I’ve received several calls lately from Clay County residents concerned about the health of their trees. Most people assume drought is responsible for a sick or dying tree. But that is not entirely the case.
Drought may be a contributor to tree kill but it is not directly responsible. Relatively few trees died directly from dehydration last year. Instead, the 2011 drought severely weakened mature trees, making them susceptible to opportunistic pathogens and insects.
Dr. Eric Taylor, Texas AgriLife Extension Service forestry specialist, said that in most instances, trees that died in 2011 were already stressed from a number of pre-existing environmental factors such as over-crowding, growing on the wrong site, age, soil compaction, trenching or inappropriate use of herbicides.
An important concept to remember is that the best defense against drought is to promote a tree’s health and vigor through proper care and management.
This is not to say that water is not important. Water, particularly soil moisture, is critical for all a tree’s physiological processes which include making and transporting food, taking in and releasing carbon dioxide, conducting biological reactions, building tissue and more.
Trees can also die from heat stroke. Extreme temperatures, not only during the day but also in early evenings and at night, have negative impacts to tree physiological processes. As do humans, trees sweat to cool off, a process called transpiration. During transpiration, water evaporates primarily from leaves. Inadequate soil moisture coupled with hot temperatures lessens the tree’s ability to transpire.
“As a result, the cells in leaves and small branches can ‘cook’ to death,” Taylor said.
Stress brought on by drought and heat also creates opportunity for macro fungi, such as hypoxylon canker, to invade trees. Hypoxylon canker is a white-rot fungus that is usually considered a weak pathogen – not aggressive enough to take over healthy trees.
Often, the first symptom that may be observed is the dying back or thinning of the crown. As the fungus develops underneath the bark, it causes the bark to break loose and slough off, exposing a matt of grey, tan, olive green or reddish-brown powdery spores. By the time the spores become visible, the tree is dead. There is no cure for hypoxylon canker and there have been cases reported in Clay County.
Another odd observation this year is the dropping of seemingly, healthy green leaves. This could occur if the tree lost part of its root system as a result of last year’s severe drought. If this is the first year for this to occur, the tree may eventually recover.
Homeowners can reduce tree stress by paying attention to over-crowding, proper pruning, minimizing damage to stem and roots and proper watering.
A rule of thumb is to begin supplemental watering if significant rainfall has not occurred in the past seven to ten days. Taylor recommends using a soaker hose or by trickle or drip irrigation, and water just outside the drip line of a tree’s crown. It is not necessary to encircle the entire tree. A good watering on half or even a quarter of the root system can be very beneficial. Do not concentrate the water at the base of a tree as this can lead to root diseases.
The water should soak into the soil without run-off. If it does, reduce the flow rate. Water until the moisture has soaked in to the soil to a depth of 8 to 10 inches. The best time to water is during early evening and at night.
During a drought, plan to water mature trees twice a month. Younger trees still growing should be watered once per week.
Remember, during a drought, the goal for tree and shrub irrigation is twofold; reduce water use to save precious water and money, yet use enough water to preserve your substantial investment in your landscape trees and shrubs.