Extended drought may threaten tree health

Depending upon its severity, extended drought can have long-term effects on tree and shrub health. Injury from secondary pests such as insect borers or root rots may attack – and kill – trees weakened by drought and other stressors.

As most people are aware, the summer of 2012 has been extremely dry – especially in parts of southern Michigan. Unfortunately, extended droughts such as the one Michigan has been experiencing can pose serious threats to the long-term health of many trees and shrubs.

Trees and shrubs can usually tolerate short periods of drought without much trouble. Typically there is some period of soil moisture stress during the course of many Michigan summers and most trees and shrubs survive it. However, when a droughty period extends over several months – not just a few weeks – and is accompanied by high ambient air temperatures, this prolonged stress can begin to take its toll. Therefore, homeowners should closely observe the trees and shrubs growing around their homes or on their property for signs of drought stress.

The most obvious symptoms of initial drought injury to deciduous plants are brown leaves, curled leaves or scorched leaf edges, and in some cases, premature leaf drop. On conifers, brown needles and wilted, drooping growth (twigs) are usual signs of prolonged moisture stress. As drought extends over several months, dieback in the upper crown of the tree may also be noticeable or sucker-twig growth along the trunk on some species may occur. See the article “Tough times for trees: Heat, drought and storms take their toll” for more information on what signs of drought damage looks like. 

From a longer term perspective, extended drought can lead to additional tree problems that may not become evident until the following growing season – even if normal precipitation patterns return. Typically trees that are experiencing other stresses at the same time as drought are usually vulnerable to attack from boring insects or root-rot diseases. These are often described as secondary pest or pathogens, but are usually what cause the “killing blow,”so to speak, in a weakened tree. However, these secondary pest problems are slow to develop and may take more than one year to invade and actually kill a plant.

When treating a tree or shrub, efforts must be directed against both the primary stress factors (such as drought) in addition to the secondary stress factor. For trees and shrubs growing around the home, it may be possible to water them to keep the soil moisture stress to a minimum. However, watering an extremely large tree or trees growing in a forest of greenbelt is usually difficult. The best way to water a tree is very slowly. Let a hose or sprinkler run very slowly or gently underneath the drip line of the tree. Also, a thin layer of composted mulch around the base of a tree is another way to conserve moisture loss, once the soil moisture has been restored to near normal levels.

Overall, homeowners should keep a close eye on their plants and monitor them for any signs of unusual symptoms or conditions – especially early next year at the beginning of the 2013 growing season. Be especially on the lookout for signs of boring insects. Most boring insects are either members of the moth or beetle family. Adult insects lay eggs on the bark or in bark crevices. Once the eggs hatch, the young larvae bore underneath the bark and begin to tunnel and feed. Eventually, they girdle a tree and cause dieback or complete mortality. Treat your plants promptly when signs of borers become visible before a plant becomes too weak to respond to treatment.