Drought is a major abiotic stress for crop production and landscape plantings in the world. Arid, semi-arid and desert climates are be found in the United States, as well as semi-tropical areas that see frequent drought with high temperatures. Arid and desert regions receive precipitation below the potential transpiration rates of plants. There are three strategies that plants use to cope in low precipitation climates: Succulence, drought tolerance, and drought avoidance.
Succulent plants store water in fleshy leaves, stems and roots. They are often adapted to absorb large quantities of water in short periods, such as desert rains. Thickened leaves and globose stems minimize the surface area through which water is lost. Waxy cuticles provide a physical barrier to water loss. Surface hairs or spines provide a microclimate that also reduces transpiration. Some succulents have vestigial leaves and rely on the stem to harvest solar radiation. Nearly all succulents have extensive, short root systems which are adapted for increased water absorption. Often their leaves have trichomes to absorb atmospheric water in dew, mist and fog. Some species of succulents can survive for months without rainfall. Succulents may occasionally appear as epiphytes, growing on other plants with limited or no contact with the ground, and are dependent on obtaining and storing water in storage organs. Succulents are also found on sea coasts where they are exposed to high levels of dissolved minerals toxic to other plants. Succulents commonly use Crassulacean Acid Metabolism (CAM) as a biochemical strategy to minimize water loss during photosynthesis.
Drought tolerant plants can withstand desiccation without drying. They often shed leaves during dry periods and enter dormancy. Most water loss from these plants is transpirational loss from leaves, and dropping leaves helps to reduce this loss. Some drought tolerant plants have resinous coatings. These plants typically make use of C4 carbon fixation or CAM to conserve water during photosynthesis. Other adaptations include reduced number of stomata, trichomes (small hairs) on leaves, adaptations in the root system to increase water absorption, and water storage organs such as tubers. Drought tolerant and succulent plants often have a thick, waxy cuticle to reduce water loss, reduced leaf surfaces and the ability to accelerate senescence.
Drought avoidance or “drought escape” plants are often annuals that escape unfavorable conditions by completing their life cycle during times of higher precipitation. One strategy are the spring ephemerals found in deserts. Desert ephemerals take advantage of spring rains, and the species survives through seed dormancy. Other strategies include tight folding or rolling of leaves to reduce transpirational water loss. Perennials will die back to their underground parts and become dormant.
Many drought tolerant and succulent plants take advantage of C4 carbon fixation or CAM pathway to fix carbon during photosynthesis. Both are improvements over C3 carbon fixation and are more energy efficient. The CAM pathway is particularly good for arid conditions because carbon dioxide can be taken up at night allowing the stomata to be closed during the heat of the day. This reduces water loss through transpiration.
Xeriscaping, a landscaping style which requires little or no irrigation, was first developed by Denver Water in Colorado because of pressure on it’s water system. Xeriscaping is now recommended in regions that do not have easily accessible, plentiful or reliable sources of fresh water. Also known as water-conserving and drought tolerant landscaping, these designs often use plants that are native to the local climate, but not exclusively. This allows for a lower consumption of water and also reduces maintenance. There are seven principles of xeriscaping:
- Orient your plot by grouping watering zones according to watering needs. Take into account north/south and east/west facing areas, as well as sun in water-conserving landscape and shade. Study natural contours and drainage of the plot and determine if terracing is necessary.
- Ideal soils for water conserving landscapes drain quickly and store water. It is important to increase organic matter in your soil, as well as aerate it.
- Create minimal turf areas, while retaining turf for activity areas.
- Right Plant, Right Place! Check nurseries for drought resistant and native plants
- Mulch using leaves, coarse compost, pine needles, wood chips, bark or gravel.
- Irrigate using soaker hoses and drip irrigation. Water deeply, and less frequently, and avoid watering during the heat of the day
- Maintain your landscape by keeping weeds from growing up through the mulch. Avoid over fertilization.
For more information on planning drought tolerant landscaping and recommended native plants for these areas:
Beneficial Drought Tolerant Plants for the DC Area:
Native Plants for Northern Virginia
Drought Tolerant Plants for Virginia
Drought Tolerant Plants for the Mid-Atlantic
Creating a Waterwise Landscape
Waterwise Plants and Landscaping