing cold injury to roots during snowless winters. Snow is a great insulator and moderator of soil temperatures. In its absence, several inches of mulch will help to moderate soil temperature.

Protect your trees and shrubs from animal damage. Rodents, rabbits, and deer feed on twigs, bark, and foliage during winter. In severe cases, their feeding can girdle and kill stems or entire plants. Deer rubbing their antlers on trees can also cause damage. Protect tree trunks and shrubs from rodent and rabbit feeding damage by using tree guards or a hardware cloth wrap. Start your protection a few inches below the ground for mice and extend it 24 inches above the average snow line for rabbit protection. Or protect entire beds from rodents, rabbits and deer with wire fencing, repellent sprays, or by hanging repellant-drenched rags.

Prune to prevent snow and ice damage. Prune to eliminate multiple leaders and narrow branch angles (Photo 3) between branches or branches and trunks of trees. Removing these weak branch attachments makes trees less susceptible to snow and ice damage.

New Oak Leaf Blight Confirmed in Minnesota

Michelle Grabowski, UMN Extension Educator

A new fungal leaf blight of bur oak (Quercus macrocarpa) trees has been confirmed in Minnesota. Bur Oak Blight, also known as BOB, was collected by Jill Pokorny of the USDA Forest Service in several central Minnesota counties and confirmed by Dr. Harrington of Iowa State University.

How to Recognize BOB

• Bur oak blight only infects bur oak trees. Leaf spots or blights on other types of oak tree are caused by different pathogens.
• On bur oaks, symptoms first appear in July or August.
• Infected leaves have brown angular or wedge shaped lesions. Leaf veins turn brown and small black spots (fungal spore producing structures) can be seen with a magnifying glass within the brown lesions.
• Infected leaves are often found first on the lower branches. After many years of infection, however, the entire canopy can be infected.
• Healthy bur oak trees typically drop all of their leaves in the fall. Trees infected with BOB, however, often have many leaves that remain attached throughout the winter.


Will BOB Hurt my Oak Tree?
Bur Oak Blight affects leaves late in the season. Because the leaves have had several months to do photosynthesis before disease becomes severe, the tree has had an opportunity to store up energy. One year of infection, therefore, will not hurt the tree.

Unfortunately trees that are infected one year are likely to have the disease in following years as well. Over several years, the disease becomes more severe, slowly affecting more and more of the canopy. This repeat infection puts stress on the tree, weakening it and making it more susceptible to secondary pests like two lined chestnut borer and Armillaria root rot. Both of these secondary pathogens can cause severe damage to the tree.

Where Did BOB Come From?
Reports of damage to bur oaks similar to BOB were reported in Minnesota as early as the 1990's. Before this time, the problem had not been noticed on bur oaks. It is only recently that the fungus that causes BOB was identified as a new species of Tubakia. It is possible that this fungus was present in Minnesota but never noticed, or perhaps was present but the environment was not conducive to disease prior to the 1990's.

On individual trees, spores of Tubakia sp. overwinter on infected twigs, leaves and petioles that remain attached in the canopy. Rain splashes spores from these old infections to the new emerging leaves, resulting in year after year of disease. New trees can be infected by spores splashing onto leaves from a neighboring infected tree. Overall the spread of the disease within a canopy and within a group of trees is surprisingly slow given the large number of spores produced by the fungus.

What Can I Do To Protect my Tree from BOB?
There are currently no fungicides recommended for treatment of BOB, but research is being conducted to determine if a fungicide injection or spray could protect uninfected oak trees and reduce disease in trees already suffering from BOB.

For now the only thing gardeners can do is reduce stress on infected trees as much as possible. Removing turf grass from below a tree and replacing it with wood chip mulch will help to keep the soil moist. In years of low rainfall, trees should be watered to reduce drought stress. Care should be taken not to wound trees with lawn mowers or other yard equipment. Avoid driving cars or other heavy vehicles over the root zone of mature trees. This can compact the soil and cause root damage.

For more information about bur oak blight read 'BOB in Minnesota' by Jill Pokorny, USDA Forest Service.

Knowing when it's time to say goodbye.

Gary Johnson, UMN Extension Professor, Urban and Community Forestry, Department of Forest Resources


Preventative cabling such as this dynamic system greatly reduces the likelihood of failure during wind, ice or snow loading events.
Gary Johnson.

No one willingly lets go of old friends, whether they're human, pets or (ahem) trees. But the reality of life is that there's an aging process that's inevitable and not all problems can be diagnosed and successfully treated. So that old friend that sheltered you from harsh winter winds, shaded you on hot summer afternoons when the heat index was over 100 degrees, greeted you in the spring with a bouquet of flowers...must eventually or prematurely be removed.

Four Questions Before Letting Go

Gardeners of all shapes and degrees tend to be incredibly hopeful. For example, when news comes along that there's a new cure for emerald ash borer (have you heard about the fruit juice and dish soap treatment yet?), the urge is to try it without considering four "critical" questions .

· Is the tree healthy?

· Will the tree remain stable if it stays in the landscape?

· What is the relative value of the tree to you?


Once a crack has started on a mature, large tree, cabling should not be considered a reliable option. Remove the tree. Beth Jarvis.

· What can your budget reasonably handle?

By the time these four questions are answered, the fate of the tree's future should be obvious.

Is the tree healthy?

Health is a measurement of a tree's ability to photosynthesize normally, store energy for growth and tolerance to stresses, and the eventual release of that energy. If a tree has a species-characteristic crown density, leaf color and size as well as a sufficient live crown ratio, it's generally considered to be healthy. Stand under the tree and look up through the foliage. If it's a Norway maple, you shouldn't be able to see a lot of sky. If it's a honeylocust, you should see blue clearly through the crown. That's an example of characteristic crown density. The same goes for leaf color and size. Don't compare oaks with service berry.

Live crown ratio refers to the vertical mass of foliage. A healthy tree should have at least a 60% live crown ratio, that is, 60% of its height is photosynthetic foliage. If a tree has abnormally small leaves, a thin crown, a deficient live crown ratio, lots of dieback (therefore, less foliage) and defoliates early due to insects or's unhealthy.


Would you consider this a reliable treatment to prevent this large ash from tipping any further? No? Good! Remove the tree.Unstaged photo courtesy of Mike Meisch.

Trees in Minnesota have a relatively short amount of time to create chemical energy (e.g. sugars) and store them up for normal functions (e.g. starches provide energy to grow, leaf out, tolerate defoliations). Full leaf expansion doesn't occur until mid-June (sometimes later) and the photosynthetic season is essentially over by the end of August. Weakened trees have difficulty tolerating or recovering from common stresses. If your tree mirrors the image of an unhealthy tree and it's been that way for awhile, it's not likely that investment s of chemicals and money will turn it around.

Declines in health are most often associated with repeated defoliations (e.g., Japanese beetle or gypsy moth, hail storms, or anthracnose) or chronic drought (several years of seasonal drought). If nothing has been done in the past to intervene and lessen these stresses, trees progress into a decline spiral from which they rarely recover. Healthy trees on the other hand can recover from problems that are shorter termed (termed inciting events) especially with a little care such as watering, mulching, and controlling defoliating insect pests.

Will the tree remain stable?

One of the more difficult decisions to let go happens when a tree is obviously "healthy" but is unstable, too risky for the landscape. What? How can that happen? How can something be that healthy looking be bad for us?


Many trees can live for years after root loss on one side, but this drastically reduces a tree's ability to stay vertical during wind storms.Jacob Ryg.

Decay. Decay is the most common reason trees fail in wind storms (well, other than the wind). A healthy tree can have extensive decay in the trunk or buttress roots, which makes it an unacceptable risk in the landscape...too likely to prematurely or suddenly fail and cause property or personal damage. If the decay is extensive and especially if there's an opening to the cavity, don't think twice, find someone with a chainsaw. A good arborist can calculate the strength loss due to decay for you. If it's greater than 33%, the risk isn't worth keeping the tree in the landscape.

Dysfunctional roots. Stem girdling roots, roots of big trees squeezed into tiny spaces (like narrow boulevards or small planting spaces) are often the causes for complete tree failures during wind storms. The bigger the tree, the more severe the root problem, the more likely massive damage will result...not may result, but will result.

Severed root systems. Street widening projects, new or repaired buried utilities, roots cut during house construction activities cause instability issues. Most trees (if they're healthy...ironically) recover from construction activities or anything that cuts the normal root spread...only to topple during the next wind storm or the next one or the next one. It's not worth it. If roots are cut within a few feet of a large tree on two or more sides, it's unstable. If it's near a house, utility wires, roads, etc, it's an unacceptable risk.


When large trees are left on narrow boulevards after street widening projects remove much of their roots, this is a pretty common scene for several years. Gary Johnson.

What's its relative value?

You could hire a certified arborist to calculate the monetary value of the tree, but that's not what I'm referring to. Is there an emotional or sentimental value to the tree? Did Grandpa plant it with you on Arbor Day when you were a kid? Did Mom love the apples it grew and made the best pie from them? Oooh, tough call if it's removal time.

Relative values for everything are as unique as the individuals that own them. Some people collect objects, take pride in weed-free lawns or drive pick-up trucks with carpeting in them. Others purge, are willing to clean bathrooms every day if it means avoiding lawn work or buy trucks for work, not show and wouldn't pay an extra nickel for carpeting.

Trees that are special (unusual species, extra large size), well -placed for shade or blocking a nasty view, showy in the spring or autumn or host a tire swing or tree house are trees that are hard to let go. You may never see another tree like that again, so the cost of care may not be an issue. Whatever it takes to save it will be long as it's relatively healthy and stable.


This tree has a live crown ratio of more than 90%. A healthy tree should have 60% or more as a measure of its photosynthetic potential.Gary Johnson.

Individual windbreak trees, individual trees that are part of a woodland or trees that are so far from your house or normal activities area have less value. Their absence will not be as noticeable as the sentimental trees or the tree that your always rely on for shading the patio in the afternoon. Younger or smaller trees tend to tug at the heartstrings less, too. If the tree has only been in the landscape for a few years, there's usually less of an emotional attachment and it probably isn't shading the landscape that much.

What's your budget?

It often comes down to this: groceries or the ginkgo. Tree care doesn't come cheap, especially if it's a large tree and the care is long term. Trees are an investment, part of the infrastructure of your landscape, just like fences, garages and patios. Deferred maintenance has never worked for building longevity and quality and it doesn't work for trees either.

If a tree is regularly straining your budget by demanding life support maintenance, you'll probably be in favor of removing it. Why on earth would someone on a limited budget invest gj7.jpg

Yes, this tree is alive but along with having less than 30% live crown ratio, it supports half of a crown of leaves. Gary Johnson.

money year after year controlling apple scab on a crabapple when there are crabapples that are disease resistant? Get rid of the money pit and plant a new, lower maintenance crabapple...they're just as pretty.

If, however, the tree is well-placed, provides a valuable service, is healthy and stable, the investment to keep it healthy and stable is probably money well-spent. Inject that American elm with fungicides that will prevent Dutch elm disease. Treat those bur oaks with oak wilt...they can be saved. Don't let gypsy moth or Japanese beetle repeatedly defoliate that linden...there are both chemical and biological controls for those problems.

A reality check is needed for comparing control options to giving up and removing the trees. The expenses related to keeping trees free of oak wilt or emerald ash borer may seem onerous, sometimes $100-300 every 2-3 years. However, removing large trees near homes doesn't come free, either. A large tree within dropping distance of a home can cost $1,000 to $6,000 or more to remove...and then you're left with nothing but fire wood. No shade, no fragrance, no privacy. All of a sudden, maintenance money seems a bit cheaper.


This stressed maple has one of the perennial, fungal "target" cankers that are so common on maples in Minnesota. This will not get better, so remove and replant with something other than a maple.
Gary Johnson.

The final decision.

That well-placed, healthy, mature tree needs some significant pruning and cabling work on it, as well as some other health management it worth it? Most likely. Keep in mind that it could be an ash. There are very effective treatments for preventing or treating ash trees for emerald ash borer. Don't give up if the tree is worthy of saving just because it's an ash.

That tree has been repeatedly topped under the power lines for years and looks like a mop on a tree trunk. Get rid of it! It's most likely filled with decay and there are many better alternatives such as smaller trees or trees not planted under the lines.

Construction activities have cut the roots within 4-5 feet of your mature silver maple on three's too risky for it's own good and it's time to replace it. Don't take a chance.

The apples on that Yellow Transparent are unbelievably delicious and you can't buy them in stores anymore! Control the apple maggots and apple scab.

Your male ginkgo has a little surprise for's a female and the odor gets a bit stronger each year if you don't clean up the mess in the autumn! What to do? It's your call on this one.

Rose Classes and Their Performance in Minnesota: Part 1

Kathy Zuzek, UMN Extension Educator

The number of rose cultivars in the world defies logic. If you open a copy of Modern Roses 12, the most recent edition of the American Rose Society's rose cultivar list, you will find thousands of rose cultivars or varieties listed along with each rose's class, year of release, the breeder who developed the cultivar, parentage, and descriptions of each cultivar's floral and foliage traits, plant habit and thorns.


Photo 1: Flower of R. acicularis, one of Minnesota's native roses. Kathy Zuzek.

Before cultivar selection and development, there were only the species or "wild" roses. There are 120 or more rose species in the world and they are found in the temperate and sub-tropical regions of the Northern Hemisphere: the Middle East, Oriental Asia, Europe, and America. The oldest species have single flowers with only 5 or occasionally 4 petals and bloom only once each year in spring. As mutations occurred in some rose species over the course of time, stamens or the pollen producing part of the plant were replaced by more petals. This gave rise to newer species with semi-double or double flowers that have more than 5 petals. Eventually mutations also arose that led to repeat-flowering rose species that bloom throughout the growing season.

There are 18-20 roses native to the United States. Four of these (R. acicularis, R. arkansana, R. blanda, and R.woodsii) are native to Minnesota. All 4 of these species are single-flowered, pink, and bloom in spring (Photos 1 & 2). Photo 2-1.jpg

Photo 2: R. acicularis.David Zlesak.

Given the thousands of rose cultivars in the world, you might think that many of the rose species were used in developing all of these cultivars. With few exceptions, only 8 species are ancestors to our rose cultivars and all 8 are from Asia.

As rose cultivars are developed, they are placed into classes. There are 36 classes of roses. Classes that Minnesota gardeners might be familiar with are the Hybrid Teas, the Shrub Roses, or the Hybrid Rugosas. Every rose cultivar is placed in a class with other roses who share common ancestors and/or similar floral, foliage, or plant habit traits.

Which classes and which cultivars can be grown in Minnesota? That depends on a gardener's taste in rose appearance, the choice of how much time he or she wants to devote to maintaining their roses to insure good performance and long term survivability, and a willingness or reluctance to spray pesticides. Some of the biggest factors that impact these decisions are choosing between repeat-blooming and spring-blooming cultivars, cold hardiness and pest tolerance of individual rose cultivars, and the pH of your soil.

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Photo 3: Spring flower display of 'Prairie Wren', a spring-blooming shrub rose. Kathy Zuzek.

Most gardeners today are looking for repeat-blooming roses. These are the roses that bloom repeatedly throughout the growing season. There is nothing wrong with this except that it does eliminate the potential for enjoying some of our hardiest and largest cultivars that can produce hundreds of blooms during their one season of bloom to provide a spectacular spring display (Photo 3). Entire classes of roses that bloom only in spring can be eliminated from your list if you want a repeat-flowering rose.

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Photo 4: Green canes below the snowline are alive while canes above the snowline are brown
and dead.Kathy Zuzek.

Gardeners in Minnesota also need to consider cold hardiness. Our hardiest roses that show no cane injury after a Zones 3 (northern Minnesota) or Zone 4 (southern Minnesota) winter where minimum temperatures fall to somewhere between -20 degrees and -40 degrees F belong to only a few classes of roses. Some of these hardy cultivars are repeat bloomers and some are not. Other "hardy" repeat-blooming cultivars within those same few classes will have part of their canes killed by winter injury each year after a typical Minnesota Zone 3 or 4 winter. Oftentimes these cultivars have canes alive in the lower portions of the plants that were protected by snow cover while cane portions above the snowline are winter-killed (Photo 4).

What is important in Minnesota is a plant's ability to re-grow vigorously during the following growing season after experiencing some winter injury. Because repeat-blooming roses produce flowers on the current year's wood, a repeat-blooming plant that grows vigorously in spring and summer after some winter injury can perform beautifully in spite of our harsh winters. There are also entire classes of roses that are not hardy in Minnesota. Unless a gardener is 1) willing to consider a rose as an annual plant or 2) tip and bury roses or provide some other measure of winter protection, these roses should not be grown in Minnesota.
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Photo 5:Blackspot on a rose leaflet. Dave Hansen.

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Photo 6: Swellings on a rugosa cane indicate where rose borer larvae have girdled the rose cane. Dave Hansen.

Roses are hosts to many pests. The pests that most impact rose survival in Minnesota are blackspot, the rose stem borer and a wasp that causes mossy rose gall.

Blackspot is a fungus that results in defoliation of rose plants across all classes (Photo 5). Roses without leaves cannot photosynthesize to produce and store the energy reserves that a plant lives on. Many roses that defoliate from blackspot in early or mid-summer also try to produce a new second set of leaves in late summer. This depletes the energy resources of the plant even more. Plants that are susceptible to blackspot are severely weakened by repeated rounds of defoliation and have little ability to survive our harsh winters. Plant size and vigor is diminished with each year of blackspot incidence until finally the rose is too weak to survive over winter. Along with blackspot's impact on winter survivability, few gardeners are willing to tolerate a defoliated rose in their garden. This leaves two options: planting blackspot-tolerant roses or repeated fungicidal sprays during the growing season. Some classes of roses have a higher percentage of blackspot-tolerant cultivars than other classes.

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Photo 7: Mossy rose gall on a rugosa rose. Dave Hansen.

The rose stem borer (Agrilus aurichalceus) (Photo 6) and the cynipid gall wasp (Diplolepis spinosa) that causes mossy rose gall (Photo 7) can be bothersome pests on cultivars within the Hybrid Rugosa class, especially when rugosas are mass planted. Both pests girdle a cane, resulting in cane mortality from the swelling or gall to the tip of the cane. Large infestatations of these pests that are allowed to re-infest year after year can eventually cause so much stress to rugosa roses that they die.

Roses prefer a soil pH of 6.0 to 7.0. As pH increases above these levels, iron chlorosis becomes a problem and can lead to plant stress, low vigor or mortality. Cultivars within the Hybrid Rugosa class are particularly susceptible to iron chlorosis on high pH soils (Photo 8).
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Photo 8: Iron chlorosis on a rugosa rose growing in a high pH soil.
David Zlesak.

Coming August 1 in Rose Classes and their Performance in Minnesota: Part 2: Descriptions of some common rose classes planted in Minnesota gardens and a look at some attractive, low maintenance cultivars within classes.

Ash Anthracnose or Emerald Ash Borer?

Michelle Grabowski, UMN Extension Educator and Jeff Hahn, UMN Extension Entomologist

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Photo 1: Ash tree defoliated by ash anthracnose Photo by J. O'Brien USDA Forest Service,

With the discovery of emerald ash borer (EAB) in Minnesota in 2009, many homeowners are keeping a close eye on the ash trees on their property. This has resulted in a great deal of concern as many ash trees began to drop their newly formed leaves early this spring. In most cases, however, the cause of this early leaf drop was a common fungal disease known as ash anthracnose.

Emerald Ash Borer

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Photo 2: Thin canopy of an EAB infested tree Photo by Jeff Hahn, UMN Extension.

If a tree is suffering from infestation with EAB, the tree's canopy will appear thin, with few to no leaves. Eventually dead branches will be noticed within the tree. Cracks and D shaped exit holes can be found in the bark of infested trunks, and woodpeckers may be noticed frequently visiting infested trees to feed on EAB larvae. The emerald ash borer itself is a slender, ½ inch long, iridescent green beetle. It is active anytime from late May into August.

Ash Anthracnose

In contrast, trees infected with ash anthracnose will have dark brown to black water soaked blotches on leaves and young shoots. These leaves are often distorted and curled around the infected area of the leaf. Infection on petioles and young shoots can result in leaf drop. In some years, leaf drop can be quite severe, resulting in almost complete loss of the first flush of leaves. Infection is often most severe in the lower and inner branches of the tree, where high humidity favors fungal growth. Ash anthracnose is very common in cool wet spring weather, but does not persist in warm dry summer weather.

Help Diagnosing the Problem

Concerned homeowners have many resources available to help them determine which problem is occurring in their ash tree. What's wrong with my ash? Is an online diagnostic tool that helps gardeners diagnose ash problems through a series of simple questions about what symptoms they are seeing. Gardeners should also visit the UMN Extension EAB resource webpage. This page includes many useful tools including a step by step guide to determine if EAB is a possibility in your tree, a publication on insects that may be confused with emerald ash borer, and maps showing where EAB has been found in MN. Those without internet access can call Forest Resources Extension at 612-624-3020 for further assistance.

What is Ash Anthracnose?

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Photo 3: Leaf symptoms of ash anthracnose Photo by M.Grabowski UMN Extension.

Ash anthracnose is caused by the fungus Discula fraxinea, and is a common problem on Minnesota ash trees early in the growing season. Whereas Emerald Ash Borer is a serious threat to ash trees, ash anthracnose is closer in severity to a common cold. It's not pretty, it can be stressful to the tree, but it is rarely life threatening.

The ash anthracnose fungus can infect most species of ash trees (Fraxinus spp.). In the Great Lakes states, green ash (F. pennsylvanica) is often most severely infected. The ash anthracnose fungus overwinters in last year's infected leaves and twigs. New fungal spores are produced in early spring in these old infections. This coincides with the opening of tree buds and the emergence of new leaves and shoots. These young undeveloped plant parts are highly susceptible to the ash anthracnose fungus. If cool wet weather persists, ash anthracnose can be quite severe.

The good news is that mature ash leaves are relatively resistant to the ash anthracnose fungus (although infection can occasionally be seen associated with insect feeding or other wounds). As the weather turns warm and dry and tree leaves mature, ash anthracnose is no longer able to spread rapidly throughout the tree canopy. Trees that lost their first flush of leaves, replace them. By midsummer, symptoms of the disease are often difficult to find.

What to Do About Ash Anthracnose?
Ash anthracnose is considered a minor stress to the health of a tree. A mature vigorously growing tree can tolerate complete leaf loss for 2-5 years. Reducing other stresses on the ash tree throughout the growing season can help the tree recover. Simple activities like watering trees during periods of drought, mulching the soil at the base of the tree to reduce competition with turf grass, and avoiding wounding trees with lawn equipment will help the tree recover its strength. Raking up and removing infected leaves at the end of the growing season will help to reduce the amount of fungi that survive from one season to the next, although some fungi often survive in infections within the tree canopy.

Although there are fungicides that will prevent ash anthracnose, it is too late this year for fungicide applications to be effective. Fungicides must be applied early in the growing season to protect young emerging leaves and shoots. Homeowners should not apply fungicides to mature shade trees. The spray equipment available to homeowners will not provide effective coverage in a mature tree, and serious safety risks can occur to homeowners that attempt fungicide application with inappropriate equipment. Fungicide applications are only recommended in trees that have suffered severe leaf loss due to ash anthracnose several years in a row. In this situation a tree care company with a certified pesticide applicator (licensed by the MN Department of Agriculture) should be contracted to safely apply the fungicide at the appropriate time of year.

Planting Bare-Root Woody Plants

Kathy Zuzek, University of Minnesota Extension Educator

Spring is here bringing planting season. Early spring between the time that the ground thaws but before bud break is one of two optimal times during the year for planting bare-root trees and shrubs.

What is a bare-root plant?


Photo 1: A bare-root tree dug from a nursery. Gary Johnson.

The name says it all. Bare-root nursery stock are trees and shrubs that are field grown for one to three years, undercut and dug in fall and spring, handled with no soil left around roots (Photo 1), and stored with moist roots and dormant tops at a temperature a few degrees above freezing until they are planted. If you have never seen undercutting in action check out this You Tube video:

Advantages and Disadvantages of Bare-Root Plants

Bare-root stock offer several advantages:

  • Bare-root plants are usually ½ to 2/3 of the cost of containerized or balled & burlapped plants because bare-root plants are easier to handle, store, and ship.
  • Longer root lengths are possible on bare-root plants since weight of the soilless root ball is minimal.
  • The entire root system of a bare-root plant can be inspected so deformed, circling, and broken roots can be detected and corrected or removed.
  • Appropriate planting depth is easy to gauge because the root system is visible.
  • Because there is no soil around the root zone, there is none of the dramatic change in soil interface between the rootball and native soil that can hinder plant establishment.

There are also disadvantages to planting bare-root trees and shrubs:

  • The range of plant sizes and plant types in bare-root plants is smaller. Bare-root trees are usually a 2" caliper or less, because larger sizes do not transplant well as bare-root plants. Caliper is the diameter of a tree stem, measured 6" above the ground. If that stem diameter at 6" above the ground is greater than 4", move up the stem another 6" and measure the diameter at 12" above the ground for your caliper measurement. Evergreens are not sold as bare-root plants unless they are very small seedlings.
  • Bare-root plants should be dormant when planted so there are seasonal restraints to planting.
  • Early spring between the time that the ground thaws but before bud break is one time to plant bare-root plants. Autumn is a second good time to plant bare-root stock. Soil temperatures and moisture levels encourage active root growth at these times of year and lower air temperatures and dormant crowns help to minimize transplant shock.
  • Careful handling of bare-root stock is important. The exposed root system cannot be allowed to dry out during handling, transporting, or planting.

How to Plant Bare-Root Woody Plants


Diagram: Planting a bare-root tree. SULIS.

Never allow the roots of your bare-root plant to dry out between purchase and planting.

Keep the roots moist and protected from wind and sun. If you can't plant immediately, place the plant in a cool, shaded, sheltered location and cover the roots with moist straw, hay, damp burlap, or loose moist soil.

Bare-root plants lose up to 95% of their roots when they are undercut and removed from a nursery. After transplanting it is hard for this reduced root system to absorb enough water to meet the needs of the plant. Until the root system grows and reestablishes to its normal size, a newly planted tree or shrub often experiences transplant shock, which is primarily drought stress. You should plant and care for your bare-root plant in a way that provides the optimal environment for root growth and replacement during the first few years after transplanting.

Optimal planting and care include:

  • A planting hole only as deep as your root system's height. This prevents settling and all of the stresses caused by deep planting.
  • A planting hole at least 2-3 times as wide as the root ball that allows for rapid root growth through the backfill soil before hitting growth-slowing compacted soil outside of the hole.
  • A planting hole with sides that slope towards the base of the hole. The majority of a woody plant's roots grow in the top foot of soil and a planting hole with sloping sides encourages new roots to grow horizontally and towards surface soils.
  • A planting hole backfilled with the original soil. Adding amendments to improve soil quality doesn't help and sometimes hurts by causing poor water drainage in the planting hole. Your time is better spent digging a wider planting hole than amending soil.
  • Adequate watering until the plant replaces missing roots. Water is usually the most limiting factor affecting plant growth after transplanting. Because your bare-root plant has lost the majority of its root system, it relies heavily on water in the root ball through the first growing season. For a bare-root tree with a caliper of 2" or less that is planted on a well-drained site, apply 1 to 1 1/2 gallons of water per inch of stem caliper daily during the first week after planting, then every other day for 1-2 months , and weekly after that until the plant is established.

Establishment Tips for Bare-Root Woody Plants

How long does it take for a bare-root tree to become reestablished? That depends on genetics, environmental factors, and tree size. A good rule of thumb for Minnesota though is to assume that it will take 1 ½ years of time for each inch of stem caliper. So a 1" caliper tree will replaces its roots in 1 ½ years while a 2" caliper tree will take 3 years.

  • A 3" layer of organic mulch instead of turf under the canopy of your tree or shrub. Organic mulch eliminates the competition for water and nutrients that sets up between roots of grass and woody plants, suppresses weeds, retains soil moisture, buffers soil temperatures, protects stems from mechanical injury, and adds organic matter to the soil. Make sure that your mulch is pulled back a few inches from stems to eliminate direct contact.
  • No pruning except to eliminate problems and to ensure good branch structure. Remove diseased, dead, broken, crowded, and crossing or rubbing branches or to encourage a central leader, to eliminate narrow branch crotches with included bark, or to remove basal sprouts on trees. Leaving as much of the crown intact as possible maximizes photosynthate production that can be used to promote root and trunk diameter growth.
  • No quick-release nitrogen fertilizers in the planting hole. Direct contact between quick-release fertilizer and roots will burn the roots. Slow-release and organic fertilizers can be incorporated into the backfill soil. See the trees, shrubs, and fruits section of for more information.
  • Staking if your new tree is densely-crowned and planted on sites with lots of wind exposure. Attach stakes with flexible web belting or any other strips of wide, soft, but strong materials low on the tree trunk. This will prevent movement of the lower trunk and the root system, but allow for movement and resulting strengthening in the top of the tree. Staking may be necessary for 1-3 years while roots are growing and beginning to stabilize the tree. Check the attachment points of the webbing or strips on the stem every 3 to 6 months and loosen if necessary. For more information see: We are sorry, this link is no longer available.
  • Trunk protection for smooth-barked species such as crabapples, lindens, and maple will prevent injury from sunscald. Apply paper tree wraps or white wraps made from synthetic material from the bottom up in an overlapping pattern until the first major branch is reached. The wrap can be secured with duct tape or expandable plastic tape. Apply tree wraps in late October or early November and remove in March or early April.

Hackberry Witches' Broom

Michelle Grabowski, University of Minnesota Extension Educator and Jeff Hahn, University of Minnesota Asst. Extension Entomologist

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Photo 1: Hackberry tree with witches brooms. W. Cranshaw,

As you look up at the trees this spring, watching for emerging buds or perhaps a returning song bird, you might notice many small clumps of short twigs scattered through the branches of some hackberry trees (Celtis occidentalis). These clumps of twigs are called witches' brooms. Although witches' brooms are present within the trees canopy throughout the year, they are most easily observed in the winter or early spring, before leaves emerge. Hackberry trees growing in open areas, like a yard or along a street, are more likely to have witches' brooms than hackberry trees in a forest. Often one hackberry tree will have many witches' brooms while its near neighbors have none.

Witches' brooms occur when the bud of the tree is injured or infected. Normally, a healthy bud opens to produce one shoot. However, when a bud is damaged or killed, multiple weak shoots may develop from the same point on the branch. Witches' brooms in trees can be caused by a variety of problems. Trees growing alongside roads where salt is applied in the winter may have buds damaged or killed by splashing salt. In some cases, infection of the tree by a fungus, a phytoplasma or even a parasitic plant like mistletoe can cause witches' brooms to form within the tree's canopy.

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Photo 2: Witches broom on hackberry tree, up close. W. Cranshaw,

The exact cause of hackberry witches' broom remains unknown, although two organisms are consistently found within these twig clusters. The first is an eriophyid mite, Eriophes celtis. Eriophyid mites are tiny, measuring no more than 0.5 mm (1/50th inch) long. Even with magnification, people are unlikely to see these mites. Little is known about their life cycle. We do know that eggs are laid in May and mites cluster on the buds, developing until the end of the summer. The second organism is the powdery mildew fungus Podosphaera phytoptophila. The fungus may be seen as a white cobweb like coating growing on the young shoots and leaves within the witches' broom in spring or early summer. Throughout the year tiny brown to black round fungal resting structures can be found on infected buds, but these are best observed with the help of a magnifying glass. How these two pests interact with the hackberry tree is uncertain. One theory suggests that the eriophyid mite causes the witches' broom to form and the powdery mildew fungus takes advantage of the weakened plant and starts an infection secondarily. It is clear that hackberry witches' broom causes little damage to the health of the tree. Trees with numerous witches' brooms have been found to grow vigorously for years.

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Photo 3: Several hackberry witches brooms, up close. Jeff Hahn. 

Since the true cause of the witches' broom remains uncertain, there is no known method to prevent or to control the problem. Gardeners who are concerned about the affect of the witches' brooms on the ornamental value of the tree can prune off severely infected branches. In most cases however, hackberry witches' broom is only an aesthetic problem in the winter months. The flush of new leaves soon to be produced will hide the witches' brooms, leaving only a beautiful green canopy to be seen by the casual observer.

Pruning Flowering Shrubs to Maximize Bloom

Kathy Zuzek, University of Minnesota Extension Educator

The optimal time of year to prune deciduous shrubs is in late winter and early spring before bud break. Healing of pruning cuts is rapid as spring growth starts and pests that could infect or invade open pruning wounds are dormant in winter. But if maximizing bloom on deciduous shrubs and trees is important, pruning times may change.

Generally, shrubs flowering later in the season (hydrangeas, summer-blooming spiraea, and potentilla), flower on current season's stems, commonly referred to as new wood.  These shrubs can be pruned in late-winter to early spring without reducing bloom.

Spring flowering shrubs (forsythia, bridal wreath spiriaea, and rhododendrons) generally flower on the last year's stems, commonly referred to as old wood. These shrubs are best pruned immediately after flowering to conserve next year's bloom.


Figure 1: Viburnum 'Emerald Triumph' stem with flower and vegetative buds (click image to enlarge). Kathy Zuzek.

To more fully understand how and when spring blooming shrubs form buds on new and old wood,  enlarge the annotated images in Figures 1 through 5 by clicking on the thumbnail image.  These figures supplement the following pruning examples for Emerald Triumph viburnum and Summer Wine ninebark.

Figure 1 shows a branch of Viburnum 'Emerald Triumph'. The round flower buds at the tips of the 2009 growth will bloom this spring so this plant blooms on "old" wood. The vegetative buds that will produce new stems and leaves during the 2010 growing season are the tall thin buds below the flower buds. Leave this branch unpruned and in spring of 2010, you will have the bloom and new stems and leaves shown in Figure 2. The stems will continue to grow and lengthen through mid-summer. Along these stems, new vegetative buds will form and a flower bud will be produced at the tip. These will be the buds that produce stems and flowers in 2011.


Figure 2: When the flower bud (a) and vegetative buds (b & c) on the 2009 stem open this spring, flowers and new stems and leaves will be produced (click image to enlarge). Kathy Zuzek.

Now imagine pruning this viburnum sometime in the next 2 months. As you remove or shorten stems, you are also removing any flower buds at the stem tip that would have bloomed in spring of 2010. Pruning in late winter will not affect the plant's health, but it will decrease the number of flowers and fruit produced in 2010.

What if you waited to prune until late this summer? Bloom would be maximized this spring. But by late this summer, new flower buds for the 2011 bloom will have already developed at the tips of this year's stem growth. By waiting to prune until late summer some of next year's flower buds will be removed. Pruning also stimulates new growth and late summer pruning can result in a flush of new growth that will not harden off properly, resulting in winter injury. So pruning in late summer maximizes bloom in spring of 2010, reduces bloom in 2011, and creates the risk of a late season growth flush that would be susceptible to winter injury.

If maximum floral display is important, the best time to prune this shrub is immediately after bloom. The shrub will bloom this spring. After pruning, growth will occur until mid-summer. Next year's flower buds will be found at the tip of this growth by late summer. The plant will harden off properly and will not be injured during next winter. By pruning immediately after bloom ends, a gardener will maximize bloom in both 2010 and 2011.


Figure 3 &4: Summer Wine ninebark and Summer Wine ninebark pruned to improve plant habit (click image to enlarge). Kathy Zuzek.

Figure 3 shows the normal plant habit of 'Summer Wine' ninebark. Ninebarks are often pruned to control the rampant growth that leads to long stems and the open and loose plant form you can see in the photo. In Figure 4, you can see a specimen of 'Summer Wine' that was pruned into a dense symmetrical plant habit. This plant was pruned the previous spring immediately after it finished blooming, which gave the plant time to set floral buds that led to the next season's heavy bloom you see in the photo.


Figure 5: Ash tree branch showing a bud scar separating different years of stem growth (click image to enlarge). Kathy Zuzek.

How do you tell where "old" wood ends and "new" wood starts? The vast majority of plants form a vegetative bud at the end of a year's stem growth. As that bud opens and expands the following spring, the bud scales fall off and leave a distinct scar around the stem. Figure 5 shows an example of this terminal bud scale scar on an ash branch. This scar shows where one year's growth ended and the next year's growth started. Oftentimes there is also some difference in stem color between the new stem growth and last year's stems.

Table 1 lists major shrubs, vines, and small trees that grow in Minnesota along with information on whether they bloom on previous year's growth, current year's growth, or both. All of these plants can be pruned in late winter or early spring without affecting plant health. Plants that bloom on "new" wood can be pruned in late winter or early spring without diminishing this year's bloom. Wait to prune plants whose flower buds are produced on "old" wood until immediately after flowering if you want to enjoy the full amount of bloom this spring.

Table 1. Flower bud location on deciduous shrubs, vines, and small trees.pdf

What's Up With That?!

Birch Abnormal Growth Syndrome (BAGS) aka. Mouse Ear Disorder

Carl Rosen, Extension Soil Scientist and Karl Foord, Extension Educator

10-1-09_Med-Summer Cascade Birch 8 6 2009 cropped_KarlFord.jpg
Enter Caption Here

The strange leaf symptoms on this river birch tree, taken on August 7 (Photo 1, left) were diagnosed as birch abnormal growth syndrome or BAGS. New leaves are severely stunted and take on a mouse ear appearance. For many years the cause of this disorder was a mystery, but it is now known to be due to a deficiency of nickel.

Photo 1 (left): River Birch 'Summer Cascade' at planting time showing symptoms of nickel deficiency (BAGS). Photo taken August 7, Karl Foord.

Nickel is an element only recently shown to be essential for plant growth and is required in very small amounts. Almost all soils have enough nickel to support plant growth, but under some conditions, nickel deficiency can still occur. The mouse ear symptoms on this river birch were first seen when growing in a peat-based container mix, and were initially misdiagnosed as bud damage from a late frost. However, after the tree was transplanted into the soil in mid-May, the symptoms, after continuing for the next few months, have now begun to appear normal.10-1-09_Med-Summer Cascade Birch mouse ear.j_CarlRosenpg.jpg

Based on research conducted at the University of Minnesota and in other areas of the country, BAGS almost exclusively occurs on river birch when grown in peat-based media and can be corrected by soil or foliar applications of nickel.

Photo 2 (right): Close up of 'Summer Cascade' river birch leaves with BAGS. Photo taken August 7, Karl Foord

Research has also shown that when soil is added to the peat media (20-30% by volume), the nickel deficiency symptoms will not occur, suggesting that there is enough nickel in the added soil to meet the nickel requirements of the plant.

10-10-09_Med_nickeldefonriver birch_carlrosen.jpgIn cases where the symptoms are most severe, an analysis of the peat has shown excessively high levels of zinc. These high levels of zinc in the peat likely accentuate the nickel deficiency. Therefore, adding soil to the peat mix may help by 1)alleviating BAGS symptoms by adding the needed nickel,  and 2) by tying up or diluting some of the excessive zinc in the peat.

Photo 3 (left): Up close. Mouse ear symptoms of nickel deficiency on peat-based media. Carl Rosen.

As shown by the picture taken on September 14 (Photo 4, below), the tree has nearly recovered from its mouse ear symptoms and is expected to make a complete recovery once the roots have fully established into the native soil. 

10-1-09_Med-Summer Cascade Birch 9 14 2009 recovered_KarlFord.jpgIn general, BAGS has been a problem most apparent to the nursery industry, as trees showing the symptoms are usually not sold. However, if the problem does occur in containers, it can be corrected with nickel applications or by transplanting to a medium containing at least 20% soil. Soils in Minnesota have enough nickel to support plant growth, therefore nickel application to river birch growing in the landscape is not necessary.

Photo 4 (left): Recovery of river birch 'Summer Cascade' from BAGS.  Photo taken September 14, Karl Foord.

Components of and Factors Influencing Fall Color

Karl Foord, University of Minnesota Extension Educator

The tree's response to the decreasing day lengths in the fall is to form an abscission layer at the base of each leaf. As this layer forms, it slowly cuts off water and mineral supplies to the leaf and reduces the manufacture of chlorophyll. As chlorophyll supplies decrease, previously masked carotenoid pigments in the leaf become visible. Carotenoid pigments are split into two classes based on oxygen content: xanthophylls and carotenes.   Xanthophylls, which contain oxygen, are yellow. Carotenes, which do not contain oxygen, are orange. Carotenoid pigments absorb light energy like chlorophyll and serve to protect chlorophyll molecules from photo damage. A carotene you may have heard of is β-carotene which is a precursor to vitamin A. In humans, vitamin A is a pigment essential for good vision.

10-1-09_Med_maplesturningcolor_upclose.JPGAnthocyanin pigments are another important component of fall color, contributing reds and purples to the fall palate. These pigments are not present in the leaf during the active growing season, and form in the leaf in the fall. Anthocyanin pigment formation is a function of sunlight, which is why you may see leaves at the tops and southern facing parts of trees turning colors before the rest of the tree.  When the first leaves of the season fall, the remaining leaves receive more light and develop more color. This phenomenon can also be observed in wooded areas where trees on the edge of woods or those that are taller develop color first.  The smaller trees that have been shaded by taller trees will not develop color until sunlight reaches them.

Photo 1 (left): Maple turning fall colors. Note the outer most leaves exposed to the most sunlight are turning first. Karen Jeannette

Anthocyanin pigments are also present in many fruits. For example, if you see an apple that is highly pigmented on one side and not on the other, it may be because the colored side was exposed to light and the other side was shaded by leaves. Many of our favorite fruit species, such as blueberries, cranberries, raspberries, blackberries, cherries, grapes, and eggplant contain anthocyanin pigments.

Functions of superior fall color

One environmental component that does not add to fall color is frost. A severe frost that is premature will kill the leaf cells and not permit the colors to develop. The leaves will instead turn brown, and drop

Superior fall color displays are a function of weather and the condition of the trees. When healthy, pest free plants with sufficient nutrients and water experience bright, sunny, and cool autumn days, and cool but not freezing autumn nights, we are treated to a magnificent display of fall color. Varying weather conditions can influence the timing of fall color. To get an up-to-date fall color report go to the following website.

The following figure shows the average time "peak" color is obtained in different parts of Minnesota.

Image courtesy Explore Minnesota @


Getting Hydrangeas to Turn Blue

David C. Zlesak and Gail Soens, University of Minnesota Extension Educator and Bailey Nurseries New Variety Coordinator / Section Grower Bud and Bloom Hydrangeas & Roses

Blue hydrangea in landscapeWith so few true blue flowering shrubs for our landscape, it is no wonder so many of us are drawn to the beauty of blue hydrangeas! Only one species of hydrangea we commonly see for sale and in our northern landscapes include cultivars that can be coaxed to bloom a true blue or, if desired, a pure pink. It is Hydrangea macroplylla which is also known as the bigleafed hydrangea. Endless Summer® is the most common cultivar of this hydrangea we see for sale in the north as it is able to bloom off of both old and new wood. This is unlike most other H. macrophylla cultivars which bloom on only old wood. Since this species is marginally stem hardy in zone 4, having the ability to bloom on new wood allows it to still flower in our climate even if the plant dies to the ground and needs to regrow from the base.

Cherry Prinsepia

Kathy Zuzek, University of Minnesota Extension Educator

Princepia Kathy ZuzekCherry prinsepia (Prinsepia sinensis) is a little known shrub native to Manchuria that has been in cultivation since 1896.  It is a member of the enormous Rosaceae family, and close relatives of prinsepia that  you may be familiar with are woody ornamental and fruit varieties from the genus Prunus (cherries, peaches, plums, apricots, almonds) and pearlbush (Exochorda serratifolia).

Cherry prinsepia is one of the first shrubs to leaf out in spring, providing some welcome color after a long Minnesota winter.  Bright green leaves are alternate on current season's growth but are produced in clusters on older wood.  A thorn is found at the base of each leaf or cluster of leaves.  The small immature leaves are soon masked by an explosion of light yellow 5-petaled flowers on old wood in late April.  In Minnesota, cherry prinsepia blooms at the same time as flowering almonds (Prunus triloba var. simplex).   

Laci High, University of Minnesota Graduate Student

PJM Rhododendron is well-adapted in Minnesota. David Zlesak

After a long, dreary Minnesotan winter, gardeners anticipate and appreciate the beauty of spring flowering shrubs such as rhododendrons and azaleas (genus Rhododendron) which can be the first sign of color in many landscapes.Due to its great soil adaptability and ease of cultivation compared to other members of the genus, PJM hybrids can be found along most residential streets in Minnesota.  These plants are loaded with lavender-pink flowers nestled among inconspicuous, scaly leaves.  Even though PJM hybrids have proven to be reliable performers for home gardeners, they lack the color range and glamorous trusses of broad-leafed forms typically found in more moderate climates.

Drought-Tolerant Plants

Kathy Zuzek, University of Minnesota Extension Educator


Do you live in a geographic area with little rainfall? Do your sandy soils allow water to percolate away quickly? Are you looking for a drought-resistant landscape? Are attractive landscapes and water conservation both goals of yours? Below is a partial list of trees, shrubs, and herbaceous perennials that once established, grow well with little supplemental watering. Within most of the species listed, there are cultivar choices that will provide you with a wide variety of ornamental traits.


Michelle Grabowski, University of Minnesota Extension Educator

For most areas of Minnesota, the spring of 2009 has been an unusually dry one.  This cool dry weather has kept many of the spring leaf spot diseases of trees at bay.  Diseases like anthracnose on oak, ash and maple have been absent up until the most recent wet weather.  Anthracnose is now being reported, especially in areas that received significant recent rain like southern Minnesota. The fungi that cause anthracnose, however, may not be causing problems for long. Anthracnose fungi thrive in cool wet weather and with the recent onset of hot temperatures, the growth and spread of this disease is likely to slow down.

It's Too Late To Treat Ash Now For EAB


Jeffrey Hahn, Asst. Extension Entomologist

With the discovery of emerald ash borer (EAB) in St. Paul in May, many people have been asking for information on how to protect their ash with insecticides.  While there are several options available to home residents within 15 miles of the infestation, the question people should be asking now is when should I treat my ash.  In general insecticide applications should be made from early May until early to mid-June.  With that in mind, it is really getting late to be treating your ash any longer this summer.

Photo 1: EAB galleries in infested tree in St. Paul. Jeff Hahn

It is possible that Tree-age (emamectin benzoate), which is a professional use only product, can be applied into July because its mode of action targets the larvae and not the adults.  However imidacloprid relies on being taken up by the tree into the canopy and killing adults that feed on leaves.  Because it takes three to four weeks for imidacloprid to be translocated in trees, any applications that take place now, will have little impact in protecting ash.  This is particularly true for products available to the general public.  If you are thinking of treating your ash yourself now, don’t do it.  You will be just wasting insecticides.  The next window of opportunity for insecticide applications will be this fall or next spring.

There are many factors to consider if you are thinking about treating your ash for EAB.  For more information on insecticide options for protecting ash from emerald ash borers, please see the EAB Insecticide Fact Sheet (pdf).

Emerald Ash Borer Discovered in Minnesota

In the last issue, we alerted you that emerald ash borer was a mile away from the Minnesota border. On May 14, the Minnesota Department of Agriculture confirmed that the pest has been located in St. Paul, Minnesota. Keep up with the latest news on Extension's emerald ash borer response page.

Emerald Ash Borer On Minnesota's Doorstep


Jeffrey Hahn, Asst. Extension Entomologist

Earlier this month, on April 7, Wisconsin reported a confirmed infestation of emerald ash borers (EAB) in the town of Victory.  This town is in Vernon county, about 20 miles south of La Crosse and on the banks of the Mississippi River about one mile from the Minnesota-Iowa border.  This the first time that EAB has been found in western Wisconsin.

The Minnesota Dept. of Agriculture has stepped up their surveillance efforts in Houston county which is right across the river from this infestation in Wisconsin.  So far, their surveys have not revealed any EAB.  Remember, that at this time, EAB has not been found in Minnesota (although the odds of finding it in Minnesota soon have gone dramatically up).  Because of the imminent danger of EAB, a quarantine has been put in place for Houston county, restricting the movement of ash trees, ash logs and branches, uncomposted wood chips, and any hardwood firewood.

Simple Steps to Productive Apple Trees


Emily Tepe, University of Minnesota Research Fellow

Apples in the Home Garden

If you’ve ever dreamed about harvesting fresh fruit from your garden, apples are a great option. Sure, they require a bit more care than a typical landscape tree, but with a little TLC you could be harvesting juicy, crisp apples right from your own backyard!

There are a few things to keep in mind when growing apples that will help you achieve bountiful harvests of high quality fruit year after year. First it is important to know that apples require cross-pollination to reliably set fruit. This means planting more than one kind of apple tree. Your choice of varieties will largely depend on your priorities. If one tree will give you all the fruit you need you may want to consider planting a disease resistant flowering crab apple, such as Indian Summer or Snowdrift, for cross-pollination. Crabapples and edible apples can successfully cross pollinate. These trees are used in commercial orchards because they bloom over a longer period of time than most eating varieties, ensuring complete overlap of bloom. This equates to greater pollination potential of all the flowers, leading to more fruit. (Ensuring complete pollination and greatest fruit set might not be necessary however, which will be covered later when we talk about thinning.) These crabapple varieties and many others produce a profusion of flowers in the spring, and will then set an abundance of pretty, though inedible pea- to cherry-sized fruit which will dangle on the trees through the winter and into the next spring.  Since the tissue of the apple fruit that we eat is derived from cells of the maternal parent tree, the apple that serves as the pollinator will not affect our fruit quality.


Michelle Grabowski, University of Minnesota Extension Educator

Throughout Minnesota, purplish brown to rusty brown needles can be seen on spruce trees. A variety of problems can result in needle discoloration in spruces including insects, disease, and problems associated with environmental conditions. This time of year two common problems are Rhizosphaera needle cast and winter injury. Rhizosphaera needle cast is caused by a fungal pathogen. Winter injury is the result of environmental conditions. It is important to be able to distinguish between these two problems, since very different action is required to maintain tree health depending on the cause of the problem.

Rhizosphaera needle cast is caused by the fungi Rhizosphaera kalkhoffii and is most commonly seen on Colorado blue spruce, which are highly susceptible to the disease. White spruce and Norway spruce have greater resistance to the disease but can become infected when stressed. With the drought conditions present in Minnesota the last few summers, Rhizosphaera needle cast is showing up in a wide variety of spruce trees.

Garden Calendar for May


Contributors: Michelle Grabowski and David C. Zlesak

May is a busy month with lots of gardening fun to be had.  After a long cold winter, the excitement of getting out and enjoying our gardens feels like a reward. 

Enjoy the many spring flowering shrubs (bridal wreath spirea, forsythia, lilacs, flowering almonds, etc.) and wait to prune them, if necessary, until after they are done flowering.  Pruning them before they flower would ultimately most benefit the plant because they wouldn’t be investing their stored energy in new growth that would soon be cut off.  However, after waiting this long for those beautiful flowers we don’t want to miss them! 

New growth is starting on most of our herbaceous perennials.  If one hasn’t cut back last year’s stems, now is a great time to do so before the new growth gets larger and interferes with removing the old growth.  Come spring, last years stems tend to be more brittle and tend to be easier to remove than last fall.  Many times pushing them a bit from side to side will allow them to cleanly snap at the base of the plant without even needing to physically cut them.

The Mystery of Maple Sap Flow

Stephen G. Saupe, Ph.D., College of St. Benedict/St. John’s University
Biology Department, Collegeville, MN 56321, (320)363–2782;


An annual springtime event from Maine to Minnesota is the production of maple syrup. Each spring, syrup-makers head to the woods to collect sap from the sugar maple tree (Acer saccharum Marsh.) and then cook it into one of nature’s greatest gifts. Although this ritual has been practiced for many years since its discovery by Native Americans, surprisingly the actual mechanism responsible for sap flow is still something of a botanical mystery. So, exactly what do scientists know about why sap drips out of a sugar maple tree in the spring? To answer this question we must first understand the conditions that affect the flow of maple sap, since our final explanation must account for these observations.

Golden Canker on Pagoda Dogwood

Michelle Grabowski, University of Minnesota Extension Educator


The word golden brings about images of wealth, vitality and prosperity. Unfortunately when this word is used to describe the branches of a pagoda dogwood, the connotation is quite the opposite. Golden canker is one of the most common diseases of pagoda dogwood small trees/large shrubs in Minnesota and it can be disfiguring and even deadly.

Photo 1: Yellow infected branch areas contrast sharply with purplish red healthy bark. Michelle Grabowski

In the early spring sunlight the infected branches are bright yellow to orange compared to the dark almost purplish red healthy bark. The striking contrast is almost pretty. Sadly any branch that has completely turned yellow is already dead and will not leaf out this spring. Close examination of these yellow branches will reveal that the branch is covered in tiny blister-like orange spots. A sharp line marks the border between healthy and diseased branch tissue.

Garden Calendar for April

calendar1_600.jpgStop pruning Oaks. April, May and June are considered high risk months for Oak Wilt infections. At this time the fungal pathogen is producing spores that can be carried by beetles from the Nitidulidae family. These beetles are sap feeders and will be attracted to fresh pruning cuts on oak trees.

Photo 1: Do not prune oak trees during April-June to avoid oak wilt infection. David Zlesak

There is still time to start seeds of fast growing warm season flower and vegetable species indoors for outdoor transplanting after danger of frost. These plants include: cosmos, marigolds, tomatoes, and zinnias. Cold hardy annuals can be direct seeded in the garden during April and include: calendula, sweet peas, peas, and larkspur. Cold hardy annual transplants we have started ourselves or purchased from the garden center can also be planted and include vegetables like cole crops (broccoli, cabbage, cauliflower, etc.) and flowers like pansies, snapdragons, and stocks.

What's Up With That?!

Michelle Grabowski

Salt is a common sight in Minnesota this time of year. Unfortunately sodium chloride from deicing salt can be very toxic to garden plants when in high concentrations. Evergreen needles, tree buds, tree roots and turfgrass can all be damaged or killed by deicing salt splashing onto above ground plant parts or washing into the soil around plant roots. In addition when de icing salt washes into our lakes and rivers with spring snow melt, it reduces water quality and could harm aquatic wildlife.

What can a Minnesotan do?

Prevent future problems by reducing use of deicing salt

  • Shovel snow soon after snowfall to avoid compaction and ice formation.
  • Redirect downspouts so that water flows away from walkways.
  • Use the least amount of salt necessary to break up the ice. Then use a shovel to clear the sidewalk.
  • Reduce the amount of salt needed by mixing sand or gravel with a small amount of salt to provide traction on ice.
  • Use non sodium chloride deicing salts like CMA or other acetate de-icers which are less toxic to plants.

If plant damage does occur, water plants liberally in the spring to flush the salt out of the soil. Next year use the steps above to prevent future damage.

In the Rhododendron Fairyland: The Fantasy of Alpines


The prominent 19th century English botanist, Francis Kingdon-Ward, wrote manuscripts in great detail profiling his lifelong collection of high-altitude ornamental plants. Throughout his lifetime, Kingdon-Ward scavenged the Namcha Barwa Mountain crevices and the Tsangpo River Gorge for these select alpine plants during his some 65 explorations through the southeast of Tibet. These rare plants became the gems of his world-famous collection and ultimately, became his lasting legacy. Perhaps, the modern gardener's intrigue for alpine plants can be traced back to Kingdon-Ward's evocative descriptions of these beautifully rare plants. In any case, the botanical world has developed a great fascination for these charming ornamentals. Our fascination is firmly rooted in the marvel of botanical life that is capable of inhabiting and thriving on the so-called "Rooftop of the World."

What's Up With That?!

David C. Zlesak

wuwt_600px.jpg“Oh no, are my rhododendrons dying?” or something similar is a startling thought that comes to mind the first time many of us see this characteristic curling on leaves of our rhododendrons in winter. Fortunately, this is a normal response called thermonasty that actually helps our rhododendrons survive this difficult time of year. This curling and drooping of the foliage is in response to cold temperatures (thermo= heat or temperature and nasty= movement to a stimulus that is non-directional). As temperatures warm and cool during winter we can actually observe rhododendron leaves appearing less or more drooped and curled. As a broadleaf evergreen, the large surface area of rhododendron leaves makes them especially vulnerable to drying out during the winter. With the frozen soil this time of year, additional water cannot easily move up the plant and replace what evaporates from the foliage. Curling and drooping to prevent wind from reaching the undersides of the leaves, where stomates (openings for gas exchange) are typically more concentrated, can help prevent wind from drawing out as much moisture. In addition, many rhododendrons are native understory plants in deciduous forests. During the growing season the plants are shaded by the trees above, but during the winter when plants cannot utilize light well, leaves typically experience more intense sunlight capable of damaging exposed leaf tissue. Curling and drooping also aids the plant by reducing the overall amount of light intercepting a leaf. As we are excited for spring to come by this time of year in Minnesota, it might be fun for us as gardeners to look to our rhododendrons for a light hearted way to predict how much more winter we have left than groundhogs and how afraid they are of their shadows!

The Winter View

Kathy Zuzek, University of Minnesota Extension Educator

art1-1_800px.jpgIt’s official. Winter is more than half over. Are you feeling a bit desperate for warm temperatures and the color green? I spend every February dreaming of a trip to anywhere warm and green or of a kinder, gentler Minnesota where spring actually arrives in February. A quick look out of my window though always reminds me of how truly bleak our long winters would be without woody plants.

Michelle Grabowski, University of Minnesota Extension Educator

Photo 1: Fire blight canker on a young apple branch. Michelle Grabowski

This past summer, many Minnesotans noticed dead brown wilted leaves on apple, crabapple and mountain ash trees caused by the bacterial disease known as fire blight (Read Midsummer Trouble for Trees & Shrubs) Although symptoms of fire blight are most apparent in spring and summer months, one of the best times to manage this disease is right now.

What's Up With That?!


David C. Zlelsak

As eastern white pines (Pinus strobus) mature, they tend to develop a rounded upper canopy and interesting asymmetric form to their branches. The interesting branch arrangement, providing rich character to these trees, is due in part to how they have adapted to handle the stress imposed by the weight of snow and ice. The generally horizontal branches of white pine are somewhat brittle, and as they become excessively weighted down, especially in the presence of heavy winds, they respond by snapping and collapsing to the ground. The straight trunk typically remains in tact, helping to keep the tree standing tall. It’s impressive to see forests in Minnesota and Wisconsin where mature eastern white pines typically tower over neighboring tree species.

Photo 1: David Zlesak

Tree Care Advisor Core Course 2009


Mark your calendars for the 2009 Tree Care Advisor Core Course in St. Paul, Minnesota! Training includes education on several topics including: tree identification, plant selection, basic physiology and morphology, soils, site analysis, firewood identification, diagnosis of disease and insect problems, pruning, planting and more. These trainings are geared towards individuals who may not know much about trees but do know they want to learn.

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