Drought stress in trees and what you can do about it

By Heather Kirk-Ballard

LSU AgCenter Horticulturist

It’s all over the news, and you are likely painfully aware that we are in a major drought right now in Louisiana. The Mississippi River is at its lowest level in more than 100 years. Your lawns are probably toast if you have not been watering, and your landscape trees and shrubs are likely showing signs of stress.

Extended droughts like the one we are experiencing now can cause damage to trees if we do not step in and assist or get some rainfall soon. Despite the fact that we had daily rain in late July and August, these severe fluctuations in heavy rains followed by extended droughts can do a number on the long-term survivability and health of trees.

Water is vital to the health of all life. In plants, it is required for metabolic processes such as photosynthesis, the process by which plants make food. Water is also needed for growth and is essential to its cooling process known as transpiration (how plants sweat, if you will).

Trees signal drought stress with a number of symptoms, starting with foliage turning from a dark green to light green followed by browning at the leaf margins. Wilting and, eventually, premature dropping of branches and stems will occur.

Trees can eventually lose their capacity to absorb water as they are damaged, and dieback of branches and eventual death can occur. This year may be masked by the fact that we are officially in the fall season. With shorter days and cooler nights, deciduous trees are already preparing for winter and dropping leaves.

Drought-stressed trees will become more susceptible to diseases such as cankers, root rots, wood rots and wilt. Insects, especially wood-boring types, will also take advantage of stressed trees. Drought-stressed trees cannot produce chemical defenses without water to aid in the chemical synthesizing process.

You can help your trees through the drought with supplemental watering. The general rule of thumb is to apply 1 inch of water per week. The best practice for watering during a drought is to do so deeply and less frequently to encourage deep root growth.

Roots are generally within the top 6 inches of soil. Let the soil dry in between waterings to avoid shallow roots, weed growth and fungal disease. Drip irrigation, soaker hoses and water applied directly at the base of the plant will decrease evaporation and deliver water straight to the roots.

A typical garden hose flow rate has an average of 12 to 14 gpm, or gallons per minute, but this can vary. Many garden hoses are 1/2 to 3/4 inches in diameter. The smaller diameter the hose, the lower the gpm it will deliver.

The next thing to consider is pounds per square inch, or psi, which determines the speed that water passes through the hose. The average pressure from a home water faucet is about 40 to 60 psi but can be higher. The last thing to consider is the length of the hose. As the hose gets longer and you are moving water over greater distances, the flow rate drops.

To get an estimate, use a 5-gallon bucket and stopwatch. Open the spigot fully with your hose connected and let the hose run outside the bucket until you are ready to begin timing. Start your watch and fill the bucket to the rim. Record the amount of time to fill the bucket.

Let’s say it took 45 seconds to fill our 5-gallon bucket. Take 5 gallons divided by 45 seconds times 60 seconds (seconds in a minute), and we get 6.66 gpm. This is a rough estimate but gives you a place to start. You also can find garden hose flow rate calculators online. Try this one from Washington State University at https://bit.ly/3nccAx7.

Another very important factor is when to water. The best times to water are early in the day when the temperatures are cooler and at or after sunset. Between 8 p.m. and 6 a.m. is best. As the day heats up, water evaporates more quickly, making watering less efficient.

For shrubs, irrigate twice per week long enough to get moisture 12 to 16 inches down, and trees once a week to a depth of 18 to 24 inches. Use a long screwdriver to test the depth of the water movement into the soil. Let the lawns go dormant and enjoy the time off from mowing.

Until rain returns, hold off planting new plants, do not fertilize and do not prune except to remove dead of dying branches. Remove weeds and utilize mulches that help retain moisture at the soil level.

One way to fight future weather extremes — be it drought or excessive rain — is to choose native plants that tolerate our climate swings. They can handle it better than highly cultivated plants. Keep those zoned to special spaces to limit the area you have to water.

Tree with wilting leaves.

A major sign of drought stress is the overall wilting of leaves. As drought continues, leaves will eventually drop. Photo by Heather Kirk-Ballard/LSU AgCenter

Understory shrubs showing drought damage.

Understory shrubs often show the first signs of stress in a forest setting, as they compete with larger trees for water. Photo by Heather Kirk-Ballard/LSU AgCenter

Grasses showing drought damage.

Herbaceous plants and turfgrasses will show signs of drought first because they are shallow-rooted and often compete with large trees for water resources. Photo by Heather Kirk-Ballard/LSU AgCenter

11/4/2022 4:10:20 PM
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