Hot and dry has been the weather pattern this month. Count yourself lucky if you have been one of the few to receive a soaking afternoon rain. Excessive heat combined with below-average rainfall has prompted the National Weather Service Climate Prediction Center to issue a 2- to 4-week hazard outlook for much of Louisiana, warning of rapidly developing drought conditions. This is not a good scenario for landscapes that already suffered drought stress between late winter and early spring.
With little rain forecast over the coming week, supplemental irrigation should become common practice for trees, landscapes, gardens and orchards. The prolonged heat is causing soil moisture to evaporate rapidly, making it difficult for plants to absorb enough water to remain healthy.
When plants and trees experience drought, they begin conserving water. One of the first signs is leaf yellowing. As chlorophyll breaks down, trees may begin showing fall-like colors weeks earlier than normal. Leaf scorching or burning can also occur. Eventually, plants may shed their leaves prematurely to reduce water loss.
Typically, we seek relief from the summer heat beneath the shade of trees. Shade can make temperatures feel 10 to 15 degrees cooler. Trees also help lower surrounding air temperatures through evaporative cooling. However, this natural cooling process comes at a cost during periods of drought.
A healthy, mature tree can use up to 150 gallons of water per day. Most of this water is lost through transpiration as the tree cools itself. When little or no rainfall occurs for extended periods, soil moisture reserves can quickly become depleted. Eventually, soils can become so dry that fine feeder roots struggle to absorb sufficient water.
Late summer and early fall are important transition periods as plants begin preparing for winter dormancy. As drought conditions worsen, plants enter dormancy under stress, which can affect their health well into the following growing season. Premature leaf drop reduces the carbohydrates and nutrients normally stored for spring growth. If drought conditions persist into winter, plants may become more vulnerable to cold injury and sudden temperature fluctuations.
Drought conditions also make plants more susceptible to insects and diseases. Weakened plants are less able to seal wounds caused by injury or pruning. These wounds can become entry points for insects and disease-causing organisms. Weakened root systems and depleted carbohydrate reserves also reduce a plant's ability to defend itself from pests and diseases.
During drought, early fall color is often a warning sign rather than a welcome sight. So, what can be done to prevent injury to our plants? Water, water, water!
If you have not been watering your trees and shrubs this summer, start now. A common recommendation for established trees is to apply approximately 10 gallons of water per inch of trunk diameter. For example, a tree with a trunk diameter of 12 inches would require about 120 gallons of water.
Watering should be done slowly so the soil has time to absorb the moisture rather than allowing it to run off. Extremely dry soil absorbs water slowly, so deep watering is more effective than frequent shallow applications. This reduces drought stress and helps maintain healthy root function. Watering during the early morning hours helps plants better withstand daytime heat while reducing water loss to evaporation.
Mulching around plants is also important for conserving soil moisture and moderating soil temperatures. Apply mulch in an even layer 2 to 4 inches deep beneath the canopy of the plant. Be sure to keep mulch several inches away from trunks and stems to prevent decay and other problems.
Hopefully, we will receive relief from the heat and drought soon. Unfortunately, fall is typically one of Louisiana's driest times of the year. Even if rainfall returns to normal, many areas may remain in a moisture deficit, meaning supplemental irrigation could be necessary for weeks or even months ahead.
Quick Tips for Drought-Stressed Landscapes
