University of Minnesota Extension
Menu Menu

Extension > Yard and Garden News > Archives > May 2011 Archives

May 2011 Archives

Contents: May 15, 2011

In this issue of the Yard and Garden News...

  • EAB Awareness Week

  • Espalier - An Art Based on Science

  • Protect Yourself From Tick Diseases

  • Black Root Rot in Flowering Annuals

  • Lawn watering practices that encourage healthy lawns and help protect water resources

  • A new kind of Tulipomania is alive and well at the Landscape Arboretum

  • Garden Calendar: May 15, 2011

EAB Awareness Week

Jeffrey Hahn, Asst. Extension Entomologist

With a stroke of his pen, Governor Mark Dayton signed a proclamation officially declaring May 22 - 28 as EAB (Emerald Ash Borer) Week in Minnesota. This is a good opportunity to remind people that EAB is still a serious pest that threatens our state's nearly 1 billion ash trees. That week also corresponds with the official start to camping season as people travel for the Memorial Day weekend.

Thumbnail image for Thumbnail image for Thumbnail image for Thumbnail image for Thumbnail image for Copy of emerald ash borer 1 - Jeff Hahn.JPG

Photo 1: Adult emerald ash borer. Jeff Hahn.

The theme for EAB Awareness Week is 'Keep our trees safe. Use MDA (Minnesota Department of Agriculture) certified or local firewood.' This is such a critical message for people to understand that the one of the most important methods for EAB to be transported into areas that are uninfested is through firewood. That is why people are strongly encouraged to leave their firewood at home and buy from local, approved firewood vendors.

Currently EAB is known only in Ramsey, Hennepin, and Houston counties. MDA has enacted quarantines in these counties to try to prevent infested ash product from moving out of these areas and into uninfested sites. To supplement this effort, MDA also continues to conduct surveys using purple traps to try to detect EAB soon after it enters an area. They have also enacted management strategies to slow the rate of spread of EAB

Citizens can also help by reporting insects they suspect are EAB and potentially EAB infested ash trees. If you think you have discovered EAB go to this step by step guide. If you can still can not rule this invasive pest out by the end of the page, then contact the University of Minnesota Forest Resources Extension who will put you in contact with someone that can help you determine whether you have EAB.

For more information on EAB, see the University of Minnesota Extension EAB web site.

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 .

Protect Yourself From Tick Diseases

Jeffrey Hahn, Asst. Extension Entomologist

The Minnesota Department of Health (MDH) recently issued a news release detailing the marked increase in the number of tick-borne diseases in 2010. MDH tallied 2,069 cases of Lyme disease, Human anaplasmosis and babesiosis from last year. While the number of Lyme disease cases only went up a little, Human anaplasmosis cases more than doubled and instances of babesiosis were nearly twice as much compared to 2009.

Copy of I s 0004.JPG

Photo 1: Adult female blacklegged tick. Jeff Hahn.

Blacklegged ticks (formerly known as deer ticks) is the species responsible for transmitting these diseases. The highest risk areas in Minnesota are in the eastern, central, and southeast areas of the state. Symptoms are variable. When dealing with Lyme disease, many cases (but not all) exhibit a red, circular, bull's-eye rash. Other disease symptoms can range from no reaction to arthritis, neuropathy, headaches, fevers, chills, and muscle aches, joint swelling, cardiac and nervous system problems, and, in a few cases, death. For more information on tick-borne disease, see the University of Minnesota Extension fact sheet, Tick-Borne Diseases in Minnesota.

The risk of disease can occur any time from spring through fall. Take the proper precautions to protect yourselves from ticks.

- Avoid areas where ticks are likely to be found. Particularly stay on trails and avoid walking through woody, brushy, or grassy areas where ticks are most common.

- Wear protective clothing, such as long-sleeved shirts and long pants. Wear light colored clothes so it is easier to see ticks on you. For added protection, tuck pants inside socks.

- Use repellents for additional protection. Apply them to socks, pant legs, and parts of clothing that may brush against vegetation. DEET and permethrin are effective. Apply DEET to clothing and skin but apply permethrin only to clothing.

- Be sure to check your clothes and yourself when you have been outdoors in known tick areas. Save any suspected blacklegged ticks for identification.

When dealing with ticks in your yard, do the following:

- Keep grass and vegetation short around homes, where it borders lawns, along paths, and in areas where people may contact ticks as ticks are less likely to survive in short grass.

-. Remove leaf litter and brush, especially in areas where the lawn borders grassy, brushy areas. Also prune trees and shrubs in these areas to allow more sunlight through as ticks are more common in shaded areas.

- When large numbers of ticks are present in areas adjacent to home yards, you can treat the edges of wooded or brushy areas and paths to help reduce tick numbers. Use an insecticide labeled for a turf area, such as those containing permethrin, cyfluthrin, or carbaryl. Do not spray such an area more than once a year.

- It is not necessary to treat your lawn for ticks as ticks rarely infest maintained yards.

For more information on Minnesota Ticks, see the University of Minnesota Extension fact sheet, Ticks and Their Control.

Black Root Rot in Flowering Annuals

Michelle Grabowski, UMN Extension Educator

Hanging Y&G.jpg

Photo 1: Petunias infected with black root rotT.Burnes, UMN Dept. of Plant Pathology.

With the weather warming up but the risk of frost not entirely past, many annual bedding plants are spending their days in pots. Gardeners are working to keep these plants healthy until they reach their permanent home in a hanging basket or in the front garden bed. Poor growth and yellowing leaves on these plants may be a sign of a root rot problem.

Black root rot, caused by the fungus Thielaviopsis basicola, has recently been identified in flowering annuals in Minnesota. Thielaviopsis can result in severe root rot in cool temperatures; 55-62F being ideal. Above ground symptoms are often mistaken for nutrient deficiency. Plants are slow growing or stunted. Leaves are yellow and leaf veins may remain green. The symptoms that give this disease its name are to be found below ground. Dark sunken black lesions occur on roots of all sizes. Under the microscope dark brown cylindrical spores with several distinct sections can be seen within infected roots.

Many different plants are susceptible to black root rot. Common hosts include Geranium, Phlox, Petunia, Pachysandra, cyclamen, lupine, fox glove, Gerbera daisy and gaillardia.

Roots Y&G.JPG

Photo 2: Dark lesions on infected roots T. Burnes, UMN Dept. of Plant Pathology.

Spores of Thielaviopsis can survive for several years in soil or infested pots. Plants that are grown with unsterilized garden soil as part of the potting mix are often infected. Reusing dirty pots can also be a source of disease.

Simple sanitation is the best way to prevent problems with black root rot.

· Use only new pots or clean old pots with a 10% bleach solution.

· Do not use garden soil as part of the potting mix as it may contain fungal spores and is not easily sterilized.

· Use sterile potting soil for starting new seeds and in hanging basket or other large planters.

· Clean tools with a 10% bleach solution, Lysol or Listerine before working with annual bedding plants.

spore Y&G.jpg

Photo 3: Thieleviopsis spores T. Burnes UMN Dept. of Plant Pathology.

· Purchase only healthy vigorously growing transplants.

Examine roots for dark lesions before planting.

· Discard infected plants to prevent spread of disease to the other plants in the garden bed or hanging basket.

Bob Mugaas, UMN Extension Educator, and Dr. Brian Horgan, Turfgrass Management, UMN Department of Horticultural Science

Picture 1.JPG

Photo 1: Proper watering benefits all landscape plants, especially lawns. Bob Mugaas.

While water is essential for all plant growth, it is not always uniformly available or distributed when and where the plant needs it. Therefore, supplemental water is often needed to sustain and support healthy plant growth whether it's vegetable plants, annual flowers, trees and shrubs or lawn grasses. Picture 1. For most of us, that means getting out sprinklers and hoses and watering on occasion. There are also automated systems available to aid in the distribution and convenience of supplying supplemental water to lawns. Following are some watering practices beneficial to lawns that can help minimize unintended, negative environmental impacts.

Water friendly lawn care practices anyone can implement

  • During the hot and dry summer months, lawns will need about an inch of water per week including rainfall to remain green and growing. One inch of water per week may be sufficient for two weeks or longer during the cooler periods of spring and fall. Always remember to factor in the amount of water received via rainfall so as not to be applying more water than the plant needs or can use.

  • Picture 2.JPG

    Photo 2: Unhealthy ponding of water on lawn surface. Bob Mugaas.

  • Adjusting watering practices seasonally is necessary. In the spring and fall when roots are active and deeper in the soil, larger amounts of water can be applied per application but with longer periods of time between watering. In summer, more frequent watering is necessary due to shallower rooting depths. Shallow but more frequent application of water DOES NOT MEAN that the soils should remain saturated with water. (See the following section on the various problems associated with waterlogged lawns and soils.) Some soil drying between watering is beneficial for the plant. Slight moisture stress experienced by the plant will encourage rooting.
  • When irrigating, avoid excessive amounts of water that may cause puddles on the surface of heavier clay soils. See Picture 2. Moreover, too much irrigation on sandy soils leads to the water moving past the root system increasing the likelihood that nutrients being carried with the water will potentially contaminate groundwater.

  • Picture 3.jpg

    Photo 3: Mowing higher encourages deeper rooting. Karen Vidmar.

  • Water early in the morning when temperatures are cool and winds are light. This puts more water into the lawn for grass plant use and less lost to evaporation.

  • Mow grass plants higher (3.0 to 3.5 inches) especially during the summer to encourage a deeper, more robust root system. See Picture 3. Deeper roots can access greater supplies of water and nutrients in the soil. In other words, lawns mowed higher will better tolerate the warm and dry conditions of summer. When supplemental water is needed, it is often on a less frequent basis ultimately conserving water.

  • Core aerifying compacted soils will: improve water infiltration, reduce runoff, increase rooting and access to soil nutrient and water reserves.

  • Take advantage of irrigation system rain sensors and/or soil moisture sensors. These will prevent automatic systems from turning on during periods of rain or when there is already enough moisture in the soil.

Be careful not to keep your soil too wet!
Picture 4.JPG

Photo 4: Creeping bentgrass taking over in an excessively moist Kentucky bluegrass lawn. Bob Mugaas.

Never allow the surface soil to remain water saturated (i.e., waterlogged), even for short periods of time. See Picture 2. Healthy grass root systems need soils with equal parts water and air. Air in the soil pore space is required for both nutrient and water uptake. Too much water forces air out of the pore spaces creating a waterlogged condition. This yields root dieback and a shallow root system making plants more vulnerable to a number of serious plant stresses and diseases.

Other issues and problems associated with overly wet soils.

  • Excessively wet soils can contribute to undesirable development of thatch (greater than ¾ inch or more). Excessive thatch is associated with poor stress tolerance and increased disease and insect problems.

  • Soils that remain wet are often invaded by grassy weeds, which can be difficult to control once established in a home lawn. Examples include annual bluegrass, rough bluegrass, creeping bentgrass and yellow nutsedge. See Picture 4. Also, moist shady areas can be invaded by moss. This is another difficult plant to control unless the environmental conditions can be changed to not favor moss growth. Conversely, lawns that are allowed to dry somewhat between watering rarely have even minimal problems with any of these weeds.
  • Research studies have shown that when additional water is applied or falls on already waterlogged soils, there will be an increase in the amount of runoff volume and potential nutrient loss under those conditions.

  • During warm summer conditions, waterlogged soils can significantly increase the loss of fertilizer nitrogen back to the atmosphere in the form of nitrogen gas. Likewise nitrogen carried in water moving down through the soil past the plant's root system can also be accelerated. In either case, this can end up wasting the fertilizer applied and potentially contaminate ground water.

For more information regarding proper lawn watering practices, check out the Lawn Watering chapter of the Sustainable Urban Landscape Information Series at the U of MN Extension website.

kf1.jpgKarl Foord, UMN Extension Educator

If you have the opportunity and the interest, check out the tulip displays at the Minnesota Landscape Arboretum in the near future while they are still at peak; for lack of a word "spectacular" (Exhibit # 1).

I was reading Michael Pollan's book, "The Botany of Desire" and came upon the chapter on tulips and his description of 'tulipmania'. You may remember the history of this period but it is worth reviewing.

In Holland a great furor about tulips whose petals were colored with dramatic streaks and stripes began in 1593. It reached a high point in 1637 when a tulip bulb of the variety 'Semper Augustus' sold for 10,000 florins (exhibit # 2). At this time such a sum of money would have purchased a house, including gardens and coach house, in a very desirable location near the canal in central Amsterdam. This was also the year that the bottom fell out of the tulip bulb market and many speculators were bankrupted. During this time it was often cheaper to purchase a painting of tulips than it was to buy the bulbs themselves. To this end people often contracted painters to feature tulips in their still life paintings (Exhibits #'s 3 & 4).


These striking tulips were named after the famous Dutch painter Rembrandt (1606 - 1669) who lived and worked in Amsterdam at the time. Interestingly enough, the tulips dramatically variegated petal colors were found to be due a color disruption caused by a virus, ultimately named the tulip breaking virus (TBV). Although beautiful, the plant's vigor and flower quality decline rapidly after infection with the virus, and for this reason the original Rembrandt Tulips are no longer sold commercially. However, there are quite a few modern, virus-free, Rembrandt "look-alike" tulips available (Exhibit # 1).


On May 10th I was photographing some of the tulips outside the Landscape Arboretum's Oswald Visitor Center. Having been primed by "The Botany of Desire" I noticed some interesting looking tulips in the bed containing the cultivar 'Anne Schilder' which has uniformly orange petals (Exhibit # 5). Several individuals appeared to have variegated petals reminiscent of TBV (Exhibits # 6 & 7). This would need to be confirmed by a virus identification test (ELISA) which could not be accomplished prior to publication. But whether the colors in this tulip are due to the virus or a genetic factor they are still beautiful and fascinating. Looking at this tulip gives one the feeling of being connected to the tulipomania events in Holland in the 17th century.


Save the plane fare to Amsterdam and the jet lag, visit the Arboretum.

Garden Calendar: May 15, 2011


Fritillaria imperalis
'Rubra maxima' Karl Foord.


Fritillaria imperalis Crown ImperialKarl Foord.

We are at least two weeks behind an "average" spring. Last year at this time we had experienced a Mother's Day (May 9, 2010) frost which killed many strawberry and apple flowers. This year as of May 13th the apples are in red bud stage and will likely start flowering next week. I have not seen a strawberry flower.

New tree id app designed by the Smithsonian

This new app is a great tool for identifying trees, and it has some citizen scientist applications as well. The location of the tree and its identity are recorded automatically. See more at the Smithsonian website!

It is currently only available to iPhones but will be released for android later this summer. And it's free!

Yard and Garden News Editor: Karl Foord
Technical Editor: Bridget Barton

Contents: May 1, 2011

In this issue of the Yard and Garden News...

  • What is that Insect?

  • Zinnias for Your Yard and Garden

  • Understanding and Using Home Lawn Fertilizers - Part III: Returning to phosphorus and potassium

  • Witches's Brooms on Landscape Shrubs

  • Spring Pruning Tips for Woody Landscape Plants

  • What are your apples doing right now?

What Is That Insect?

Jeffrey Hahn, Asst. Extension Entomologist

An interesting insect, found under unusual circumstances, was reported recently. A gentleman had a large dead branch pruned out of his maple in February. It had rotted in the center and was a hazard that needed to be removed. He cut the limb into smaller pieces and stacked them in April. He noticed in one branch section where the wood had rotted an accumulation of mud.

Copy of DSCN3224.JPG

Photo 1: Rat-tailed maggots. Kyle Jensen.

He removed the mud and uncovered several pinkish larvae with long 'tails'. They were legless with no obvious head. These insect larvae are rat-tailed maggots, Eristalis spp. The most commonly encountered species is Eristalis tenax. The body of a mature rat-tailed maggot is about 3/4 inch long with the telescopic breathing tube (the 'tail') as long as two inches. This insect belongs to the family Syrphidae which are commonly called flower flies or hover flies because adults are typically found around flowers and are able to hover in place when flying.

Rat-tailed maggots typically live in stagnant, low oxygenated water with high levels of organic matter. They have been found in sewage water, manure pits, and other types of polluted water as well as ponds with a lot of algae. They are also commonly found in rotting, decaying organic matter, including animal carcasses, damp compost, and wet, decaying leaves.

They are essentially harmless to people, although there have been some reported cases where they are involved in myiasis, i.e. infesting living tissue of people and animals. Rat-tailed maggots in particular would infest gastrointestinal tissue. Fortunately, this would be considered extremely rare and unusual in Minnesota.

There is not a good explanation for why these rat-tailed maggots were found in the rotting limb of a tree. There is a precedent for rat-tailed maggots being associated with moist, decaying plant matter so it is somewhat conceivable for them to be found in rotting wood. But for them to spend their lifetime in the rotting center of a tree limb still attached would be considered unusual at best.

Zinnias for Your Yard and Garden


Photo 1: Benary's Giant Wine

Robin Trott, UMN Extension Educator

Last summer was the first year I grew zinnias in any quantity. Spaniards who first saw them growing in Mexico thought zinnias were so unattractive they called them "mal de ojos", or "sickness of the eyes"; and until last summer, I tended to agree. I thought zinnias were large, gaudy flowers that didn't even have the redeeming quality of a pleasant fragrance; and so I never grew them in my home garden. Boy, was I wrong!

Last summer I grew many tall zinnia varieties to be used as cut flowers. Benary's giants were spectacular, and the peppermint stick varieties were something else. It's no wonder that the National Garden Bureau has declared 2011 the "Year of the Zinnia."
One of the reasons for the booming popularity of the zinnia is its diversity. Zinnias have a variety of flower forms--they may be single, semi-double, or double. Single Zinnias, Zinnia angustifolia, have one row of petals with an exposed center, similar to the daisy. rt2.jpg

Photo 2: Benary's Lime

Crystal White, an All American Selections winner is a delightful example of the single flowering variety. The semi-double and double flowered zinnias have a mass of dense petals that hide the center. Button, beehive, cactus shaped and dahlia flowered zinnias all fall within this category. Profusion, Queen Red Lime and Zahara are all examples of premium double flowering zinnias. Giant Fantasy is a splendid cactus flowering zinnia, and, Benary's Giant is a spectacular dahlia flowering variety. Zinnias also come in a wide range of color choices. My favorites are the cream, salmon, magenta and lime varieties. If you have a specific color scheme in mind, there is a zinnia that will fit in your garden palette.


Photo 3: Peppermint Stick

rt4.jpgZinnias are annuals that grow easily from seed. Direct seed after all danger of frost has past and the soil has warmed; or start inside 4-6 weeks before your last frost date (May 24 for our region.) Zinnias bloom profusely throughout the growing season and can grow as short as 6" or as tall as 48". Zinnias require minimal care. Deadhead spent blooms to encourage repeat blooming, keep watered during the growing season, and fertilize at half strength monthly from July - September. Zinnias are prone to powdery mildew, so avoid overhead watering, and space them to allow air flow between plants.

Use zinnias as border plants, in containers, as tall garden focal points, dried, and for cut flowers. Choose smaller varieties, such as Zinnita at the front of border plantings and in containers. Benary's Giant and tall cactus flowering zinnias are excellent as focal plantings and for cut flowers.

To gather flowers for fresh arrangements, cut them during the cool part of the day. (Early morning or early evening.) Select blooms that have not fully opened--they will continue to open indoors. Cut zinnias into a clean bucket of fresh water, and re-cut the stems, removing the lower leaves, before arranging them.

"Cultivators of the earth are the most valuable citizens. They are the most vigorous, the most independent, the most virtuous, and they are tied to their country and wedded to its liberty and interests by the most lasting bands." -Thomas Jefferson.

All photos by Laura Trott, taken in the garden of the author, Robin Trott.

Bob Mugaas, UMN Extension Educator


Photo 1: Steps for properly taking a soil test. Karen Vidmar, University of Minnesota.

Over the last couple of articles we have explored how to better understand the sometimes bewildering array of lawn fertilizers available and make appropriate choices for one's own lawn as well as the large and diverse role that the nutrient nitrogen plays in the life of lawn grasses. Part III, the final section of this three-part series, will turn attention back to the nutrients phosphorus (P) and potassium (K) and the important roles they play in turfgrass health. Since determining the amount of P and K in a container of lawn fertilizer was thoroughly discussed in the first article of this series, this last article will focus on the importance of P in K in turfgrass health and their application as a component of lawn fertilizers. Before beginning, remember that the best way to determine the amount of P and K in your soil is to have a reliable soil test done such as that performed by the University of Minnesota Soil Testing Lab. Collecting the soil to test properly is important for accurate soil test results. See Picture 1.

Turfgrasses and the important role of phosphorus(P) in plant health.

Phosphorus is an important component of many different plant molecules and compounds responsible for promoting healthy turfgrass growth. In fact, without ample P, normal healthy growth is not even possible. Some of its more important roles include those related to the storage and transfer of energy within the plant, promotion of healthy, well developed root systems, proper formation of plant cell membranes and, it is an important component of the cell's genetic material including its DNA.

In addition to its important roles within the plant, a critical component to understanding the use of phosphorus fertilizer is how it behaves in the soil. That is, phosphorus moves very little in the soil with most of it being bound tightly to soil particles. As a result, phosphorus does not move into the soil water solution as easily as other nutrients (e.g., nitrogen). That's both good news and bad news for the grass plant. The good news is that P is much less likely to move down through the soil via leaching and be lost as a plant nutrient. Hence, its supply in the soil is relatively stable. The bad news is that with such little movement in the soil it can make it more difficult for the plant to obtain sufficient supplies of P to support healthy growth.

Fortunately, healthy, actively growing lawn grasses have an extensive, dense, fibrous root system that makes them relatively efficient as extracting P from the soil compared to many other plants. For this reason, the application of supplemental P via fertilizer to an established, mature stand of turfgrass is usually unnecessary or needed in only very small amounts. Again, this is not because our grass plants require less P for growth than other plants. Rather, they are just more efficient at extracting available P from the existing soil. It should also be apparent that when our lawn grasses have shallow restricted root systems, such as in compacted or waterlogged soils, they do not have as much access to potential soil reserves of phosphorus and hence supplemental applications of P via a fertilizer may be necessary.

Additional phosphorus at establishment is beneficial


Photo 2: Available soil phosphorus is needed for these young developing grass plants. Bob Mugaas, U of MN Extension.

Application of a phosphorus fertilizer, often purchased as a lawn starter fertilizer, is recommended at the time of establishing a new lawn or completely renovating an existing lawn. In this instance, the additional phosphorus is applied and lightly incorporated into the soil surface where it is more readily available to the very young, developing grass root system. See Photo 2. This aids in the promotion of vigorous seedling growth and quicker establishment. Once established and the grass plants have developed a deeper much more extensive root system, the application of supplemental phosphorus fertilizer can be reduced or even eliminated (at least temporarily) depending on existing soil P levels as determined by a soil test.

Fertilizer phosphorus and water quality issues

While loss of phosphorus from a lawn via leaching is considered to be very low, losses via runoff and soil erosion can occur and can create pollution problems for nearby lakes and rivers receiving stormwater runoff. Once in a lake, the additional P, even in very small amounts, can stimulate rapid and excessive algae growth. In turn, these 'algae blooms' as they are called can significantly reduce water quality for many different uses and negatively impact fish and other lake life.

The Minnesota Phosphorus Fertilizer Law:

Because of this concern for excessive phosphorus in lakes and rivers from fertilizer, the Minnesota legislature passed a statewide law that restricts the application of phosphorus fertilizer to established lawns and other turfgrass areas. The law states that fertilizers used on lawns will be restricted to 0% phosphate (P2O5) content. Exceptions include application at establishment, either by seeding or sodding, and then only during the first year of establishment. Applications can also be done when a soil or tissue test shows a need for P. In those cases, lawn fertilizers with P can be used. More detail pertaining to the law and the penalties than can be applied is found in Chapter 18C.60 of Minnesota Statues. Because of this law, soil testing becomes even more important for managing applications of phosphorus to turfgrass.

Recent U of MN research sheds additional light on the potential for lawn phosphorus losses.

Recently completed research at the U of MN, clearly showed that neglected lawns (i.e., those receiving no additional fertilizer), became thin and weed infested over the course of the study and contributed more runoff volume and more phosphorus loss during the growing season than those receiving sufficient levels of fertilizer to maintain good turfgrass density and active growth. In addition, there was essentially no difference in the amount of runoff volume or phosphorus loss between those lawn plots receiving nitrogen and potassium only and those receiving nitrogen, a low rate of phosphorus (1.0 pound of P annually) and potassium. Hence, where background levels of soil P are high on an established healthy stand of grass, there does not appear to be any significant decline in plant health or vigor, when P is no longer being added via fertilizer. In general, that result would support one of the law's assumptions that additional P is not necessary for healthy turfgrass when ample levels already exist in the soil. Likewise, neglecting a lawn entirely by applying no additional fertilizer to maintain plant health and density would not be a good practice.

For those of you interested in more details and results from this study, there is a link to the complete final report of this study in the Phosphorus section in the Understanding and Using Lawn Fertilizers chapter of the Home Lawn Care section of the Sustainable Urban Landscape and Information Series (SULIS). The U of MN Extension publication entitled Preventing Pollution from Lawn and Garden Fertilizers provides additional information on the responsible use of fertilizers to protect water quality.

Turfgrasses and the important role of potassium (K) in plant health.

Even though grass plants contain rather large amounts of the nutrient potassium there is still much we don't know about the biological role and activity of potassium in the plant. Potassium is important in the synthesis of many plant components and in the regulation of many physiological processes including the efficient use of nitrogen. While potassium is involved in many of these activities, it is usually not an integral part of the final product produced. Some of those plant processes where potassium is involved but not part of the end product include the formation of carbohydrates and the photosynthetic process, activation of various enzymes that in turn control other plant processes and, aids in the formation of plant proteins.

Where soil potassium levels are low and plant deficiencies exist there is often a reported increase in the incidence of turfgrass diseases and reduced tolerance to environmental stress. However, the visible, field detection of a potassium deficiency can be difficult at best. While research continues, our understanding of the biology and specific roles that potassium plays in the plant remains rather elusive.

Potassium is held on the surfaces of soil particles and moves little in most soils, though it can gradually move out of the root zone in very sandy soils. Its movement in soil is often considered to be between that of nitrogen, which is very mobile, and that of phosphorus, which is very immobile. Where soils are high in native potassium, supplemental potassium fertilization may be unnecessary; however, where soils are low in native potassium, supplemental applications can be very important. However, an application of additional potassium does not necessarily create obvious visible symptoms as one might associate with the nutrient nitrogen. Again, soil tests are essential to determine the potassium level of a soil and to develop a potassium fertility program.


Photo 3: Healthy, dense lawns can help protect water resources. University of MN Extension.

Concluding remarks

It is hoped that this series of three articles beginning with the March 1, Yard and Garden News issue followed by the April 1 issue and concluding with this one has provided at least some basic insight into the importance of the three major nutrients contained in most lawn fertilizers to a healthy, vigorous lawn. Selecting an appropriate fertilizer and applying it correctly may just be one of the more important things you can do for your lawn and protect the environment at the same time.

Witches's Brooms on Landscape Shrubs

Michelle Grabowski, University of Minnesota Extension

witchs broom 1 YG.jpg

Photo 1: Witches' brooms on willow M. Grabowski UMN Extension.

As spring slowly unfolds in Minnesota this year, many gardeners are anxiously watching the buds on their favorite landscape shrubs open up for the season. Some shrubs have one to several clusters of thin weak twigs arising from one point on a larger branch. These unusual growths are known as witches' brooms and they can form at the tip of the branch or lower down the stem. Many different problems can result in a witches' broom. Mites, fungi, aphids, salt or other damage to buds can all result in a proliferation of small branches, where only one should have emerged.

Another less known and little understood pathogen causes witches' brooms in Minnesota landscapes. Phytoplasmas are single celled organisms in a group known as fastidious bacteria. These tiny pathogens were unknown to scientists prior to 1967. They are very difficult and often impossible to grow in a laboratory and can only be seen with a powerful electron microscope.

As a result, little is known about phytoplasmas. Many of them do not even have proper scientific names (a genus and species) because they have not been properly described. Often they are categorized into groups of phytoplasmas that are believed to be related to one another. To truly identify a phytoplasma in a landscape shrub, DNA analysis needs to be done.

Phytoplasmas live in plant sap and interfere with photosynthesis, plant growth, development and seed production. Infected shrubs often have yellow, curled or distorted leaves. lilac foliar YG.jpg

Photo 2: Yellow leaves & dieback in a phytoplasma infected lilac M. Grabowski UMN Extension.

Branches are often stunted or unusually thin. Witches' brooms are very common in phytoplasma infected shrubs. The entire plant may be stunted and in very severe cases killed.

Phytoplasmas are carried from plant to plant by insects that feed on the sap. They can survive from year to year within the insect or the host plant. Once a shrub is infected, it cannot be cured. Some shrubs tolerate the infection; others decline for several years and then die. The diseases below are phytoplasma diseases that can occur in Minnesota.

Willow Witches' Broom

Witches' brooms in willows can be seen in willows growing alongside roads as well as in landscapes throughout the state. Black willow (Salix nigra) and pussy willow (S. discolor) are both susceptible to a phytoplasma in the aster yellows group. Willows infected with this phytoplasma have witches' brooms on one or more branches. These brooms often have stunted yellow leaves and may die during winter months. Exact identification of a phytoplasma requires a costly DNA analysis. Although the willows with witches' brooms in Minnesota have not undergone this analysis, phytoplasma is the most likely cause of the problem.

lilac broom YG.jpg

Photo 3: Brooms on phytoplasma infected lilac M.Grabowski UMN Extension.

Willows are considered tolerant of the disease and often continue to grow vigorously despite infection. In fact, it is believed that the willow is able to isolate the pathogen in the infected branch. The brooms themselves have a very high concentration of phytoplasmas, where as other branches in the same shrub are often completely free of the pathogen. Gardeners with infected willows should prune out any branches with witches' brooms.

Lilac Witches' Broom

Lilac witches' broom is caused by Candidatus Phytoplasma fraxini, a specific member of the aster yellows phytoplasma group that only infects lilac and ash trees. Over 20 species of Syringa are susceptible to lilac witches' broom. Common lilac (S. vulgaris) is tolerant and often shows no symptoms other than slower growth and shorter twigs. Japanese tree lilac (S. reticulata) and many hybrid lilacs are very susceptible to lilac witches' broom. These lilacs have yellow, distorted small leaves that often scorch brown on the edges by midsummer. Many tiny thin shoots form in clumps at the base of the plant. The shrub declines and is often killed a few years after the first witches brooms appear.

Dogwood Witches' Broom and Stunt

Witches' brooms caused by phytoplasmas have been identified on silky dogwood (Cornus amomum), gray dogwood (C. racemosa) and red osier dogwood (C. sericea). It is believed that the phytoplasma responsible for witches' brooms in dogwoods is a member of the aster yellows group. It is unknown how commonly this disease occurs in Minnesota.

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.

What are your apples doing right now?

Karl Foord, UMN Extension Educator

What are your apples doing right now?
Apple trees have two kinds of buds; vegetative buds that only give rise to leaves and shoots, and mixed buds that give rise to flowers as well as shoots and leaves. The parts of the flower in their primordial state can be identified in the dormant mixed bud as seen in this cross section (Figure 1).


Bud stages

Apple flower buds progress through a series of developmental stages. This begins with the Silver Tip stage where the bud has started to open but no green tissue is visible. The progression runs through to petal fall and the beginning stages of development of the apple fruit (Figures 2 - 10).






Apple varieties can break bud at different times, and buds can be at different stages on the same tree. For example, the buds on my Haralson apple tree are almost all between ½ inch green and tight cluster. Only the leaves are visible but the tips of the leaves are red. My Honeycrisp has a few buds beyond the ½ inch green stage like the Haralson, but many of the buds are between silver tip and green tip.

Bud sensitivity to temperatures

As we move into May and watch our apple flowers develop, what temperatures should we recognize as damaging to apple flowers? The temperature at which flower buds are injured depends to a significant degree on their stage of development. Buds are most hardy during the winter when they are fully dormant. As they begin to swell and expand into blossoms, they become less resistant to freeze injury. The temperature sensitivities of the bud stages noted previously are listed in Table 1.


The month of May can bring some damaging low temperatures. On May 9 of last year a low temperature of 25oF was reached in many locations in Minnesota. The apple flowers of many trees were in full bloom and vulnerable to temperatures below 28°F. In some orchards the frost damage to the flowers was practically complete and the whole apple crop was lost except for some late flowers on Honeycrisp and a significant number of flowers on Sweet 16, a late flowering variety.

At the present developmental stage, apple flower buds in Minnesota could survive temperatures in the low 20's. Let us hope we have seen the last of the temperatures listed in table 1, for this year at least.

Photo credits:

Calendar: May 1, 2011


Echinacea purpurea 'Magnus'Julie Weisenhorn.

Most flowering annuals are available as sturdy seedlings at garden center and nurseries, but many will grow just as well or even better when you seed them directly into the garden. Zinnias, cosmos, bachelor's buttons, California poppies and marigolds are good examples of annuals that grow rapidly from seed. But if you want annuals for containers, buy well developed transplants that will look good the minute you pot them up.

Early May is a good time to plant grass seed, but for good results you need to rough up the soil first. Unfortunately, this exposes crabgrass and other weed seeds that will sprout right along with your new grass. To stop most weed seeds, apply a specially formulated version of pre-emergence herbicide right after seeding. The label must state clearly that it's meant for newly seeded lawns, otherwise it will kill desired grass seeds, too.

Attract butterflies to your yard by planting many good nectar-producing flowers. Include coneflowers, Russian sage, Joe-Pye weed, butterfly weed (Asclepsias), beebalm, catmint, Mexican sunflower, and single or semi-double zinnias. Though butterfly larvae (caterpillars) may feed on your plants, don't use any insecticides in the garden. And don't put up "butterfly houses" unless you enjoy them as garden art... butterflies will never inhabit them!

The Yard and Garden News will be going back to a twice-monthly format for the summer! See you on the 15th!

Yard and Garden News Editor: Karl Foord
Technical Editor: Bridget Barton

  • © 2014 Regents of the University of Minnesota. All rights reserved.
  • The University of Minnesota is an equal opportunity educator and employer. Privacy