The effects of this summer’s drought will be long-lasting. Trees can take as long as three years to completely die from drought stress and related problems. Sufficient moisture is a critical need for woody plants. There is an old saying that a 90-foot oak tree with a 90-foot spread will lose more than 90 gallons of water a day when temperatures are 90-plus degrees. That’s a rather staggering statistic if it’s accurate. Sometimes we fail to recognize the effect drought can have on trees since they usually do not wilt like most other plants, but they do often exhibit other signs of damage.
One obvious symptom is leaf scorch. Some woody plants try to condition themselves against drought. For example, tulip and birch trees shed their leaves to reduce water loss, while oak and beech tree leaves develop a thick waxy coating to lessen water loss. Needled trees (spruce, yew, juniper and pine) have resins that protect them from drying out and they sometimes shed older needles. Certain trees sacrifice entire limbs to reduce drought stress. Lower and older limbs usually die before younger ones, and smaller branches are typically forfeited before larger ones.
Drought increases the susceptibility of trees to disease and insect infestation also. Opportunistic diseases related to drought stress include Armillaria root rot, Diplodia tip blight, Rhizosphaera needlecast, Verticillium wilt, and Nectria and Cytospora cankers.
Increased wood-boring insect activity is predicted for the next few years. Trees with die back actually release chemicals that attract wood-boring insects. Watch for flatheaded appletree borers, which attack crabapple, hawthorn, serviceberry, mountain ash, maple, ash, and ornamental pears, plums and cherries. Two-lined chestnut borers attack chestnut, oak and beech trees. There are also several round-headed borers that attack ash, linden, cottonwood, poplar and privet. Damage caused by moth borers, such as ash/lilac, dogwood, and carpenterworm borers will likely be higher as well.
Finally, an increased scale population may be noticeable. Check for European fruit lecanium scale, common on maple, oak, hazelnut, and crabapple trees. This scale will produce copious amounts of honeydew.
To improve tree health, keep trees well-watered going into winter. Concentrate watering at the drip line where the tree’s water-absorbing roots are located. Water deeply since most of a tree’s root system is found in the top 12 to 15 inches of soil. A 2- to 4-inch layer of mulch applied around the tree will help retain moisture.
Smith is a University of Illinois Extension Master Gardener in McLean County. For horticulture questions or information about the Master Gardener program, call 309-663-8306 or visit www.mcleanextension.org.