Plants that have adapted to live in dry climates are called Xerophytes (from the Greek “Xero,” meaning dry). Over time, they have evolved clever ways to survive without water and with too much sun and heat. Here are some of the ways.
1. Tiny leaves. Most desert plants have very small leaves compared to the large fleshy leaves of rain forest plants. Tiny leaves present very little surface area to the sun. Dessicating winds blow right through them, where larger leaves would be shredded by strong, dry desert winds and give off too much precious moisture. You might have to get up close to appreciate the diminutive specimens, but it is well worth the effort.
2. Thorns or “hard leaves”. Thorns are leaves that have been reduced so far that they don’t have any fleshy green parts left. The Fishhook Cactus has large, curvy thorns all over the surface of the cactus. These are actually reduced leaves. They also protect the plant from animals that would otherwise eat the flesh of the cactus to the ground.These thorns, or in the case of the Joshua tree, “leaves” are tough as nails and thoroughly dangerous to handle.
The purple Opuntia (and many other cacti) has minuscule tufts of thorns, called glochids, that collect in clothing and skin and are extremely difficult to remove. They emerge on the areoles of some cacti, and are the remnants of ancient leaves that have adapted so they do not forfeit moisture in dry seasons. They are extremely difficult to remove once stuck in your skin or clothing, so watch out! To remove them from skin, use a tweezers to pull each individual thorn straight back out. Don’t bend or twist the tweezers or they’ll just break off. They persist in your skin if not removed and cause itching and a little redness and swelling. It’s a great defense mechanism to keep your hands off the pads of opuntia cacti.
Joshua Tree “leaves” are thick, fibrous, and tough as nails with a super-sharp spine on the end.
3. Thick, succulent leaves. Some plants have the ability to store enough water in their leaves to survive long periods of drought. Thick, spongy leaves are called “succulent” leaves. Succulence is also an adaptation to cold, so you may see succulent leaves on plants on rocky outcroppings in upper mountain elevations. When the most desperate droughts occur, however, the plant uses the water in the succulent leaves, and if no rain occurs, will whither and die. Other plants like the Joshua Trees have very stiff, hard leaves that give off little moisture to the sun’s rays.
The Desert Four O’Clock has thick, almost succulent leaves that hold moisture for the plant.
Many succulent leaves are filled with moisture for dry times.
4. Hairy leaves. Hairy leaves reduce the water given off in transpiration and reflect away the sunlight falling on them. Hairy leaves are called “pubescent” leaves.
5. Leaf Curling. During rainy periods, the cells on leaf surfaces of some plants fill with water, fully opening and stretching out the leaves. During dry periods, they shrivel and curl up, reducing the surface area exposed to sun and desiccating winds.
Ocotillo with no visible leaves.
6. Shedding leaves. Some desert plants drop all their leaves in dry periods and don’t re-grow them until the next rains come. They give off little or no water in transpiration from their bare stems.
Ocotillos often only have their tiny leaves on the stems at the height of a good rainy season. The rest of the year, stems are bare except for spines and flowers at the tips in the springtime.
7. Leaf direction. To reduce the heat on the leaf surface, many desert plants turn their leaves away from the sun, so only the thin edges face the sun.
8. Flowers that open only when the sun is low. Some plants avoid exposing their delicate flower petals and internal workings to the sun by closing up after dawn and only re-opening after the sun is low. An example is the Desert Four O’Clock, which closes up tight during the mid-day sun. Another example is the Desert Evening Primrose, whose flowers only open at night.
The Cylindropuntias have papery sheaths covering their ferocious spines.
9. Spine Sheaths. Some of the cacti with spines, particularly in the genus Cylindropuntia, have papery sheath coverings on the spines that reflect away sunlight and prevent moisture loss. Cylindropuntias include the chollas and other relatives that that hang on to skin and clothing with utmost tenacity due to their barbed spines.
Cholla with spine sheaths.
10. Woodiness. Woody plants don’t lose as much water in the hot sun or drying winds. So many desert plants drop all leaves in summer and only expose woody surfaces to the sun until the cooler, moist temperatures return in fall and winter.
11. Thick surface coverings. The waxy layer on a leaf’s surface is called the cuticle. The thicker the cuticle, the less water is lost. Desert plants have thick cuticles to conserve as much water as possible. The Globe Mallow is coated with a thick white layer to reflect sun and reduce water loss. You can see miles of blooming orange Globe Mallows in Southern Arizona in March and April. Check out the area near the Casa Grande monument in the middle of the state. The sight is breathtaking!
12. White or blue color. White and pale blue or pale green leaf color helps reflect away the sun’s rays to lesson the blast that the leaves take during the hot, dry days. Many beautiful colors result, some of which are seen in modern homes as “designer colors!”
These Artemesia (on the left) and Caesalpinia (right) lessen the sun they collect via a leaf color that reflects away much of the damaging rays of the sun.
13. Shallow root systems. Desert plants often have widespread shallow roots that collect water at or near the surface of the ground. Desert rains run off into the washes rather than sinking into bone dry soil, so plants have to grab water at the surface as best they can. Desert plants are extremely efficient at capturing water before it penetrates deeply into the soil (it it’s able to) or washes away down a slope.
14. Extremely deep root systems. Other plants have very long, deep roots that tap into underground water.
15. Drought avoiders/escapers. These are plants that grow rapidly, completing their entire life cycle during or following a rainy period. Death Valley is full of drought-escaper wildflowers that emerge, flower, set seed, and scatter the seeds in the springtime, well before the summer heat sets in.
16. Long-lasting seeds. While many plants’ seeds do not maintain viability beyond the first year they are produced, desert plants will often be viable (able to sprout a new plant) for many years. Of necessity, they need to ensure their continued propagation and a long-lasting seed is a smart evolutionary adaptation. In times of severe drought, possibly lasting multiple years, there will not be enough moisture for the seed to sprout, so it lies dormant, only to sprout when rains finally return. So don’t give up on the seeds of your desert plants. Keep researching soils conditions, moisture levels, and, in particular, breaking dormancy for a particular type of seed. Most importantly, try again if at first they do not sprout for several months. Most have the ability to break dormancy eventually.
If a seed looks chewed or has holes in it, it has probably shared its protein with animals or insects and will never sprout. Additionally, if a seed has any mold or fungus growing on it, it won’t be viable.
There are even more ways that plants adapt to dry climates, but these are the most obvious and the most fun to observe. They make desert plants really cool to study.
[Prepared by Mary Engle, with some information adapted from Desert Wildflowers of North America, by Ronald J. Taylor, ©1998.]