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What is "Drought" and "Drought Stress?"

July 11th, 2007

What is "Drought" and "Drought Stress?"

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 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 Florida, plants may frequently encounter drought stress. Rainfall is very seasonal and periodic drought occurs regularly. Because Florida's soils are typically sandy and have low water holding capacity, many plants may experience drought stress after only a few days without water. During drought, local governments may place restrictions on landscape irrigation in order to conserve potable 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.

Drought symptoms resemble salt stress because high concentrations of salts in the root zone cause water loss from roots. Close examination of environmental and cultural conditions should help identify the specific problem.

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 high water tables, 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.

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, providing shade and creating windbreaks. 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.