What Makes a Plant Drought Tolerant?

Plants have certain anatomical characteristics that enable them to survive with little or no water for extended periods. How can you spot them?

It hasn’t rained for weeks. Humidity is low. The sun radiates heat in the high 90’s or low 100’s, as exposed soil cracks. Birds and insects group at the edge of even the smallest puddle or damp spot to quench their thirst. Gardens scorch and wither. . .except yours. It shimmers in the hot evening breeze, glistening green, blue, and silver foliage catch the low angle of the setting sun, looking as lush as though they had been rained on that afternoon. You’ve watered only minimally, as your plants have shown you their needs. You’ve selected well. Your plants are undemanding.

It is quite possible to fill a garden with plants that will tolerate drought for a while, or even for an extended period of time. Dry gardening has long been a theme for gardeners in demanding climates, and plants have survived widely varied conditions in their native habitats for eons. Climate has always changed; it is not an idea new to our time. When it changes drastically, species adapt, move, or disappear from their former ecosystems. Those plants which we see today in their native habitats, have lived through those changes and are suited to current conditions. Walkthrough the wild places near your property to see what the native plants there have to teach you about withstanding drought.

What Adaptations Does a Plant Need to be Drought Tolerant?

Just as animal species do, plants have made certain adaptations overtime to their environmental conditions. When asked for an example of a drought-tolerant plant, many would offer ‘cactus’ in immediate response. It’s true that members of the family, Cactaceae, have anatomical characteristics that accustom them to long periods of little precipitation. Their most obvious adaptation is their narrow leaves or spines, which minimize leaf surface so that the plants lose less of their system’s moisture through transpiration. Plant moisture is released from the surface of leaves, much as ours is released from our skin. A thin layer of water, released from pores, evaporates as it is transpired by plants or perspired by humans.

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Cactus may be an excellent choice for your dry garden, but many plants besides cactus have small leaves as a guard to slow transpiration. The mountain mahoganies (Cercocarpus sp.), the sedums, pussytoes, and yarrows are just a few. Photosynthesis still takes place abundantly as smaller leaved plants compensate by producing many leaf structures. However, the surface of each is greatly reduced, so that it loses less moisture, while it also receives some shade from its numerous companions.

A Leaf’s Coating Makes a Difference

A waxy (glaucous) or leathery surface provides protection from leaf transpiration in plants such as the chokeberry shrub (Aronia sp.). While this thicker leaf coat conserves moisture, it lends a lush, glossy appearance to the drier garden and a deeper green. Western sandcherry (Prunus besseyi), the Oregon grape holly shrub (Mahonia aquifolium), and various shrub roses have this leaf coating. Their foliage contrasts well with the grey-green, blue, or silver that many other drought-tolerant specimens possess.

Lighter Color and Hairiness are Defenses

You might think of hairy leaves as a more common defense against moisture loss. Fine to coarse hairs may provide a covering for leaf and stem tissue giving this foliage a grey-green or silver appearance. The lighter color of these leaves helps to cool leaf tissues by reflecting sunlight, while tiny hairs reduce transpiration in low humidity, sometimes as much as 70%. Some notables are grey-green Clary sage (Salvia sclerea), the silver artemisias, soft and furry lamb’s ear (Stachys byzantina), and low-growing woolly veronica (Veronica pectinata). Four-wing saltbush, silver-leaved lavender cotton, and some viburnums exhibit this anatomical feature, as well.

Did that Plant Point North?

Some plants actually orient their leaves throughout each 24-hour period to protect themselves from moisture loss. The compass plant, Sylphium laciniatum, a native of the tallgrass prairie, reduces exposed leaf surface area by aligning most of its leaves in a generally north-south direction, so that less surface is directly hit by the sun as it crosses the sky east to west. This, in addition to its leathery, hairy leaf and stem covering, help it to conserve moisture each day.

Cushions find Comfort in Harsh Conditions

Growing low and in compact form, the cushion plants have developed their own moisture conserving device. On the tundra or across the dry prairies, some plants have adapted to frequent drying winds and almost constant sun exposure by crowding their leaves into small domes or buns. Others lie almost flat, seeming to crawl across the soil’s surface. Small leaves clustered closely together and close to the ground aren’t buffeted in strong winds and provide themselves with a miniature “hut” or carpet of tiny leaves. Some pinks (Dianthus) grow this way, as do prickly thrifts (Acantholimon), drabas, germanders (Teucrium), thymes, iceplants (Delosperma), and many more of the plants you may generally see labeled as “alpines,” or “rock garden plants.” They may be used in a traditional rock garden site, as the front layer of a dry border, or among pathway stones.

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If your garden is subject to dry periods you may notice other methods plants use to save themselves until more moisture is available. Their leaves will wilt, their blooms will dry up early or drop, they may even retreat underground until the cooler temperatures of late summer and fall arrive. Some, like the grasses, may go dormant in order to conserve energy and reduce transpiration. Observing your plants’ various responses to drought and heat will tell you a lot about their preferences. Whereas the purple coneflower will wilt in a hot, dry spell, hens-and-chicks will thrive. A dry winter can decimate some specimens, while it will benefit others.

You Can Adapt Your Garden to Drought

Your soil, exposure, and drainage patterns all play a role in how drought-tolerant your garden plants will be, during both the growing and dormant seasons. Slight changes to your landscape may make a difference as to which plants you can grow and which you cannot. Placing even a small boulder for shade, planting in a slight depression, or utilizing taller plants to shade shorter ones can be helpful devices. You can train your plants, from the time they are put into the ground, to use less water by providing less. Their roots will then grow deeper and wider to utilize all available water beneath the surface.

Of course, researching and choosing plants that have been proven to be drought-tolerant is well worth your time. What grows natively in the dry climate of your area? What grows in geographically similar regions? What does your local reputable nursery recommend? What can you find from mail-order sources which specialize in drought-tolerant specimens? This is where the fun begins. A combination of observing the conditions in your own garden with those in the surrounding native landscape and widening your knowledge of drought-tolerant plants from around the world is a rewarding and never-ending endeavor. Enjoy the process and outfit your garden so that it’s well equipped for the precipitation that falls naturally from the skies.

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