Drought Could Have Lasting Effect On Trees

Although trees have deeper moisture-reaching roots than agricultural crops, they are not immune to this summer’s persistent drought, a Purdue Extension urban forestry specialist says.

Trees across Indiana and the Midwest are struggling in the arid conditions. Some could succumb or suffer for years to come, Lindsey Purcell said.

“Drought can have a major impact on tree health and survival by effectively slowing and reducing growth,” he said. “If drought is severe enough or lasts for a prolonged period of time – such as what we’re experiencing now – it also can cause death to all or portions of a tree.”

More common, however, is the effect drought has on a tree’s ability to withstand insects and diseases. A water-deprived tree is unable to produce its usual levels of carbohydrates, significantly lowering its energy reserves. Those reserves are needed for a tree to produce chemicals that ward off pathogens.

The drought also is providing an ideal environment for emerald ash borers, Purcell said. EAB is an invasive insect that kills ash trees.

All 92 Indiana counties are experiencing some level of drought, with many counties in the northeast, south-central and southwest parts of the state in extreme drought, according to the U.S. Drought Monitor.

Purcell said the drought already has left trees in urban areas with permanent damage, although it might not be evident this year. Warm weather earlier this year interrupted normal bud formation, which controls shoot length and expansion. The dry conditions that followed led to decreased numbers of new leaves within buds and new stem segments.

“Drought not only influences the number of leaves but also the size, as well as twig extension the following year when those buds expand,” Purcell said.

“The result of prolonged dry conditions may not inhibit the first growth but may decrease the number of stem units formed in the new bud that will expand during the second or third, or more, flushes of growth. If drought continues, all growth flushes will be affected. Thus, tree growth next year will be atypical and, again, create predisposed conditions to diseases and insects if not monitored and managed properly.”

While not something most homeowners think about doing, watering trees of any size and age can go a long way toward minimizing drought damage, especially for newly planted trees or those less established, Purcell said.

When watering younger and newly established trees, homeowners should follow the “5 plus 5” rule each week: give the tree 5 gallons of water plus 5 gallons for every diameter inch of tree trunk. For example, if a tree has a trunk diameter of 4 inches, provide 20 gallons of water slowly over the root zone.

For older, well-established trees Purcell recommends providing an additional inch of water every week or so to keep leaves turgid. To measure an inch of water, place an empty tuna or cat food can under a tree’s canopy and turn on a sprinkler system. Turn off the sprinkler when the water is 1 inch deep in the can.

“For those trees with mulch beds, you should consider adding a half gallon of water per square foot of mulch area,” Purcell said.

In addition to watering, Purcell urged homeowners to protect tree trunks from mechanical damage such as lawn mowers and string trimmers, reduce competition for available moisture with other plants, turf and shrubs, and add mulch to the root zone of trees at a depth of at least 2 inches to preserve soil moisture.

“These things are especially important to do during drought,” he said.