Prognosis for Drought Damaged Trees and Shrubs

As the 2003 drought intensifies over much of the state, symptoms of water stress on woody plants are becoming increasingly obvious. The recent period of triple-digit temperatures greatly accelerated the development of some of these manifestations of extreme moisture deficit. Trees such as birches and cottonwoods dropped many leaves over the past several weeks, and older leaves on other species such as sassafras and blackgum are now coloring early and beginning to drop. Some shallow-rooted trees and shrubs such as Japanese maple, hydrangeas and azaleas that have not received regular irrigation are showing extreme leaf scorch. Perhaps the most worrisome stress symptom to many people is leaf shedding on large trees in landscapes and forests. With all of these disturbing reminders of drought stress appearing around us, it is natural to wonder about the chances for recovery for some of the worst affected of our woody friends.

While it is difficult to predict the outcome of anything related to the weather, there are a few generalizations that can be made regarding the effects of water stress on woody plants that might provide some comfort to people concerned about the future a of drought-injured, favorite tree or shrub. First, large trees of most species are highly drought resistant and are rarely killed by a single season of drought. This is because a large tree has a massive root system that has access to a huge volume of soil. Also, a tree can store some water in its stems and roots that is protected by bark. If drought conditions persist for several years, large trees may be weakened to the point that they lose resistance to attack by insects and diseases. Then, they may decline over a period of years before they finally die. The decline of red oaks in the Ozark forests of Missouri and Arkansas is a good illustration of the effects of multi-season drought on large trees. Hopefully, the drought currently affecting most of Missouri will end soon and most of the large trees that are currently showing symptoms will recover quickly.

A second generalization is that scorching, browning and early shedding of leaves are not the end of the world. While these drought responses reduce photosynthesis and detract from the visual appeal of a plant, brown or missing leaves do not lose water to the atmosphere. Even plants that are completely defoliated by drought often recover and eventually resume normal growth once the stress is relieved. For this reason, it is best to wait until the following season before removing a drought-damaged tree or shrub. If the stems are still pliable, there is always a chance that the plant will produce a near-normal set of leaves the following season. However, it is important to water drought-injured plants carefully until new leaves are produced. Often, people only begin watering after scorch is observed and then they overwater. Keep in mind that a leafless plant uses much less water than one with leaves. The best approach is to check the soil in the root zone of the affected plant regularly with a shovel or trowel and keep it moist (not soggy) to a depth of 10 inches. This is, of course, much more easily achieved in a reasonably well-drained soil than in heavy clay. Some plants, such as azaleas and rhododendrons that are particularly drought susceptible may have their root systems injured by a severe water deficit to the point that they succumb to root rot organisms like Phytophthora. Plants in this condition rarely recover regardless of the amount of irrigation given.

If in doubt about the viability of a drought damaged landscape plant, check the stems to see if they are still flexible. Even a plant that has died back may have some viable stem tissue closer to the ground from which it can regenerate. If there still appears to be some life left, water moderately as described previously and hope for the best. Avoid excessive fertilization, since this can further stress the plant. Fertilization in spring with half of the recommended rate of a soluble fertilizer may help to promote root growth. In many cases, a plant that appears to be at death’s door in August, may look surprisingly good the following May.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630