Drought and Drought Stress on South Texas Landscape Plants
By: David Rodriguez

In nature, water is usually the most limiting factor for plant growth. This is also the case in home or commercial landscapes. If plants do not receive adequate rainfall or irrigation, the resulting drought stress can reduce growth more than all other environmental stresses combined.

Drought can be defined as the absence of rainfall or supplemental irrigation for a period of time sufficient to deplete soil moisture and injure plants. Drought stress results when water loss from the plant exceeds the ability of the plant's roots to absorb water and when the plant's water content is reduced enough to interfere with normal plant processes. In South Texas, plants may frequently encounter drought stress. Rainfall is very seasonal and periodic drought occurs. During drought, local governments may place restrictions on landscape irrigation in order to conserve water, and landscape plants may become subject to drought stress. The use of drought tolerant plants in the landscape can reduce the likelihood of plant injury due to drought stress.

How Does Drought Stress Affect Plants?
A plant responds to a lack of water by halting growth and reducing photosynthesis and other plant processes in order to reduce water use. As water loss progresses, leaves of some species may appear to change color -- usually to blue-green. Foliage begins to wilt and, if the plant is not irrigated, leaves will fall off and the plant will eventually die.

How Long Before Drought Stress Develops?
The time required for drought injury to occur depends on the water-holding capacity of the soil, environmental conditions, stage of plant growth, and plant species. Plants growing in sandy soils with low water-holding capacity are more susceptible to drought stress than plants growing in clay soils. A limited root system will accelerate the rate at which drought stress develops. A root system may be limited by the presence of competing root systems, by site conditions such as compacted soils, or by container size (if growing in a container). A plant with a large mass of leaves in relation to the root system is prone to drought stress because the leaves may lose water faster than the roots can supply it. Newly installed plants and poorly established plants may be especially susceptible to drought stress because of the limited root system or the large mass of stems and leaves in comparison to roots. Always look at a container plants root system before planting. Loosening or cutting the root system at initial planting stage will aide in establishment.

How Does Environment Affect Drought Stress?
Aside from the moisture content of the soil, environmental conditions of high light intensity, high temperature, low relative humidity and high wind speed will significantly increase plant water loss. The prior environment of a plant also can influence the development of drought stress. A plant that has been drought stressed previously and has recovered may become more drought resistant. Also, a plant that was well-watered prior to drought will usually survive drought better than a continuously drought-stressed plant.

What Changes Can Be Made to Reduce Effects of Drought in the Landscape?
The landscape environment can be modified to reduce or prevent drought stress by irrigation, mulching, and providing shade. Reducing the overall water requirements of the landscape is best achieved by initially designing the landscapes for water conservation, including efficient irrigation systems, proper watering and the use of drought tolerant plants where appropriate.

What are the Characteristics of Drought Tolerant Plants?
Some species have an inherent tolerance of drought because they have evolved in arid areas, regions with frequent drought, or regions with soils of low water-holding capacity. Some species have anatomical or physiological characteristics that allow them to withstand drought or to acclimate to drought. All plants have a waxy coating on their leaves called "cuticle," but some species have developed exceptionally thick cuticles that reduce the amount of water lost by evaporation from the leaf surface. Leaf hairs, which reduce air movement at the leaf surface, are another means of reducing evaporation from the leaf. Since the amount of surface area exposed to the atmosphere affects evaporation, leaf size and thickness are other adaptations, with thicker leaves and smaller leaves being more resistant to water loss. Some species have evolved large surface root systems to quickly absorb rainfall, while other species grow deep root systems to tap deep water tables. Some plants avoid drought by dropping their leaves during droughts and quickly regrowing new leaves when environmental conditions improve.

Lists of Drought Tolerant Plants
The plants listed tolerate drought stress better than most landscape plants. Although these plants are considered drought tolerant, new plantings will require regular irrigation for 6 weeks to 6 months or more before they become established well enough to be effectively drought tolerant. Trees larger than two inches caliper will take longer to establish.

Plants are listed by common and scientific names (alphabetized by scientific name) and are divided into categories such as trees, shrubs, groundcovers and vines. Those marked with **** are considered to be Deer Resistant. Also, trees not so marked are vulnerable until they reach a size sufficient to withstand the rubbing of the buck deer.

COMMON NAME/SCIENTIFIC NAME

Remember, Learn and Have Fun!

David Rodriguez is the County Extension Agent-Horticulture for Bexar County. He represents Texas Cooperative Extension with the Texas A&M University System For any landscape or gardening information, call the Bexar County Master Gardeners AHotline@ at (210) 467-6575, email questions to mg-bexar@tamu.edu, or visit our County Extension website at http:bexar-tx.tamu.edu

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