Trees and shrubs that died over winter are quite obvious now. Most other vegetation is covered with green foliage, while winter-killed material is stark or has small tufts of leaves scattered throughout the plant. Advice in the spring would be to give the plant time to recover. By now, plants should have developed some new leaves if the plant had any chance of recovery. The Plant Clinic has received many complaints of trees and shrubs that did not survive. What factor or factors caused the death of so many trees and shrubs this winter?
The usual complaint we hear is that the tree or shrub looked fine in the fall and was dead this spring. If that were the case, the logical assumption is that winter injury is the culprit. Freeze injury during dormancy is one possibility. Another possibility is the injury that occurs following a sudden drop in temperatures in the fall. Cold acclimation usually occurs as temperatures fall. If temperatures fall rapidly (overnight), however, plant tissue are often injured. Cold damage can also occur in the spring when there is a cold snap just as tissues begin to expand. In that case, plants have begun deacclimation and are more susceptible to cold temperatures. Authors Sinclair, Lyon, and Johnson, in Diseases of Trees and Shrubs, discuss these possibilities. Although individual cases of cold injury occurred, the general temperature conditions of the past winter in Illinois do not seem to account for the number of winter fatalities of trees and shrubs. Besides, we would expect all similar species in an area to be affected if cold injury were the only factor. That was not the case.
It is possible that the plants that supposedly looked fine last fall were already under stress. Drought stress is a likely key player in Illinois. Acute drought stress is obvious to most gardeners because plants wilt during the day and rehydrate at night. Symptoms are sudden and easy to see. A slow, continual lack of water as experienced last summer and fall, on the other hand, causes a reduced growth rate, reduced leaf size, off-color foliage, and stem dieback. These symptoms might not be obvious because they happen slowly over the season. We saw these symptoms on many plants last year, especially white pine, birch, and lindens. In many cases, frequent, shallow irrigation kept trees alive but also caused shallow root development. Trees with a shallow root system are not able to withstand more drought stress as readily as trees with deeper roots. The past winter was dry for most of Illinois; and winds are thought to have been a factor in the final desiccation of already drought-stressed trees.
Illinois has fared well this spring with frequent rains. When drought hits again, help your trees by watering deeply on a weekly basis as long as drought occurs. Watering turf around the trees is usually not adequate for tree needs. A long, slow soak in the drip line is helpful, as are hose end root injection tools. Check tree books to determine the relative water needs of your tree. Birch, for instance, requires more water than most other trees, so it shows deficit more quickly. Most tree feeder roots are in the top 18 inches of soil. When watering the roots via the soil surface, apply at least an inch of water in each watering. If you use a soil injector, put the injector into the soil to about 18 inches. Injectors give a slower release of the water, but it is deeper into the soil with less loss to evaporation and surrounding turf. A disadvantage to injectors is that you canít be certain how much water you are applying.