Published on 08/18/99

Heat, Drought Take Toll on Landscape Plants

This summer has been rough. Day after day the heat has topped 95 degrees. Little to no rain has fallen for three straight weeks. And the combination has led to serious, and sometimes fatal, plant stress in Georgia landscapes.

Some of the plants now showing major heat and drought stress include dogwood, Japanese maple, oakleaf hydrangea, fothergilla and azalea.

leafburn.jpg (40346 bytes)
Photo: Gary Wade

Marginal leaf scorching is an easy-to-see symptom of drought and heat stress.

Heat/Drought Stress Symptoms

Symptoms range from wilting and pale yellow color to marginal leaf scorching, leaf cupping and defoliation. Trees and shrubs planted in the past year and those with other stresses (root damage from construction, for instance) are the most seriously affected.

Annuals in nonirrigated areas are a struggle to maintain. They fail to provide an effective color display when they wilt day after day.

In some counties, restrictions on outdoor water use and irrigation bans make matters worse.

Drought stress is often compounded by an increase in insect and disease problems. Powdery mildew, ambrosia beetles and spider mites are three common predators of stressed plants.

A Matter of Balance

Droughts stress plants when the foliage demands more water than the roots can supply. The stress usually results from roots' dehydrating and dying in extremely dry soils.

Sometimes one part of the root is affected before another and the plant discards a branch here and there to compensate proportionally for the volume of roots lost.

The plant walls off the branch from the remaining live limbs. It shuts down production in that branch in an effort to survive.

On the other hand, sudden death syndrome, when an entire tree or shrub dies suddenly, is a most disappointing root-death symptom. It's particularly disheartening if the plant is an irreplaceable, 100-year-old oak.

Sudden Death

Sudden death may come days, weeks or even months after the imposed stress. The plant appears to survive, even though the root system is dead, by drawing on water and carbohydrates stored in the roots. Then it dies suddenly when the reserves are depleted.

Sudden death in spring following a drought the season before is one of the hardest environmental injuries to explain.

How do you help plants cope with heat and drought stress? Do everything possible to conserve soil moisture and prevent root injury or death.

Mulch Conserves Moisture

Mulching is one of the best ways to conserve moisture in the soil. Fine-textured mulches such as pine straw, pine-bark mininuggets or shredded wood mulch hold moisture in the soil better than coarse-textured mulches.

Spread mulch not just under the canopy, but as far beyond the canopy as you can, since roots extend two to three times the canopy spread.

Pruning Can Help

Summer pruning may be necessary to reduce the leaves' demand on the roots. If a tree or shrub wilts or begins to show leaf scorching or other stress symptoms, thin the canopy by one-third to one-half, depending on the severity of the stress.

With selective thinning cuts, you can reduce the size of the canopy without destroying the plant's shape.

Annuals and herbaceous perennials showing moisture stress can be cut back to within 6 inches of the ground. Most will return with vigor, assuming they get some moisture to sustain what growth is left.

Do No Harm

During periods of heat and drought stress, avoid any further stresses on the plants' roots.

Fertilizing a drought-stressed plant is one of the worse things you can do. Chemically, fertilizers are salts. They will pull water from the roots, dehydrating them further.

Avoid disturbing the roots by digging, too, or suffocating them by placing soil over them.

When the weather breaks, regular rains return and cooler fall temperatures arrive, apply a phosphorus-based fertilizer, such as Superphosphate, at one-half pound per 100 square feet around stressed trees and shrubs. That will help them rebuild their roots during the fall and winter while the top is dormant.

Gary Wade is a Cooperative Extension horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.