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Extension > Yard and Garden News > August 2010

Tuesday, August 31, 2010

Contents: September 1, 2010 and Monarch Caterpillars

This just in! Monarch Caterpillars spotted by Karl Foord at the Minnesota Landscape Arboretum in Chaska.

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Yard and Garden News Editor: Karl Foord

Technical Editor: Bridget Barton

Knowing when it's time to say goodbye.

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


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

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

Four Questions Before Letting Go

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

· Is the tree healthy?

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

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


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

· What can your budget reasonably handle?

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

Is the tree healthy?

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

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


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

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

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

Will the tree remain stable?

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


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

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

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

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


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

What's its relative value?

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

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

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


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

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

What's your budget?

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

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

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

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

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

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


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

The final decision.

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

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

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

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

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

September Home Lawn Care To-do List

Bob Mugaas, UMN Extension Educator

Thumbnail image for bob2.JPG1. One of the very best times of the year to be fertilizing your lawn is from about Labor Day through the middle of September. Applications that put down about one pound of nitrogen per 1000 square feet will help provide the necessary nutrition through the late summer / fall period - a time of active grass shoot and root growth.

2. It is important to avoid serious water stress on lawns this time of year. As noted above, the late summer /early fall period is a time of active growth. Hence, not only is sufficient nutrition important but ample soil water is just as important to sustain and encourage growth. Early in September, the average rule of thumb of one inch of water per week including rainfall should be sufficient. As we get later into September and early October, that same one inch of water may be sufficient for two or even three weeks depending on weather conditions. That is, the warmer and drier the weather the more frequently plants will need watering. The cooler, and more moist the weather, the less frequently plants will need to be watered. Remember, soils should be just damp to the touch not soaking wet to provide a healthy place for roots to grow and beneficial soil microbes to flourish.

3. Early September is an excellent time to be doing some overseeding of damaged or thin areas of the lawn. Seed germination is usually much quicker due to the warmer soil temperatures. Hence, seedlings are able to get out of the ground and more quickly establish the area prior to the onset of colder conditions of late fall and early winter. If you're struggling with trying to get some grass growing under the shade of some maturing shade trees, try growing some of the fine fescues. They are well adapted to dry shade conditions and are tolerate much lower inputs of fertilizer and water while still remaining healthy. If seeding areas shaded by trees, be sure to keep lightly removing tree leaves as they fall. That will help ensure that seedlings receive sufficient sunlight throughout the fall resulting in better establishment. Keep newly seeded areas damp during the germination process and gradually back-off the water as they begin to get established - usually about three to four weeks.

4. One of the best times to be aerating the lawn is right around Labor Day. This minimizes the amount of germination from unwanted weed seeds making for less competition to the new grass seedlings. It also provides increased soil oxygen levels that encourage better root growth and a healthier soil microbial community. Lawn aerifiers that pull a core of soil and deposit that core on the lawn surface are the most effective units that are still relatively easy to use for homeowners. These soil cores can be left to decompose naturally over the next few weeks. There is usually not a need to remove these from a home lawn. If you are also planning some fertilizing, and/or want to do some overseeding, an excellent time to do that is right after you have aerified. Aerification can also be used to control the rate of thatch build-up as the decomposing soil cores help to reinoculate the underlying thatch with soil. In turn, that helps break down the thatch and keep the amount of thatch build-up to below damaging levels (i.e., less than ½ inch) Thumbnail image for bob3.JPG

When you think of fall as an active period of growth for your lawn grasses, the extra effort to ensure good growing conditions during that time helps ensure a healthy lawn going into the winter and a lawn quicker to recover and resume growth in the spring. Besides, late summer and early fall are some of the nicest conditions of the year to be tinkering with your lawn. Enjoy!

Dogwood Sawflies

Jeffrey Hahn, Asst. Extension Entomologist

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Dogwood sawfly defoliation
Chris Bauer
Dogwood sawflies been found defoliating dogwoods during late August. In all of the reported cases, defoliation was severe and apparently appeared suddenly. However, despite the apparent sudden appearance, these sawflies have a actually been present for weeks but just went unobserved when they first started feeding.

Adults are active any time from late May through July. After the larvae first hatch, they are covered in a whitish material which some entomologists believe helps the sawflies resemble bird droppings, protecting them from predation. The young larvae skeletonize leaves, i.e. feed between the veins. Older larvae consume the entire leaf except for the midrib. As mature larvae, dogwood sawflies are about one inch long. Also, the whitish material comes off, revealing their greenish - yellowish, conspicuously spotted bodies.

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Dogwood sawfly (note white material on body)
Chris Bauer
When they are done feeding, they wander off looking for places to pupate, preferring rotted wood. Dogwood sawflies have also been known to bore into homes to pupate. They remain as pupae through the winter and in the following spring There is one generation of dogwood sawflies each year.

If you find your shrubs are being defoliated now, there is not a lot that you can do as dogwood sawfly feeding is either done or is nearly finished. However, if you did have a problem with them this year, watch for them next year starting in June and treat if they are abundant. There are a variety insecticides than can be used if you catch them while they are small, including insecticidal soap, spinosad, horticultural oil, permethrin, bifenthrin, and other pyrethroids. Safari (dinotefuran) and imidacloprid, both systemics, are also effective but need some time to be taken up by the shrubs.

Woolly Oak Gall

Jeffrey Hahn, Asst. Extension Entomologist

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Woolly oak gall
Brittany Kock
People have been commonly finding about one inch long fuzzy or woolly looking growths on the leaves of oak trees, especially species in the white oak group. These galls are either reddish or whitish or sometimes both. Despite their appearance, these growths are not disease, but is abnormal plant tissue due to the feeding of the larvae of tiny non-stinging insects called cynipid (sin-IP-id) wasps. These galls are common and like other leaf galls, have very little, if any, impact on tree health, especially if they are vigorous, mature oaks. These galls generally do not become very abundant and their numbers vary from year to year on individual trees. By the time you see leaf galls, it is already too late treat them, just ignore them.

Mildew Aplenty on Minnesota's Pumpkins, Squash and Cucumbers

Michelle Grabowski, University of Minnesota Extension Educator

Two types of mildew can be seen on pumpkins, squash and cucumbers in Minnesota this August. Powdery mildew and downy mildew have both been reported. Despite the similarities in their names, these two diseases are caused by very different pathogens and have very different symptoms and control strategies.

Powdery Mildew
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Photo 1: Powdery mildew on squash M.Grabowski, UMN Extension.

Powdery mildew is caused by the fungus Podosphaera xanthii. As its name implies, this disease can be recognized by the powdery white fungal growth on leaves and stems. Infections often start as a few white powdery spots but can quickly grow to cover the entire leaf in a layer of powdery white fungal spores and mycelia.

Although the powdery mildew fungi do not commonly attack squash fruit, yield of infected plants can be reduced because infected plants have less energy to invest in fruit. Often severely infected plants produce fewer and smaller squash, cucumbers or pumpkins.

Spores of the powdery mildew fungus blow in on the wind and are impossible to keep out of the garden. The best way to prevent powdery mildew is by planting disease resistant varieties. Many powdery mildew resistant varieties of pumpkin, squash and cucumber are available. Home gardeners can also use a fungicide with sulfur as the active ingredient to protect susceptible plants. In order for fungicides to be effective, they must be applied when the first small spot of powdery mildew is observed and the fungicide must be sprayed to cover both the upper and lower leaf surfaces. Perhaps the simplest solution for the home gardener is to include a few extra plants in the garden, to make up for the yield lost to powdery mildew.

Downy Mildew
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Photo 2: Downy Mildew on Cucumber D.Moen, UMN Extension.

Downy mildew is caused by Pseudoperonospora cubensis, a fungus like organism, often called a water mold because it thrives in moist conditions. This disease can be recognized by the almost square yellow to brown spots that appear on leaves. Leaves infected with Downy Mildew often look like a patchwork quilt of yellows, greens and brown. On the underside of infected leaves, a purplish gray fuzzy mold can be seen. Under warm wet conditions, downy mildew can spread rapidly. Many leaf spots grow together, turning infected leaves brown so quickly; they almost appear to have been hit by frost.

The downy mildew pathogen cannot survive Minnesota's harsh winters. Each year new spores must move into the state on moist wind from areas to the south. This means that gardeners experiencing problems with downy mildew this year may not see it at all next year.

Although there are several varieties of cucumber that are resistant to downy mildew, gardeners may not be able to find squash or pumpkin varieties with good resistance. Once downy mildew has started an infection, it is very difficult to control. In a home garden, the best solution is to remove infected plants as soon as symptoms appear to reduce the spread of the disease to other cucurbits in the garden.

Winter Damage in Apples

Karl Foord, UMN Extension Educator

Preparation for winter

Plants acclimate or develop hardiness to freezing temperatures in response to changes in light duration and temperature. Acclimation is a two-stage process. The first stage is initiated by decreasing day length and results in partial hardiness. The second stage is initiated by subfreezing temperatures and results in full hardiness and acclimation. The hardening response in a plant may vary from year to year because of variation in temperatures over that year. The degree of cold hardiness of a plant is determined by the genetic capacity of a plant to acclimate to freezing temperatures and transform its tissues from a non-hardy to a winter hardy state. Plants are often given a cold hardiness rating based on the lowest midwinter temperature that plant tissues can endure relative to the USDA winter hardiness zone temperature bands. For example, Honeycrisp has a winter hardiness rating of USDA hardiness zone 3 (-40 to -30 F).

However, extreme low temperature is only one of a number of factors affecting winter survival. Winter damage frequently occurs during late fall or early spring due to extreme changes in temperature when the plant is not at its maximum hardiness. Injury is a function of the acclimation status of the plant at the time of the radical temperature change, and is often difficult to determine, and may not show for several years.

No two winters are the same

Every winter may have an episode that could cause some damage to some trees. For example the winter of 2008-9 was considered to be a more stressful year than the winter of 2009-10. This is borne out by the temperature graphs of the two years. Note that the winter of 2008-9 had a great deal of significant temperature fluctuation while 2009-10 was much smoother (Figures 1 & 2). Dr. Jim Luby noted three potentially damaging events that stand out in the winter of 2008-9 as follows: 1. The 43 degree temperature drop on December 15, 2008, 2. The 37 degree temperature drop on March 11, 2009, and 3. The five nights of low temperatures below - 20 from January 13 - 17, 2009. Many areas experienced a freeze event on Mother's Day (May 9, 2010) with varying amounts of damage. Some Minnesota apple orchards lost almost all their apple flowers and reports from Iowa noted lots of damage with bark splitting off the trees.



Carryover environmental effects

A dry season or a particularly heavy crop can reduce the vigor of the tree and make it more susceptible to winter injury. As Kathy Zuzek would tell you a rose variety that is defoliated by black spot in July is more susceptible to winter injury then one that has not been defoliated.

Other influencers of hardiness

Lack of snow cover during the coldest period can lead to root damage.

Pruning apples before they have accumulated their full hardiness can set trees up for winterkill, especially so with cultivars that are less winter hardy. In general, pruning cuts are dehardening in the early winter and the larger the cut the more dehardening occurs. Prune fruit trees in late spring before the buds become active.

Orchard topography is important because it affects cold air drainage. Lower areas where cold air accumulates can cause frost to settle and damage trees.


High intensity sunlight on a sunny winter day heats up the south and southwest side of thin-barked young apple tree trunks causing the cells to come out of dormancy and become active. After sunset temperatures can drop precipitously to levels well below freezing which kills active cells and conductive tissue. This often appears as a longitudinal crack running up and down the trunk.

Commercial tree wraps made of crepe paper, plastic spiral wraps or longitudinally cut drain pipe will intercept the sun and insulate the bark preventing sunscald. Wrap the trees from base to the lowest branch in the fall after leaf drop. I have used all methods. I have left the plastic wraps on for at least three years until the bark thickens and is less prone to sunscald. I removed and rewrapped the crepe paper each year.

The Taste of Minnesota Apples - An Adventure

Karl Foord, UMN Extension Educator

I did an apple tasting exercise with 24 young people last summer and we tasted the following apples traditionally available at grocery stores, Granny Smith, Golden Delicious, Jonagold, Tentation, Fuji, American Cameo, Royal Gala, and Red Delicious. More then half of the young people chose Fuji as their number one choice. Given this array of apples my taste buds agreed with theirs.

Then I started tasting local Minnesota Apples. Quite frankly I was astonished. These were excellent apples and in my opinion outclassed all of the commercially available apples previously tested. Granted the store apples had either been grown in Washington State and stored for 10 months, or had been shipped from either Chile or New Zealand. The Minnesota apples were fresh. Certainly tastes differ significantly among people and a recommendation is simply my opinion. You must discover for yourself as it is your taste and opinion that will render the final decision.

We are still early in the apple season. Some of the earliest of varieties have come and gone like State Fair. This is mostly due to the warm spring we had. However, many of the best are yet to come. My favorite early apples are Chestnut Crab and Zestar. I thought Paula Red was a good apple when captured within its optimum taste window. It doesn't keep that well and that is why my memory of Paula Red is one of a mushy apple. Check out our Early Apple Tasting video with Mike Dekarski (linked to this Y&G issue) to see some early season apples that we tested last week.

I look forward to SweeTango but we will have to see how many apples make it to the market. I also look forward to Honeycrisp, Sweet Sixteen, and SnowSweet.

Don't miss out on the Minnesota fresh apple season. Most orchards will let you taste before you buy so you can get the apple that tastes best to you.

I highly recommend that you give yourself a treat and experience the taste adventures available in local Minnesota Apples.

You can find an orchard near you at the Minnesota Grown .

Saturday, August 14, 2010

Fungus Among Us

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Photo 1: Stinkhorn fungi Photo by M.Grabowski UMN Extension.

Frequent summer rains, wet soils and humid conditions have created a favorable environment for a wide variety of fungi this summer. Gardeners are noticing mushrooms, shelf fungi, and other odd and interesting fungal spore producing structures of all shapes, sizes and colors sprouting in the landscape. The question then arises, which of these fungi should a gardener be concerned about?

Although it is true that more plant diseases are caused by fungi than any other type of pathogen, only 11% of all fungi are capable of causing disease in plants at all. The grand majority of fungi are saprophytes. That is they survive by breaking down organic matter and absorbing nutrients from it. In many cases, the mushrooms sprouting in the woodchip mulch or pushing up through the lawn are not harming the nearby plants, but are working on breaking down woodchips, plant debris or other organic matter.

Two saprophytic fungi commonly found in Minnesota landscapes are stinkhorns and birds nest fungi.

Several kinds of stinkhorns can be found in Minnesota. These fungi start as round to oval egg like structures. When mature a spongy looking stalk with a slime covered cap emerges. Often the remains of the 'egg' can be seen at the base of the mature stinkhorn. Stinkhorns get their name from the smelly sticky slime that caps the mushroom. This slime is full of fungal spores. The smell attracts flies, which will carry the spores to new locations. Stinkhorns can be found in mulched beds, under trees and shrubs and occasionally in lawns.

Bird's nest fungi
There are two common genera of bird's nest fungi found in Minnesota; Cyathus sp. and Crucibulum sp. These two fungi form tiny cup shaped structures with small round disks inside known as peridioles. The cups are designed to catch rain drops and send the peridioles (full of fungal spores) flying to a new location. These hard dark disks can often be found nearby clinging to plants, siding, or whatever else is in the way. Bird's nest fungi commonly grow on woodchip mulch in landscapes.

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Photo 2: Cyanthus striatus - Bird's nest fungi Photo by M.Grabowski UMN Extension.

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Photo 3: Crucibulum laeve - Bird's nest fungi Photo by M.Grabowski UMN Extension.

Mushrooms and Shelf Fungi of Plant Pathogens

Mushrooms or shelf fungi that are growing directly on the trunk of the tree, out of the root flare or right at the base of the tree indicate that the tree is suffering from heart rot, root rot or butt rot. Trees suffering from internal wood rot may or may not have symptoms in the canopy. For example a honey locust with Ganoderma root rot may have a shiny brown shelf fungi growing at the base of the tree and several dead or wilting branches within the canopy. These branches have died because the trees rotted roots were no longer able to provide the nutrients and water they needed. In contrast a cottonwood tree with heart rot may have a full healthy green canopy despite internal rot in the trunk of the tree. Regardless of how healthy the canopy appears, all three types of rot can greatly weaken the tree due to internal decay.

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Photo 4: Car trapped below a fallen branch from a cottonwood suffering from heart rot Photo by M.Grabowski, UMN Extension.

This year's strong winds and thunderstorms have resulted in many broken branches from trees weakened by decay. In severe cases, the entire trunk may break or the tree may fall over. If you suspect a tree on your property has been weakened by a decay causing fungi, consult a certified arborist ( as soon as possible. These professionals can help determine the structural stability of the tree and recommend appropriate action.

Be On the Watch For Yellowjackets

Jeffrey Hahn, Extension Entomologist

The early spring has contributed to an above average number of yellowjackets this summer. Yellowjackets can be found nesting in a variety of different places, depending on the particular species. They range from aerial nests, e.g. hanging from tree limbs or the eaves of homes to subterranean nests, e.g. nesting in old mouse burrows to nesting in various voids in buildings. These nests, especially the cryptic ones, are present all season, but may not be noticed until late summer when the workers are more numerous.

Aerial yellowjacket nest
Jeff Hahn
If a yellowjacket nest is located in a site away from human traffic and is not a risk for stings, then just ignore it. Yellowjackets are beneficial because of the insects on which they prey. All of the inhabitants of the nest will eventually die in the fall when freezing temperatures arrive. If however, the nest is located somewhere where people could get stung, then the nest should be eliminated. There are some situations where you can eliminate the nest yourself. A professional pest control technician is always an option any time you do not want to deal with a wasp problem.

Nests that are out in the open are fairly easy to deal with. Wait until dark when the wasps are much less active. Use an aerosol can of insecticide, something that is labeled for wasps and hornets (or something similar) and spray into the opening. Check the next day to see how effective the treatment was. If you still see wasps flying, repeat the spray the following evening.

Ground-nesting yellowjackets
Jeff Hahn
Yellowjackets nesting in the ground are more challenging. You do not see the nest itself but you see an opening in the ground where they yellowjackets fly in and out. It is not unusual to walk by a nest in the ground all summer without knowing it is there. Then one day, the yellowjackets will react to a disturbance, e.g. mowing the lawn, and will come out to vigorously defend their nest. The most effective way to control a subterranean nest is with a dust labeled for ground dwelling insects (e.g. Bonide Eight Garden Dust), although these dust formulations are generally difficult to find. Apply it at the entrance of the nest at night when yellowjackets are less active. Check after a day to see how effective the treatment was and repeat if necessary. Another option is to use a liquid insecticide, pouring into the nest entrance, but this is less effective.

Hidden yellowjacket nest in home
Jeff Hahn
The most challenging yellowjacket nests are those found inside homes in wall voids, attics, concrete blocks, or similar spaces. You can not see the nest, similar to a subterranean nest, but you can see the workers flying in and out of an opening or crack. These nests are very difficult for a homeowner to control on their own. A dust labeled for use in homes is the most effective or a foam formulation, but unfortunately these products are generally not available to the general public. A liquid aerosol, while readily available, is generally not effective. Sometimes an aerosol spray can cause the yellowjackets to look for another way out, which often leads them to the inside of the home. Also, don't seal the nest opening until you know all of the yellowjackets are dead as you can cause the same reaction. The best method to control hidden nests in buildings is to have a professional pest control company treat the nest.

Close up of photo 3
Jeff Hahn
Remember that yellowjackets are annual nests, i.e. the old queen and the workers only live until the weather gets below freezing, then they die. If you are dealing with a yellowjacket nest late in the season, it might be easiest to wait until the cold temperatures kill them. Nest are also not reused the following spring.

Plant Hardiness Zones Revisited

Karl Foord, UMN Extension Educator

If a particular temperature recording station has an average minimum low temperature of between - 30F to - 25F then the station would be in 4a. If the range was - 25 F to - 20 F then this station would be part of the 4b hardiness zone. The same logic determining the temperature ranges for Zone 2b [-45 to -40 F], Zone 3a [-40 to -35 F], and Zone 3b [-35 to - 30 F].


The average part of this calculation gave me concern because if it is an average then there must be numbers less than and greater than the average. The question then becomes how wide is the distribution around the average?

Minneapolis is in the 4a plant hardiness zone with a low minimum temperature range of -30 F to - 25 F. Over the last one hundred and eleven years low minimum temperatures have exceeded this range 10% of the time, but only by four degrees at the most1. Temperature data for the last 10 years (2000-2009) places Minneapolis in hardiness zone 4b. If we are willing to accept a 10% chance of a low minimum beyond the -20 F to -15 F range, then Minneapolis would be placed in zone 5a. The question is how many of your 5a plants would have died given the low temperature of -24 on January 30, 2004. The relatively milder recent winters gave rise to a revised plant hardiness map that can be viewed here. This map was subsequently rejected by the USDA.

I think it is critical to point out that the low minimum temperature is only one of a number of factors that come to play on a plant's winter hardiness.

Other factors include snow cover, temperature patterns favoring the development of dormancy, moisture conditions, and microclimate effects to name just a few.

Snow cover
Snow functions as an insulator protecting the root system of overwintering plants. Nine inches of snow can lead to a 42 F differential between a - 14 F air temperature and a 28 F soil surface temperature.


Reliable snow cover increases the temperature that the plant experiences and puts the region in a higher plant hardiness zone. This is why locations like the Upper Peninsula of Michigan are functionally zone 5 due to consistent deep snow falls, whereas without this snow the UP would likely be zone 3a.

Temperature Patterns Favoring the Development of Dormancy

The plant's ability to withstand cold temperatures is a function of the metabolic status of the plant. If the plant experiences gradually decreasing temperatures and is allowed to achieve full dormancy then it has achieved its optimum genetically programmed degree of winter hardiness. However in winters that are warm in the beginning of December followed by a significant temperature drop to average temperatures at that time of year, the plants are not metabolically prepared and will be damaged.

Moisture Conditions

Cold dry winds tend to desiccate plants especially evergreen plants with exposed leaves. Plants entering winter under draughty conditions are further stressed leading to weak plants and mortality.


Local conditions can modify the climate experienced by the plant. Protected locations reduce the stress caused by cold dry desiccating winds. Highly exposed locations can increase plant stress. Slope affects airflow as cold air sinks into lowland areas called frost pockets. South facing hillsides capture more heat which can be advantageous for plants like grapes, but disadvantageous if the heat stimulates early flowering subjecting the plant to the risk of late spring frosts. There is an art to finding the right plant for the right location. I am a big fan of Japanese maples. Their microclimate over the winter is my garage. I for one am not willing to accept the risks of the one in ten chance of a true 4a or even 3b winter that we might experience in the Minneapolis area zone 4a. Especially consider microclimates if you live in 4a or 4b and have a lake home or cabin in 3b or 3a.


Data from the Minnesota Climatology Working Group for Minneapolis

Garden Calendar: August 15, 2010

  • The apple harvest is starting, with Paula Reds and State Fairs already ripe. The Zestar in Southern Minnesota should be ready for harvesting within the next week or two.

  • Sooty blotch and fly speck fungi have shown up. (See pictures)

  • Disease and insect pressure has been high this summer.





Photo credits: Karl Foord

Yard and Garden News Editor: Karl Foord

Technical Editor: Bridget Barton

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