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May 2009 Archives

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.

Consumers Beware!!

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Michelle Grabowski, University of Minnesota Extension Educator

Downy Mildew, a potentially devastating fungal disease of roses has been found on several roses for sale in Minnesota. The disease has been identified on Double Knock Out®, Pink Knock Out® and Europeana®, but all rose cultivars are susceptible to Downy Mildew. This disease results in irregular purplish red leaf spots that eventually turn tan in the center. Infected leaves often turn yellow and fall off. Under very humid conditions gray fuzzy fungal growth may be seen on the underside of infected leaves. Downy Mildew also causes purplish black streaks on rose stems. Downy Mildew thrives under cool humid conditions. It spreads easily on the wind. If infected plants are brought into the garden, the disease could easily spread to other roses and raspberries in the area. Do not purchase roses with dark leaf spots!


Emily Tepe, University of Minnesota Research Fellow

If you grow strawberries you are familiar with the spring and summer task of crawling through the rows pulling weeds. It’s a time consuming, arduous task, but a very necessary one. Weeds rob strawberries of valuable water and nutrients, resulting in reduced vigor and fewer, smaller berries. Hand weeding is just about the only way to remove weeds in a strawberry plot because mechanical removal can easily damage the low growing strawberry plants, and approved herbicides are declining in number and becoming more and more expensive. Additionally, consumer interest in local foods grown without such chemicals is rapidly increasing, leading growers to look for alternatives. So with few options other than growing strawberries with plastic mulch (which poses its own environmental problems), growers get on their hands and knees each year in an endless battle against the unrelenting weeds.

But now there is hope for the achy knees, sore backs, and tight schedules of strawberry growers. Wool mulch, cleverly named Woolch™, is marketed by the Minnesota Lamb and Wool Producers Association and has been studied for the past ten years at the University of Minnesota West Central Research and Outreach Center in Morris, and at several local farms. The results of 10 years of trials have shown overwhelming success of Woolch™ in reducing weeds in strawberry plots.

Late May Lawn Care Tips


Bob Mugaas, University of Minnesota Extension Educator

Usually the middle to end of May is the prime time for putting down preemergence weed killer for crabgrass. In general early in the month is appropriate for the southern 1/3 of Minnesota while later in May is fine for middle to northern sections of the state. But what if I miss the prime window of application, how do I know if it’s too late to apply the product? That’s a good question. For all practical purposes, once the crabgrass seedlings have emerged from the ground it is too late for a preemergence product to effectively be put down. There is one notable exception and that is the preemergence weed killer known as dithiopyr. It is known in the trade by its product name Dimension. It is a common ingredient in many homeowner formulations. This product does provide some control of seedling crabgrass plants up until the 2 or 3 true leaf stage. That is still a pretty small plant. It is important to distinguish between tillers and leaves. Tillers are secondary shoots that also arise from the crown of the plant. See accompanying picture for a comparison of a two to three tiller stage compared to a two to three leaf stage. Applying dithiopyr at the two to three tiller stage is useless. It must applied prior to or at the two to three leaf stage to have any control effect. It should also be noted that dithiopyr will continue to have its preemergence effect once it’s applied as well as very early postemergence effect.


Michelle Grabowski, University of Minnesota Extension Educator

Whether you are looking for tomato transplants, annuals for a front garden bed or a new tree or shrub, one of the most important things you can do to ensure the future success of the plant is to start out with a healthy disease free plant.

Some plant pathogens live in our gardens in plant debris or soil, waiting for the right plant and the right environmental conditions to come along. Other plant pathogens come into the garden on wind, rain, or are carried by insects. Unfortunately many plant pathogens can be brought into the garden on infected plant material.

This later group of plant pathogens can be avoided by a disease management strategy known as exclusion. Exclusion is a strict ‘no pests allowed’ policy. For gardeners, this is one of the simplest pest management strategies to implement.

Garden Insects


Jeffrey Hahn, University of Minnesota Asst. Extension Entomologist

Flea beetles are active on crucifers now. These insects are about 1/16 - 1/8 inch long and an iridescent black violet (flea beetles on other plants are the same size and can vary in color). They overwinter as adults and are active in the spring, feeding on the leaves. They chew small, shallow pits and holes into the leaves. A heavily infested plant looks like it got shot with a BB gun.

Plants are most susceptible to damage in spring - seedlings are more vulnerable than transplants. If your plants are suffering 10 % - 30 % damage, you should treat plants to protect them from flea beetle damage. Apply a garden insecticide, such as permethrin, spinosad, or carbaryl. Different flea beetle species also attack potatoes, spinach, beans, squash, corn, and other plants so be on the watch for feeding injury on these plants as well. More information on flea beetles is available at this link (



Jeffrey Hahn, University of Minnesota Asst. Extension Entomologist

A couple of caterpillars were noticed recently. Eastern tent caterpillar is a common insect on apple, crab apple, cherry, and other fruit trees. They have a dark colored, hairy body with a yellow stripe down their back and grow to almost 2 inches in length. They overwinter as eggs on branches and emerge in the early spring. They construct webbing in the forks of branches which is where they rest at night and during cloudy, rainy days.

Cankerworms have also just emerged recently. A type of inchworm, they are yellowish green with a smooth body and grow up to 1 inch long. Cankerworms skeletonize leaves, i.e. they feed between the major veins. When they first start to attack leaves, this damage will begin as small oval holes between veins. As the caterpillars become larger, entire areas between the veins are consumed. Cankerworms feed on a variety of trees, including apple, linden, elm, ash, and hackberry.

Be on the Watch for Ticks


Jeffrey Hahn, University of Minnesota Asst. Extension Entomologist

We are well into the beginning of tick season. There are two ticks that are of particular importance to people, the American dog tick, commonly called wood tick, and blacklegged tick, formerly called deer tick. Both ticks commonly bite humans. However while the American dog tick is basically just a nuisance and essentially does not transmit disease to people, the blacklegged tick is a known vector of Lyme disease as well as human anaplasmosis (formerly known as human granulocytic ehrlichiosis) and babesiosis.

Both ticks are found in hardwood forests and fields and other grassy, weedy areas, especially along trails and paths. If you are out in areas where ticks are found, take the proper precautions to avoid them. Stick to trails when you are walking and try to avoid moving through grassy areas. Wear protective clothing, such as long-sleeved shirts and pants. You can maximize your protection by tucking your pants into your socks.

Harvesting Your Rain


Three picnic shelters at the Minnesota Landscape Arboretum are in the beginning phase of a transformation as the arboretum prepares for its 2009 summer exhibition called Waterosity.  When complete, the picnic shelters will be a new permanent display called Harvest Your Rain.  Each shelter is being modified to show three different ways of managing stormwater runoff from your property’s impervious surfaces such as rooftops, driveways, and sidewalks.  During rain storms and snow melt, rain barrels, rain gardens, and green roofs all “harvest your rain” decreasing the amount of runoff and non-point pollution that would otherwise pour over these impervious surfaces and into sewers and adjoining water bodies such as lakes, rivers, and wetlands.  Below are articles on rain gardens and rain barrels.  In June look for more Waterosity-related articles on green roofs, the new Cutting Edge on Lawns display at the Minnesota Landscape Arboretum, porous paving, and water-wise irrigation tips.

Rain Gardens


Eleanor Burkett, University of Minnesota Extension Educator

Whether you live in the city or reside on a lake or river shore, managing stormwater runoff is worth considering for your landscape. Rooftops, roads, driveways and sidewalks create hard impervious surfaces which rainwater and melting snow cannot penetrate through to soak into the soil. Additional runoff created by impervious surfaces often is channeled into depressions on your property, often eroding soil along the way. The additional runoff also increases the amount of nutrients and sediment that are carried into lakes, rivers, and wetlands.

Simply put, a rain garden is a shallow depression filled with plants designed to collect rainwater runoff and allow it to filter into the soil, removing nutrients, sediments and other pollutants before reaching the groundwater. Small shrubs, flowering plants and ornamental grasses within a rain garden absorb nutrients, and the sediments settle to the bottom. Rain gardens add beauty to the landscape and may attract butterflies and birds.

What's Up With That?!


David C. Zlesak

The core or pith of plant stems are typically either hollow or contain loosely packed, spongy parenchyma cells.  The pith of this butternut or white walnut (Juglans cinerea) is distinctive in that it is chambered with dark bands of sclerenchyma plates separating hollow zones. Black walnut (Juglans nigra), a close relative, also has chambered pith and has bands that are typically a bit lighter in color. Parenchyma cells are relatively large, of variable shape, and have thin cell walls.  As developing stems elongate, parenchyma can tear and disintegrate in many plant species.  Another cell type found in plants is sclerenchyma. Sclerenchyma cells help provide support to plant tissue and have thickened, secondary cell walls containing cellulose and are often impregnated with lignin.  As stems of white and black walnuts grow, parenchyma cells eventually collapse leaving mainly sclerenchyma cells in distinctive plates.  

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.

Meleah Maynard, University of Minnesota Master Gardener

“Just opening the gate and stepping into the garden filled me with peace. My little plot looked like a bush and I brought my granddaughter [the see it] and she just felt everything with her hand.”—Sabathani gardener


They aren’t visible from the street, but just north of Minneapolis’ Sabathani Community Center, members of the Urban Gardener program are already busy working on plots that will soon be bursting with vegetables. Launched in 2007 with a grant written by Extension Urban Director Barbara Grossman, the program is about much more than gardening. It also aims to build community, help families stretch their dollars a bit further by growing some of their own food and offer information about nutrition and healthy eating.

Hennepin County master gardener Mollie Dean, who has been coordinating the program since its inception two years ago, says interest has grown quickly with over 275 people attending gardening classes so far and more than 55 people crowding into the program’s first class this April. Most participants are first-time gardeners and many of them live in the surrounding neighborhoods. “We’re seeing a lot of young couples this year, but we also have a lot of parents and children, as well as grandparents and grandchildren,” she says.


David C. Zlesak, University of Minnesota Extension Educator

Taking plants from the relatively low light and moderated temperatures of the home environment and plunging them suddenly outdoors in bright sunlight, wind, and temperature extremes can result in severe injury. Depending on the extent of the injury the plant may be capable of recovering relatively quickly, after multiple weeks, or in extreme cases not at all.  As plants grew indoors, the tissues that they produced were adapted to those environmental conditions.  With a gentle transition period, plant tissue can adapt to some degree when conditions change.

Plants use environmental cues to help them adapt their tissues to their current growing conditions.  For instance, leaves produced in shade typically have more surface area and are thinner in order to better intercept light and best invest energy resources into the most efficient type of tissue.  Leaves grown under brighter light are often thicker with additional layers of photosynthetic cells within them and have a more developed waxy cuticle layer in order to better conserve moisture.  Higher light levels are often associated with greater heat and therefore a greater potential for water loss. Leaves grown under higher light conditions also are typically higher in pigments like carotenoids and anthocyanins that serve in part to defend leaves from damage from excessive light, especially UV light.


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.

Early May Lawn Care Tips


Bob Mugaas, University of Minnesota Extension Educator

Early May usually finds most Minnesota lawns well into turning green and in need of a first mowing.   Remember that the early part of the grass growing season, late March through the first part of May, is when our lawn grasses produce the best root growth of the year.  Therefore it is important to not be mowing too short as shorter mowing heights reduce the amount and depth of those roots.  Maintaining an average mowing height for around 2.5 to 3.0 inches, even at this time of year, is appropriate.   Those larger, more robust root systems are important in providing the plant with the necessary water and nutrients to sustain the plant through flowering and increase the plants ability to withstand summer stresses.

For average home lawns consisting primarily of Kentucky bluegrass and growing in sunny conditions, early May is a good time to apply that first application of fertilizer.  Usually this will coincide with about the time you plan to mow for the first time.  This will be especially true if there was no fertilizing done the previous fall.  As a rule of thumb, the amount of fertilizer put down should provide about one pound of actual nitrogen (N) per 1000 ft2 of lawn area.  It’s best to apply about ¼ to ½ inch of water following the fertilizer application to help it dissolve and move into the soil where it will be less apt to runoff and be available for plant roots to take up.

Waterosity Comes to the Arboretum


Carolin Dittmann and Kathy Zuzek, Owner of Verbena Design and Landscaping and former University of Minnesota Former Graduate Student and University of Minnesota Extension Educator

Water is the essence of life.  All living beings on earth depend on water for survival.  Water is also a source of joy and beauty.  Here in Minnesota with our 10,000 lakes, the headwaters of the mighty Mississippi, and the beauty of Lake Superior and the rivers that cascade into it, it is easy to celebrate the beauty of water.  It is also far too easy to ignore how wide-reaching our impact on water resources is.  From agricultural and industrial uses that benefit everyone to our individual use of water in our daily lives, we all consume and pollute water.  All of us – individuals, communities, and agricultural and industrial enterprises – need to act with wisdom and stewardship to manage and conserve our water resources, ensuring that safe water will be available as a source of delight and health today and in the future.

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.

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