In the Garden:
Southwestern Deserts
July, 2009
Regional Report

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Cactus spines are modified leaves. Their limited surface area reduces moisture loss.

How Desert Plants Survive

When the thermometer shows triple digits for months on end, it's hard to imagine how plants can survive, let alone thrive in our conditions. Desert plants have evolved with ingenious adaptations to reduce moisture loss while standing tall in blazing sun and intense heat. When you are in the garden or hiking in the desert, take a moment to notice some of these features.

Reducing Moisture Loss
Have you wondered why individual leaves on most desert plants are so tiny? Compare our dalea, ironwood, and mesquite leaves to red oak, silver maple, or magnolia foliage native to regions with more plentiful moisture. Small leaves create limited surface area, which reduces a plant's overall moisture requirements.

Transpiration refers to a plant's loss of water vapor through stomata (small pores), typically during the day. Cacti and other desert plants reduce water loss by opening stomata at night when temperatures have cooled.

A tough waxy coating is another adaptation that reduces moisture loss on such plants as creosote and cacti. Cacti also reduce moisture loss by foregoing traditional leaves. Cacti spines are actually botanically modified leaf parts with sharp points. Spines protect and shade the cacti's tissue.

In times of extreme drought, ocotillo and palo verde trees drop their leaves completely, inducing dormancy. This reduces the amount of live plant material to which the root system must supply water. When rainfall returns, the plant responds with fresh growth.

Harvesting Rainwater
Agave, hesperaloe, and yucca use their funnel-like or rosette shapes to collect and direct rainfall to their main stem and roots. Cacti spines act as "drip tips" for the same purpose.

Cacti develop very shallow, but wide-ranging, root systems that quickly absorb even minimal rainfall. Saguaro roots may extend up to 100 feet in all directions from the trunk. When water is available, cacti and other succulents such as agave, aloe, and yucca store it in their roots, stems or fleshy leaves, ready to tap as needed during dry spells.

Sunscreens
Many desert plants, such as globe mallow, Texas sage, and wooly butterfly bush, have pale green, gray, or silver foliage that reflects sunshine and reduces heat build-up on leaf surfaces. Brittlebush and Superstition mallow have fuzzy hairs on their leaves, which act as miniature sun reflectors to cool plant tissue. Jojoba angles its leaves vertically to minimize surface area exposed to the sun.

Because desert plants are adapted to arid growing conditions, they thrive with limited water and develop few problems. Research shows that stressed plants, such as non-natives poorly adapted to intense heat and sun, are more likely to be attacked by pests and diseases, which in turn requires intervention. If you prefer a low-maintenance, water-thrifty landscape, choose native and desert-adapted plants for the bulk of your design.


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