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Extension > Yard and Garden News > May 2009

Friday, May 15, 2009

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!!


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!

The Wonders of Woolch™ - A New Mulch for Farm and Garden


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.

Photo 1: Rolls of wool mulch. Emily Hoover


Photo 2: A row of strawberries growing through wool mulch. Emily Tepe

Woolch™ is a value-added product of the Minnesota lamb and wool industry. It is manufactured using short fiber waste wool from the carding process, in which raw wool is cleaned and separated before spinning. Similar short fiber waste wool is collected from the napping process of blanket-making at local woolen mills. These short fibers are combined with wood shavings from Minnesota timber mills through a heat and roller process, and the result is a lightweight felt-like mat that is completely natural and biodegradable.

Woolch™ works well with strawberries for several reasons. First, water and nutrients can move through it, daughter plants can root through it, but weeds neither grow up nor root down through it (except for quackgrass, which should be eliminated before planting). Second, it helps maintain soil moisture and moderates soil temperature. Third, it creates a barrier between the low growing strawberry and the soil which may harbor fungal diseases that can lead to reduced growth, poor yields and damaged fruit. In a similar vein, berries resting on Woolch™ remain cleaner and drier than those conventionally grown. Finally, Woolch™ breaks down over a couple of years, meaning it can then be tilled into the soil to add valuable nutrients.


Photo 3: Wool mulch in the garden keeps the soil moist and cool, and deters weeds. Emily Tepe

Though not part of any research project, I can personally vouch for the success of Woolch™ in the home garden. Last season I planted vegetables and flowers in a veritable weed patch, and I expected to spend the summer battling the continual onslaught of weeds. Not this gardener! I ran a soaker hose throughout the beds to allow for direct and easy watering. I then cut pieces of Woolch™ to place around the plants, and cut slits to fit pieces around stems or rows of stems. I shoveled a bit of soil over the edges to keep the Woolch™ in place, and gave it a good watering from above to help it adhere to the soil. Throughout the season the soil remained moist and cool under the Woolch™, and the plants were vigorous and abundant. The only weeding I had to do was is in the paths between beds, which was quick and easy. This season Woolch™ will be used in a new edible landscape planting in the beds outside the Plant Growth Facility on the St. Paul Campus. Stop by and take a look at this innovative, effective, and totally natural product in action!


Photo 4: Fitting wool mulch around the stems of plants like this pepper keeps weeds at bay and helps hold moisture in the soil. Emily Tepe

Woolch™ is available directly from the Minnesota Lamb and Wool Producers Association. For more information visit their website at You can support this local industry while benefiting your garden or small farming operation!

Late May Lawn Care Tips

Bob Mugaas, University of Minnesota Extension Educator
Photo 1: The crabgrass plant on the left side of the picture is the 2-3 true leaf stage while the one on the right is at the 2-3 tiller stage. Bob Mugaas

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.

For creeping Charlie, second to fall treatment, spring time, at or during full bloom, is a very good time to apply postemergence herbicides to control this common lawn weed. However, rather than just simply reaching for an herbicide to kill the plant, stop and consider for a moment as to why this weed seems to be getting worse or expanding in your lawn. Creeping Charlie does best in a moist, partly shaded to fully shaded environment. As shade increases it becomes less and less favorable for sustaining a turfgrass cover and more and more favorable for weeds like creeping Charlie to encroach and take over. It might be that doing some pruning or other practices to get more light to the soil surface will improve growing conditions such that turfgrass can survive and thrive there. Then, if creeping Charlie is controlled with an herbicide you can come back and overseed with a shady lawn mix and expect it to be more competitive and vigorous thus helping to keep creeping Charlie from reestablishing. If the area is simply too shady to grow turfgrass, why fight it, consider other ground covers or a shade garden of perennial flowers. They are not necessarily less work than a lawn, but can provide a very attractive area in the landscape. If you are considering the use of an herbicide for controlling creeping Charlie, select one that contains the active ingredient triclopyr. Research has shown that the addition of this product to a broadleaf herbicide is more effective than those not containing it. You can even purchase products where triclopyr is the only ingredient. They are usually sold as a weed control products specifically for creeping Charlie, white clover, oxalis and other difficult weeds.

Photo 2: Typical growth habit and foliage of creeping Charlie (Glechoma hederacea). Bob Mugaas

Hot, dry periods during the latter half of May are not uncommon. Since spring is a very active period of growth for grass plants plus we are often putting down fertilizers and weed killers, it’s important to keep grass out of drought stress. Broadleaf weed killers such as those used for dandelions can injure or even kill lawn grasses that are under drought stress. A general rule of thumb for lawn watering is about 1 inch of water per week which includes what we receive through rainfall. Remember, it may be necessary to apply a couple of ½ inch applications on compacted soils, heavier clay soils and sandy soils rather than a single 1 inch application. In the compacted and heavier clay soils, too much water per application may cause ponding or water saturated conditions on the soil surface due to very slow water infiltration rates. This can damage or even cause dieback of roots in a matter of hours that are growing in the shallower regions of the soil. Sandy soils usually allow for rapid water infiltration and drainage; so much so that a significant amount of water can be lost down through the soil and unavailable to grass roots. Hence, for lawns growing on sandier soils a couple of ½ inch applications of water during the week may be more beneficial to the grass plants and less wasteful of the water applied. If you’re uncertain about how much water your sprinkler or sprinkler system is putting out, place a few flat bottom cans (tuna fish cans work well) in the water distribution area of the sprinkler. Leave the sprinkler on for about an hour or one cycle of an automated system then go measure the amount of water in the bottom of the can. That will give you the rate at which water is being applied either through a manual or automated system.

Photo 3: Spring flowering stems of creeping Charlie. Bob Mugaas

Since spring is a very active period of grass growth, there is greater demand for nutrients, particularly nitrogen (N), to sustain that growth. However, it is equally important to not be over zealous with the application of lawn fertilizers, especially N, in the spring. Remember that most of the growth emphasis of mature grass shoots (which will be most of the lawn) is to produce a flowering stem and then die. Most of us see little of the actual grass flowers because we are always mowing them off. Nonetheless, a light application of nitrogen fertilizer can be beneficial. In most cases this means about ¾ to 1.0 pound of actual N per application. If possible, about half the fertilizer should contain ‘slow release’ nitrogen along with some that is readily available. Once water is applied, the nutrients dissolve and move into the soil where they can be taken up by the grass roots. The slow release fraction provides an additional supply of N over a longer period of time. Very high application rates of nitrogen fertilizers can cause too much top growth at the expense of root growth. Weakened and stressed root systems are less able to supply shoots with the necessary nutrients and water creating even more stress in the grass plant. This also sets up the plant to be even more vulnerable to injury from mid-summer heat and drought stresses likely to come.

If you have been mowing your lawn a little shorter during the early spring period, now is the time to increase mowing heights prior to the onset of summer stresses. This helps encourage a more robust and deeper root system allowing the plant to access a greater volume of soil and therefore larger soil water and nutrient reserves. Higher mowing heights in combination with good turfgrass density can sufficiently shade the soil surface such that weed seeds won’t receive the necessary sunlight to begin germination. Even though other factors such as moisture and nutrients may be favorable for weeds to grow, the reduced sunlight prevents those seeds from germinating and beginning to grow. Mowing should follow the “1/3” rule which states that no more than 1/3 of the turfgrass height be removed at any one mowing. Since growing back that 1/3 will take more time at a higher height of cut, mowing frequency can also be reduced. In other words, if the desired mowing height is two inches, it will take longer for a grass plant to grow from 2 inches to 3 inches, than it will from a mowing height of 1.0 inch to 1.5 inches; both of which are increases of 1/3 in grass plant height.

While there are many other competing interests in the yard and garden during this time of year, regular attention to basic lawn care should still allow ample time for the flower and vegetable garden activities that many of us also enjoy.

No Pests Allowed - Purchasing Healthy Plants to Start the Season Right


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.

Photo 1: Petunia plants wilting due to Rhizoctonia root rot. RK Jones NCSU,

First, purchase plants from a reputable nursery or seed company. In Minnesota, the Nursery Law (MN statutes, Chapter 18H) protects consumers, by requiring that all perennial nursery plants be inspected annually and certified pest free before being sold. This includes perennial flowering plants, trees, shrubs and other perennial landscape plants. Anyone selling perennial nursery plants in Minnesota must have a nursery certificate. You can verify any vendor’s certificate status at the Minnesota Department of Agriculture’s website.

Although you may find a great deal on plants on e-bay, Craig’s list or at a garage sale, remember these plants have not been inspected and many gardeners have found that their bargain plant came with a free plant pathogen. These unwanted pathogens can harm the new plant and potentially spread to other established plants in the garden.


Photo 2: New Chysanthemum plants wilted from root rot. RK Jones NCSU,

At the nursery or garden center, carefully inspect every plant prior to purchase. Look at the upper and lower leaf surfaces for spots, discoloration, unusual growth or other signs of disease. Examine the stem. Are there any open wounds, excessive sap, discolored or soft mushy areas? These could all indicate a disease problem. Either in the store or prior to planting, examine the roots. They should be firm and light colored. Dark mushy areas may be infected with a root rotting pathogen. Reject any plants showing symptoms of disease. If many plants in a group appear diseased, do not purchase any of the plants within the group. Nursery plants are often grown close together and share many maintenance practices like pruning, watering and fertilizing. If there are infected plants within a group, it is easy for the plant pathogen to move from one plant to another in a nursery setting.

Read over the plant tag. Is this plant hardy in your area? Will the plant thrive in the conditions of your garden? Plants stressed by cold, drought, too much sun or shade or other environmental problems often are more susceptible to disease problems in addition to environmental stress.


Photo 3: Tomato plant with a large crown gall. FLDACS,

Whenever possible purchase plants that are bred to be disease resistant. Many common disease problems like apple scab on crab apple, powdery mildew on phlox and Verticillium wilt on tomato can be avoided altogether by growing disease resistant varieties. Disease resistance is often listed on the plant label.

In addition, some plants like roses and hostas are screened by some nurseries to verify that they are free of viruses like Rose Mosaic Virus and Hosta Virus X. Ask at the nursery if these plants have been screened or ‘indexed’ for virus prior to purchase.

Careful selection of plants early in the season can help gardeners avoid future disease problems.

Splitting and sharing perennial plants is a tradition as old as gardening itself and can be a cost effective way to grow your garden while connecting to friends in the gardening community. If you are receiving plants from a split perennial, accept plants from someone you know and trust. Volunteer to help dig and split plants. This will allow you to see the mother plant. Select vigorously growing plants for division. Examine the plant carefully for any disease signs or symptoms. Look for spots, discoloration, unusual growth, or rot on all plant parts. Reject plants that appear to be infected with a disease. Healthy looking, vigorously growing plants can be split and shared. Gardeners may choose to take extra precautions against introducing a disease problem by isolating the new plant in an area away from the main garden bed for a year to watch for potential problems. This way, if a problem does come up, the plant is easily removed and it is less likely to spread to your other well established perennials.

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 (

Photo 1: Flea beetle on crucifer. Jeff Hahn

An interesting insect was found on rhubarb recently. At first glance, this very small (1/16th inch long), orange insect appeared to be an aphid. However upon closer inspection, the insects were quite mobile and apparently not feeding. The shape was also not quite right, this insect was more globular than the typical pear-shape you find with aphids. In fact the insect we found was a springtail, more specifically a sminthurid springtail.


Springtails are an extremely abundant group of insects but because of their small size and the fact they are most common in the soil and leaf litter, they are infrequently noticed by people. Springtails are considered beneficial insects because they feed on decaying plant material, helping in the decomposing process. However, infrequently they are found on plants and can actually be found chewing on them. No damage was found on the rhubarb that was inspected. At worst we would not expect anything more than minor feeding. Management for springtails is not necessary.

Photo 2: Springtail on rhubarb. Jeff Hahn



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.

Photo 1: Eastern tent caterpillars on Prunus. Michelle Grabowski


Mature, healthy, well established trees can tolerate caterpillar feeding, even when it is severe. Young trees or those that are stressed are less able to tolerate feeding and should be protected from caterpillars. A non-chemical method to eliminate eastern tent caterpillars is to wait until they have retreated to their webbing and then pull the webbing out along with the caterpillars. You can then destroy them by crushing or burning them. You can also treat the leaves with a variety of insecticides. Less toxic choices include Bacillus thuringiensis, insecticidal soap, and spinosad.

Photo 2: Cankerworms on linden. Jeff Hahn

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.

Photo 1: American dog tick. Jeff Hahn

Use a repellant for the most effective protection. Products that contain DEET (N,N-diethyl-meta-toluamide) can be applied to your clothes and exposed skin. It is not necessary to use high concentrations of DEET — there is no evidence that increased percentages are more effective. Do not use products containing more than 15% DEET on children. Another effective repellent is permethrin. Apply this repellent only to clothing. Do not overapply any repellent!

Lastly, be sure to check yourself for ticks after you have been outside in areas where ticks occur so you can promptly remove them. This is particularly important because of the risk of disease. Remember if a tick is on you but is not attached and biting, it can not transmit disease to you. Also, there is only one tick, blacklegged ticks, that can transmit disease. They not only need to be biting but they need to be attached for about 36 hours to be able to pass on disease organisms. The more quickly you discover and remove ticks, the lower your risk for contracting disease. Because ticks can be challenging to identify, be sure to have unknown ticks identified by an expert.

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.

Photo 1: There are many opportunities to redirect and utilize runoff water from impervious surfaces. David Zlesak

Rain Barrels: A Way of Collecting and using Rainwater


Have you ever watched a river of rainwater run down your driveway into the lake or storm sewer? Or even worse, seep into your basement? Collecting roof runoff in rain barrels is a good solution to these problems and it also helps alleviate stressed water systems and conserve limited resources. Although rain barrels have been around for thousands of years, people are now encouraged more than ever to use them as a way to protect our lakes and rivers while saving money on water bills.

For the rest of the article on rain barrels please see:

Photo 2: Rain barrel setup off of a garage. Rebecca Chesin

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.

Photo 1: Raingardens help to limit the amount of nutrients and sediment that are carried into lakes, rivers, and wetlands. David Zlesak

Rain garden designs can be simple or elaborate, depending on your gardening interest and experience. When designing a rain garden consider placement, soil type, size, desired shape, and plant species. To insure satisfaction, sketch a design before you start.

Rain Garden Placement

Rain gardens should be placed 10’ or more away from buildings to prevent water from the rain garden from entering basements and damaging foundations. They should also be 35’ or more from septic system drain fields, 50’ or more from drinking water wells and well away from underground utility lines. Call Digger’s Hotline (800) 242-8511to locate electrical, gas or telephone lines before designing your rain garden.


Photo 2: Rain garden at the edge of a forest. Brian Peterson

Test Your Soil

Of utmost importance is the soil in which a rain garden is constructed. The purpose of rain gardens is to infiltrate runoff water, so the soils need to be porous enough to quickly soak up water, ideally emptying within 48 hours. Forty-eight hours is the standard because most plants can survive inundation for this amount of time and it prevents a rain garden from becoming a mosquito breeding area. A simple test of a soil’s ability to infiltrate water is to dig a wide hole 10” deep and fill it with water; if the water disappears within 48 hours, the site is suitable for a rain garden. For clay soils mix in organic matter before planting.

Rain arden Size and Shape

Often several rain gardens are designed into the home landscape. For roof top runoff each down spout usually drains to one rain garden. Each rain garden should be about one-third the size of the roof area being drained. Rain gardens typically range from 100 to 300 square feet in size. Applying these same concepts, rain gardens may be built to capture runoff from other impervious surfaces such as driveways and sidewalks.

Rain gardens can be designed in any shape - crescent or kidney shapes are attractive. A long and narrow rain garden may be better suited to fitting between closely spaced buildings, sidewalks and roads.

Select Plants

Choose plants that are suitable for your soil type and will tolerate standing water for up to 48 hours. Many native plant species are well suited for rain gardens. If locating a rain garden near a lakeshore or riverbank it is recommended to use native plants, it may even be a requirement, so be sure to check with your local Soil and Water Conservation District or County Environmental Department.


Photo 3: There are many plants to select from that can tolerate periods of standing water such as Rosa palustris, the swamp rose. David Zlesak

Install the Rain Garden

Once the size, shape and location of the rain garden have been decided, construction can begin. Lay out a rope or garden hose in the shape desired as a guide for digging. On a slope the soil from digging may be used to create a berm on the downhill side of the rain garden. Otherwise, excess soil should be removed from the site. The depth of the depression may vary from 4” to 8”. For best infiltration the bottom of the rain garden should be flat and level. Finally, use plastic pipes, installed above or below ground, or a rock lined spillway to connect a downspout or other source of runoff to your rain garden.  

For more information:

Downloadable “how to” manual by the University of Wisconsin Extension:

Blue Thumb:

Friday, May 1, 2009

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.

Photo 1: Minnesota quarantine area. Note, the yellow do is a confirmed EAB infestation. MN Dept. of Ag.

Minnesotans should be greatly concerned about EAB.  Minnesota has one of the largest concentrations of ash in the U.S. with about 900 million found in our forests and urban landscapes. This insect is such a severe pest as it attacks all species of ash, regardless of size or state of health, killing them after three to four years of tunneling under the bark.

Photo 2: Emerald ash borer. Jeff Hahn

What can you do to help?  First and foremost don’t move firewood from outside of Minnesota into the state.  In fact don’t even move firewood within Minnesota.  If you are going camping or any activity that involves firewood, buy it locally from an approved vendor, don’t bring firewood from home.  And when you return home, don’t bring extra firewood back with you.

This is so important because firewood is the primary method EAB has for being moved long distances.  By itself it only flies about ½ mile a year.  But with peoples’ help it can travel hundreds of miles at a time.

People can also help by being aware of what emerald ash borer looks like and the symptoms of an EAB infested ash tree.  An emerald ash borer is a slender, ½ inch long, iridescent green beetle.  It is active anytime from late May into August. There are other insects ( in Minnesota that are also green so look closely.

Be alert to trees that are suddenly showing signs of thinning foliage and dead limbs.  An emerald ash borer infested tree will show signs of D-shaped exit holes (although they may not always be easy to see).  If you were able to look under the bark, you would find a series of S-shaped tunnels backed with frass (sawdust and insect droppings).  Although there are other problems that can cause to ash to dieback (, it should be a red flag that should cause one to look more closely.  Use the ‘Do I Have Emerald Ash Borer’ diagnostic web page ( on the Minnesota Dept. of Agriculture web site for help in deciding whether you have EAB.

For more information on emerald ash borer see,

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.

Photo 1: Cluster of apple flowers. Dave Hansen

Photo 2: Crabapples produce a lot of flowers and pollen over an extended period of time. David Zlesak

If you’re interested in more fruit to eat, you may want to choose another eating variety for cross-pollinatio. Zestar!, Snowsweet, and Honeycrisp apples are highly recommended for home gardens. These popular University of Minnesota varieties offer home growers a variety of flavors and harvest times. Planting an early-season variety such as Zestar! with Snowsweet – a late-season apple – means a full season of fresh fruit. The University of Minnesota hosts several websites that may be helpful as you choose varieties and plan your planting.

University of Minnesota Apples:

Commercial Fruit Production:

Garden Information:

Improving Yields

Do you have a couple of apple trees in your yard that just aren’t producing much fruit? Do you get a lot of apples each year, but they’re smaller than you would expect? Thankfully there are some simple things you can do to improve yields and increase fruit size. Before that though, it is good to remember that apple trees take about 5 years to reach maturity. This means until then they might produce little or no fruit. So when it comes to apples, patience is definitely a virtue.

There are a few things to understand about the behavior of an apple tree. An apple tree’s primary goal each season is to produce seeds…as many seeds as possible. It starts this process by producing flowers, lots and lots of flowers. Once these flowers are pollinated and set fruit, the tree will naturally put much of its energy into developing that fruit. That seems great, right? Well, not exactly. What we can’t see is that shortly after fruit set, flowers are actually forming for next year inside buds. The tree will ignore these flowers, putting everything it has into developing a lot of the current fruit filled with seeds. This is why most apple trees, when left to their own devices, only produce a large amount of fruit every other year. In other words, they are naturally biennial. We can change that to a large extent through fruit thinning. Thinning involves removing excess fruit to allow space for remaining fruit to grow large, and to allow flower initiation and development for the following year. Thinning also promotes improved fruit uniformity, color, flavor, and reduces limb stress and breakage.

Photo 3: Thin fruit when it is about ½ “ in diameter.

Fruit Thinning

So when is the best time to thin? This is the tricky part. There is a short window during which you should thin an apple tree, which falls between fruit set and flower initiation. Fruit set occurs after the petals have fallen off, and the remaining ovary begins to swell. That’s pretty simple. But if we can’t see the flowers, how do we know when initiation happens? Thankfully flower initiation is dictated by day length, which is quite reliable, and generally occurs around June 20 in this region. Fruit thinning should be done before then or next year’s harvest will be compromised. A good rule of thumb is to thin the tree when the fruits are about ½” in diameter, or about the size of a dime.

Most apple trees will self-regulate to a small degree, meaning they will drop some fruit to reduce the burden. This is called the June drop period, and in this time the tree will naturally abscise some of the tiny fruit. Abscised fruit is recognized by a yellow pedicel – the stem that connects the fruit to the tree. These fruits become loosely attached and can be removed with a flick of the finger. June drop may happen before or after the ideal thinning window, so don’t rely on it as a guide. Just remember the ½” rule.

How much should we thin? If you look closely, you’ll see that each bud produces a cluster of about 5 flowers. The first and largest flower in each cluster is called the ‘king bloom’ and it will go on to produce the ‘king fruit’, the largest fruit in the cluster. Ideally, this is the one to keep, but it can sometimes be difficult to determine when the fruits are so small. Generally, fruit should be thinned to a spacing of about 6”. This may seem excessive when looking at those tiny apples, but consider when they grow to 3” or so how close together they’ll really be. And that is when they’ll need a lot of light to mature, and will be weighing down the branches. A spacing of 6” will allow the tree to produce large, uniform fruit while conserving some energy to work on flower buds for next year.

Photo 4: Aim to have the final fruit spacing be about 6" apart. David Bedford

So how exactly do we thin the fruit? Thin by carefully plucking the tiny fruits off the ends of their pedicels (stems). This prevents any injury to the spur which is holding next year’s buds. You can use a thinning shears to make this a little easier. Just snip the fruit off right at the top.

One note of caution: Haralson apples are very prone to biennial fruiting. If you have a Haralson tree and you want it to produce fruit every year, you may have to thin so excessively that you get only a small crop. In this case it may be best to permit its biennial nature and get a large crop every other year.

Pruning and Training

Photo 5: Young 'Honeycrisp' apple tree being trained in the central leader system. David Zlesak

Pruning is essential to reliable fruit production on apple trees. Again, left to its natural course an apple tree will send out branches every which way and will fill them with fruit, resulting in small apples, uneven development, and overall reduced productivity. Proper pruning spaces out the fruit, allows more sunlight to reach the fruit, increases airflow in the canopy (which reduces disease potential), and focuses the tree’s energy into bigger and better-tasting apples. Pruning is the key to productivity, and if done consistently every year is a rather simple and enjoyable task. Understanding a little about tree growth and following a few simple guidelines will give you the confidence to prune without fear!

Before we get into pruning, we should first talk a little about training. Training is done primarily when the tree is young, and starts right after planting. Think about training a puppy. You want to develop good habits right from the start, because trying to go back later to change bad behavior will be far more difficult. It is similar with apple trees. Proper training will help to establish a well-structured tree that will be easier to prune and maintain in the future. Attempting to straighten a leaning trunk or reposition branches that are several years old will often prove difficult.

A common training system for apple trees is the central leader system. The goal of this system is to create a strong, vertical central stem, or leader, off of which grow strong, evenly-spaced side branches. The result of this training system is a well-proportioned tree which is narrow at the top and wide at the bottom, similar to the shape of a Christmas tree. The open, evenly spaced braches allow for good light penetration and air circulation, both of which promote fruit production and lower the risk of many diseases. Training is accomplished by tying the main stem to a sturdy, vertical support and then pressing young side branches to an angle of about 45° to 60° from the stem. Branches at this position will produce a balanced amount of vegetation and fruit. This training is done using things like clothespins, rubber bands, wooden spreaders, ties, and weights. Young, supple branches that are 3-6 inches long can be easily positioned in June. Older branches can be positioned any time between late winter and early summer. An excellent publication from the University of Wisconsin offers detailed explanations and diagrams of how to properly train a tree to the central leader system. This publication can be found at

During the training phase – the first 3 or 4 years – careful pruning allows you to select the branches that will create the structure and foundation of the tree, which will determine its form from here on out. The strongest branches are the ones to keep, and should be evenly spaced and staggered so they are not directly above or across from one another.

Photo 6: Dormant pruning. David Bedford

As the tree matures, pruning is done to maintain the shape of the tree and encourage fruit production. If the tree has been well-trained it will require little pruning, but should definitely be pruned every year. For this kind of maintenance pruning, here are some tips to follow:

  • Remove weak, broken, diseased or unproductive branches, including any that are growing directly upward or downward.

  • Keep branches at the top of the tree shorter than those at the bottom.

  • Remove water sprouts (thin twigs growing vertically from trunk or branches).

  • Remove suckers (vertical shoots growing around the base of the tree). These can be removed throughout the growing season as they appear.

  • Remove branches competing with the main stem. The main stem must remain the tallest part of the tree.

  • Leave just the collar of the branch you remove. Don’t cut too close, but don’t leave a big stub either.

  • “Head back” if you need to limit the growth of the main stem. Cut the tip of the stem back to a weak bud or twig. Avoid “topping” or “shearing” the top of the tree, which means cutting a large amount from the main stem. This will result in excessive vegetative growth, will significantly reduce your yield and fruit quality, and can permanently damage the tree.

  • Remember to always use clean, sharp tools specifically made for pruning.

Photo 7: Prune suckers from the base of apple trees anytime of the year. David Zlesak

Finally, pruning is done when the tree is dormant, in late winter or early spring when risk of severe cold damage is past. March and April are the best months to prune. If you wait until growth begins, the risk of infection and disease is much greater. Prune early and don’t be afraid to prune a lot. Your tree will thank you.

The following websites offer detailed training and pruning recommendations:

Cornell University:

Ohio State University:

University of Wisconsin:

A Word about Fertilizing

A soil test every two or three years will provide accurate recommendations for what type and at what rate you will need to fertilize your apple trees. The University of Minnesota can conduct a soil test for you.  The most important time to test your soil is before planting, because this is when it is easiest to amend your soil with lime, phosphorus and potassium if they are needed.  These amendments are most effective when incorporated in the soil.  Once the tree is planted, incorporation is difficult. The U of M soil testing lab website ( offers guidelines on how to submit samples for testing, and how to read and interpret the results. Follow the fertilization requirements on the test and you will maintain healthy, productive trees.  Avoid over fertilization with nitrogen, which is especially important with apple trees. Over-fertilizing with nitrogen will cause excessive vegetative growth at the expense of fruit production.

It is best to apply fertilizer in early spring after the ground has thawed. Spread a wide band of fertilizer around the tree’s drip line – the outline of the widest extent of the branches – and about 4 inches beyond. Keep fertilizer at least 6 inches from the trunk, as the fertilizer may burn the tree. Detailed advice on fertilizing apple trees.

A Garden of Plenty

Growing apples in the home garden is a wonderful way to bring fresh, delicious fruit to your family with the satisfaction of having grown it yourself! It’s also a great opportunity to teach children about where fruit comes from, about fruit trees and tree care, and about the fulfillment that comes from growing your own food. With a bit of patience and a little effort your trees should offer abundant harvests year after year.

Urban Gardeners at Minneapolis' Sabathani Community Center

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.

Photo 1: Group gardening instruction. Terry Straub

To help participants gain the skills they need to be successful gardeners, the program is based on a six-class curriculum (which is currently in the process of being replicated for use by other counties statewide), including lessons on soil, planning a garden, design, selecting plants and maintenance. Those who complete all six classes are declared Certified Urban Gardeners and awarded a certificate, which is presented during a graduation ceremony held in mid May each year (this year it will be May 16th). Last year’s graduation drew a large crowd of proud family members and neighbors, Dean says.


Photo 2: Classes cover many important topics including soil management and composting. Terry Straub

About 25 master gardeners are currently involved with the program (more are definitely needed) and they work with people on everything from planning how big a garden will need to be to feed their family to actually sketching out a design plan on graph paper. Because gardeners come from a variety of ethnic backgrounds, the master gardeners who volunteer with the program spend time working with people to ensure they are able to grow things that suit their needs. Master gardeners also maintain their own demonstration plot, which helps them in explaining a lot of the ins and outs of vegetable growing. The plot is staffed every Saturday in June, so interested gardeners can stop by and ask questions.


Photo 3: Gardening is a great family activity. Terry Straub

Within Sabathani’s established community garden, which includes fifteen 40 ft. x 40 ft. plots, the Urban Gardener program has two 40 ft. x 40 ft. squares, which are divided into 10 ft. x 10 ft. or 10 ft. x 20 ft. plots. “We try to get people to choose the smaller size their first year because we want them to succeed,” Dean explains, adding that it’s often hard for participants to carve out the amount of time it takes to maintain a plot all summer. Twelve people worked plots in the community garden last year while other gardeners in the program chose to use their skills in their own backyards, or in other community gardens. One older woman, who took the Urban Gardener classes and enjoys taking care of container plants, brought her five adult children along last year because she wanted them to learn how to garden, too. This year, Dean expects to have about 25 people tending plots in the garden. Terry Straub, program coordinator for the Hennepin County master gardener program, hopes Sabathani gardeners may also consider becoming master gardeners in the future.


Photo 4: There is a strong sense of community among members of the Sabathani Community Center Urban Gardener Program. Terry Straub

While some people do plant a few of their own flowers, the program supplies everyone with vegetables at no cost. The bulk of plants come from donated seeds. But master gardener Joan Onffroy has also helped out by starting over 200 seedlings for participants for the past two years. To encourage a sense of community, everyone is asked to help each other throughout the season. During the program’s fall harvest festival, all of the plots are cleaned up and everyone sits down to a big potluck meal, which includes plenty of dishes made with produce participants have grown. “People are so proud of what they accomplish,” says Dean. “And right now, with everyone being so interested in locally grown food, this really seems like the right program at the right time.”

Easy Does It: Help Your Bedding Plants and Houseplants Transition to the Great Minnesota Outdoors


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.

Photo 1: These sweet peas were scorched by the sun and are severely injured. Notice the silvery haze over the leaf in the foreground and the completely sun bleached younger leaves. David Zlesak

When tissue adapted to one environment is suddenly transitioned to a vastly different environment, it can suffer injury.  For instance, when we take houseplants indoors in fall where they experience lower light, leaves produced under the higher outdoor light levels often cannot suddenly adapt to lower light and have a tendency to fall off.  In the spring we are generally forcing plants to endure the opposite transition by taking them from lower light into higher light levels.  If the transition to higher light is too fast, the tissue can suddenly become scorched.  Scorched tissue has a silvery white cast and is often irreparably damaged. It can remain silvery-white or soon may transition to brown and die.  The chlorophyll, which causes plants to be green, can be damaged so severely by sudden high light (along with the cells housing this life sustaining pigment) that they no longer can undergo photosynthesis.  Such plants will hopefully produce new foliage adapted to the new light conditions and recover.


Photo 2: These plants are taken outdoors on the deck for an increased time each day and are brought in at night. David Zlesak

In addition to severe and sudden changes in light levels, sudden changes in air movement, water availability, and temperature also can significantly shock plants. Plants in small containers can quickly dry out outside and wilt on a warm spring day and a dry wind can speed water loss. Wind can also whip young plants and result in bruised and broken tissue. Temperature extremes can also stress and stunt plants, especially low temperatures for warm season species.  Many tropical species especially can experience chilling injury at temperatures below 50F.  The lipids or fats in their cell walls are typically more saturated than those of temperate species. Lipids with greater saturation transition from a liquid to solid phase at warmer temperatures (like margarine, a relatively saturated fat, is a solid at room temperature, while corn oil, a less saturated fat, is a liquid). Solidification of lipids within plant membranes compromises their ability to function and can result in severe and in some cases irreparable damage. Because of this, it is not recommended to place warm season crops like tomatoes and squash out too early as cold temperatures can dramatically stunt their growth.  Often later plantings of such crops can outgrow earlier planted, cold stunted plants of the same species, thus negating the benefit of trying to get a jump start on the growing season.

Fortunately, a gradual transition from indoor to outdoor conditions can allow changes to occur in existing plant tissue to allow it to better adapt to the new environmental conditions and then start producing new tissue more fully adapted to the new environment.  Often a gentle hardening off/transition period of one to two weeks is sufficient to allow existing tissue to adapt to the extent possible. Here are some tips to harden off your bedding plants and houseplants you want to transition outdoors:

  • Slowly transition plants to higher light conditions by incrementally increasing exposure to higher light intensity and duration at the higher light levels.  Consider placing plants outdoors in bright indirect light first and then after a couple days begin to allow them to experience full sun.  Slowly increase the duration in full sun over the hardening off period.  One technique that can work well is to first place plants on the North side of a building where full sun is not experienced and then over time pull the plants further away from the building where there will be continually increased exposure to sun considering the time of day and shadows of the building.  This is a useful technique especially for people who are away from home during the day. 

  • Check plants often for watering knowing that especially bedding plants in small pots can dry out quickly. 

  • Provide plants with protection from strong winds by placing them in protected locations.  Tall or otherwise weak plants can benefit from staking to prevent stem breakage.

  • For especially warm season crops, slowly transition them to cooler temperatures.  Studies have pointed to the benefit of placing such plants in cooler temperature (~50F) conditions for only a couple to few hours a day during the transition period.  This provides the signal to plants to adapt their lipids while limiting tissue injury. 


Photo 3: The commercial greenhouse environment is often high in light and has good air movement. David Zlesak

Plants we purchase from the garden center are often grown in greenhouses with relatively high light and good air movement and the transition from that environment to our garden is less severe than from our home.  Even so, sometimes a hardening off period can be beneficial, especially if we can tell that the plant was struggling for some time in the retail environment from perhaps being stored temporarily under a bench or in another less than ideal environment.  Helping plants gently transition to their summer home will pay off in quicker adaptation and resumed growth.

Distinguishing Disease from Winter Injury on Spruce Trees


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.

Photo 1: Notice how the lower branches of this Colorado blue spruce are most severely discolored by Rhizosphaera needle cast. Michelle Grabowski

Trees suffering from Rhizosphaera needle cast can be recognized by browning of the older needles. The older needles are located at the base of the branch closest to the trunk, while the new needles grow from the tip of the branch. Diseased spruce trees often have branches with green needles at the tip of the branch and brown needles towards the base. In addition, the branches closest to the ground tend to be more severely infected, because humidity is highest there. Later in the summer the discolored needles will fall off. If a spruce tree has been suffering from Rhizosphaera needle cast for several years, it may appear sparse and have dead branches at its base.


Photo 2: This branch infected with Rhizosphaera needle cast shows discoloration of the older needles, while younger needles remain green. Michelle Grabowski

The fungal pathogen of Rhizosphaera needle cast can be seen on infected spruce needles. Use a hand lens to closely examine discolored needles. Tiny black pimple like spore producing structures can be seen arising from the stomates, or air holes in the needle.

Spruce trees suffering from winter injury often have needle discoloration on the needles at the tips of the branches. Frequently this damage occurs on the south or west side of the tree due to excess wind and sun on those sides. In some cases winter injury is observed on trees receiving reflective light from a nearby building or car. The discolored needles often appear bleached or faded, with the tip of the needle most severely discolored.  Winter injury can occur under several conditions. Needles can be killed by cold temperatures, desiccated by the wind, or bleached by the sun. If a spruce tree did not have time to harden off properly in the fall or is not fully adapted to Minnesota’s winters, complete browning of all needles may be observed.


Photo 3: Spore producing structures of Rhizosphaera needle cast on infected needles M.Ostry USDAFS

If the problem is clearly winter injury, not much can be done for the tree at this point. Luckily winter injury rarely kills the buds of the tree and as weather warms, new growth resumes improving the overall color of the tree. Make a note to water trees throughout the summer to prevent drought stress and help the tree harden off next fall. In very exposed areas, spruces can be protected from future winter injury with a simple burlap barrier to block the wind and sun. Read ‘Protecting trees and shrubs from winter damage’ ( ) to learn more.


Photo 4: Notice how the south side of this tree is most severely affected by winter injury. Michelle Grabowski

If the problem is clearly Rhizosphaera needle cast, management strategies should be implemented to protect this year’s needles from infection. When new needles are half the length of mature needles, spray the tree with a fungicide whose active ingredient is Chlorthalonil. Completely read the label and follow all instructions when using a fungicide. Apply the fungicide once more at the interval recommended on the fungicide label (typically 3-4 weeks later). These two sprays will protect the needles from infection.


Photo 5: Concolor fir is a good alternative to Colorado blue spruce. David Zlesak

In addition several cultural practices will help to reduce the risk of future problems with Rhizosphaera needle cast. When planting new spruce trees choose Norway spruce or white spruce instead of Colorado blue spruce because they are more resistant to the disease. Concolor fir (Abies concolor) is another alternative to Colorado blue spruce and has relatively large, blue-green needles. Reduce moisture on spruce needles by controlling weeds around the base of the tree and redirecting lawn sprinklers to avoid wetting the needles. Reduce stress on spruce trees by mulching the soil around the tree and providing trees with water during periods of drought. Avoid planting new spruce trees near old infected spruce trees.  


Photo 6: The needles at the tips of this branch are most severely affected by winter injury. Michelle Grabowski

If unsure what is causing needle discoloration in the spruce tree, send a sample to the UMN Plant Disease Clinic ( before implementing any management strategies.

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