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Practice fall tree health

Practice fall tree health

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Webworm awaits

Fall webworm is abundant, particularly along the river.

Like all living things, trees are influenced by many outside forces such as weather and climate, diseases, insects, pesticides, trauma, nutrient levels and of course old age.

Some forces affect only a certain species; think of Emerald Ash borer. Some affect many or even all species in an area; think damaging late spring frost or wildly abnormal precipitation.

Many do not require action on our part because their impact is minimal. Some do require action. Addressing something affecting your yard tree may be more important to you than the same problem in your forest.

Here are a few of the common issues in this region right now.

Fall webworm

Fall webworm is abundant right now, particularly along the river and especially in black walnut trees, but other species are attacked, too.

The larval stage are caterpillars that hide in a “web tent” while they eat leaves. While the webs are unsightly, they do not merit action on a landscape level. If they are on your favorite yard tree though, you may want to act.

If the webs are low enough, pull them out of the tree and put in a bucket of soapy water to kill the caterpillars (or squash under foot if you want instant gratification).

A horticultural oil spray or a spray with Bacillus thuringiensis var. kurstaki in late June may reduce the problem in the fall of that year, but sprays are ineffective if caterpillars are in nests. Insecticides are seldom used because many are toxic to other (beneficial) insects, and trees can tolerate defoliation.

Early fall coloration

Trees turning fall colors in late July or early August is a sign the tree is under some kind of stress. Stress makes them stop making chlorophyll allowing underlying fall colors to become visible. Maples tend to have this issue often. Coloring may be caused by an injury to the roots or trunk, water stress (too much or too little), nutrient deficiency or an insect or disease problem.

Frequently the best action is supportive care. In very dry conditions, supplemental deep watering may be needed. A light trickle of water overnight under the drip line is recommended. This should be done only once per week or 10 days.

Give your yard tree a good place to live. Proper bark mulch can help protect against mower or weed whacker damage. A soil test can help determine if the pH is off or if a certain nutrient is lacking. Do not simply apply some fertilizer you have on hand. That may worsen the problem.


Chlorosis is a general term that refers to the symptoms of uniform yellowing of leaves. It usually is from nutrient deficiency from either suboptimal soil pH, too little water, or too much water. Severe stem and root diseases can also cause chlorosis (e.g. white pine blister rust and Armillaria root disease).

Iron chlorosis is a common form of chlorosis. It is the result of a lack of iron in the new growth of the plant. Iron is not necessarily deficient in the soil — it may be there, but just in an unavailable form for absorption through the root systems. A University of Illinois article explains this issue in detail.

For this issue, you need to monitor the problem. The current season’s weather may be the culprit. As mentioned above, giving your yard tree a good growing environment is often the best preventative.

Dying tops

Cumulative stresses to older trees, such as root damage, late spring frosts, past droughts, and past floods (even from many years ago) cause trees to develop large dead limbs, and they ultimately may die altogether.

There may not be one specific cause of this dieback. Southeast Minnesota has had unprecedented amounts of precipitation in the past decade.

Even when the yearly total is “average,” the timing has been unusual, and this has been very stressful to our trees, especially those growing near drainages, streams, rivers and wetlands. There may be old construction damage or disease or insect attack that alone was not a huge impact, but intense precipitation tipped the scales toward dieback and decline. Stresses compound and mount over time.

Taking good care of your trees throughout their lives gives them the best chance to be around for as long as possible. Good care includes proper pruning from an early age, avoiding root disturbance and mulching for protection.

Valiree Green is a forester with the Minnesota Department of Natural Resources based in Caledonia.


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