Some Like It Hot
PLANT LOVERS ARE USUALLY GAME TO TRY SOMETHING NEW. But how do you know a plant won’t need constant weeding and feeding, be deer candy or wimp out in the heat? Many experts claim that you don’t really know how to grow a plant until you’ve killed it at least three times. With a little understanding of evolution, though, you can beat those odds by a long shot. There’s more to survival of the fittest than meets the eye, but you can tell a lot just by looking at plants and observing their role in native habitats.
As Charles Darwin wrote in his 1859 “Origin of the Species by Means of Natural Selection,” plants and animals “are bound together by a web of complex relations.” Plants coevolved with insects, microorganisms and wildlife, each one competing for limited resources, each struggling to be nourished and avoid being eaten long enough to reproduce. Random changes happen in the genes of plants all the time. A genetic trait that consistently gives a plant survival and reproductive advantages can lead to genetically distinct subpopulations and, potentially, new species.
What does this process of natural selection have to do with choosing the best redbud tree for that hot, sunny spot in your Southwestern yard or an understory tree for your East Coast hideaway? When you learn to read wild landscapes, you begin to see features and patterns of adaptation in the vegetation that can help guide landscape design decisions. The presence of particular plants indicates where the water is; where soil is deeper, richer or rockier; or where shade moderates light. The more you can emulate the structure, patterns and plant composition of local natural plant communities, the more successful and sustainable your plantings will be.
Right Features, Right Place
Consider the Sonoran Desert. Rainstorms are short, intense and infrequent; water runs off fast. The saguaro cactus (Carnegiea gigantea), with wide-reaching, hair-covered shallow root systems able to slurp up to 200 gallons of water during a storm, anchors this landscape. A water-filled saguaro can weigh tons. Wide spacing between saguaros limits their direct competition, but mesquite (Prosopis spp.), desert ironwood (Olneya tesota) and paloverde (Parkinsonia spp.) — small trees whose roots reach deeper into underground reserves — grow in close association. These feathery-leafed thorny legumes are nurse plants: Their canopies form a protective microhabitat essential for germination and growth of saguaros and other plants. They moderate temperatures, add structural diversity, provide leaf litter that improves soil fertility and water retention, and promote untold wildlife interactions.
Everything in the desert stings, sticks or stinks.
Mesquite plays a unique role by both drawing nitrogen from soil (via some extraordinarily deep roots) and by making atmospheric nitrogen useable, aided by nitrogen-fixing bacteria colonizing its roots. Fallen nitrogen-packed leaves and pods of mesquite and other legumes decompose, creating nutrient-rich oases. In the words of Arizona ethnobotanist Dr. Gary Paul Nabhan in his book “Gathering the Desert,” “Mesquite islands are essentially self-fertilizing.”
“Nitrogen is an essential element in proteins,” writes Mark A. Dimmitt in “A Natural History of the Sonoran Desert,” “so nitrogen-fixing plants can make large crops of seeds with high protein contents.” Abundant large, nutritious seeds of desert legumes, especially mesquites, are important wildlife food. Animals such as coyotes, packrats and certain birds that stop by for a meal of seed or a shaded nap leave behind seeds of other plants that have passed through their digestive systems or dropped from fur. The seeds germinate and grow in the favorable mesquite island soil — far from the parent plant, reducing competition.
Between these hospitable islands, succulents, grasses, and tough, resinous small-leafed shrubs and forbs thin out to reveal patches of seemingly barren soil. Ephemeral plants emerge from those bare places after winter rains. Many are annuals, which germinate, bloom, set seed and die before sufficient rains return.
The Sonoran ecosystem is a good model for sustainable landscaping in dry places. From observing it, you can see that desert succulents — ocotillo, yuccas, dasylirions, agaves and cacti — benefit from wide spacing and association with woody desert legumes such as acacias, paloverde and mesquite. These shrubs or small trees may be thorny, but their delicate leaves, light shade and profuse, beautiful flowers bring a note of grace — as well as pollinators and other wildlife — to gardens in hot climates. Having a lush look requires water and soil high in organic matter. In a hot, dry climate, it’s better to landscape with compelling plants that evolved to prefer lean soil and little water.
Looking at Leaves
Leaves reveal secrets for success in challenging climates. They need to collect (sunlight, for photosynthesis) and protect (from sunscald, moisture loss, insects and other herbivores). Many dryland plants have silvery or leathery leaves and thick waterproof cuticles that keep water inside. Most have small leaves, limiting surface area exposed to sun and drying winds. Delicate compound-leafed pea-family plants, including Acacia, Lespedeza, Dalea and Lupinus species, abound. Relatives from more temperate climes (Baptisia australis, false indigo, for instance) can afford larger, darker, more sun-absorbing leaves.
Compare pine-leaf milkweed (Asclepias linaria) or slim milkweed (A. linearis) to the larger-leafed swamp milkweed (A. incarnata) and it’s easy to tell which are desert plants and which inhabits wetlands. Narrow-leafed mountain mint (Pycnanthemum tenuifolium), a perennial found in dryish upland soils from Texas to Maine, works for a low-water garden; wider-leafed P. muticum, a meadow-dweller, prefers a moist, fertile spot.
Water-rich landscapes turn green in summer, but dry places display a muted palette for a reason. In extreme environments — deserts, windswept plains and rocky cliffs, salt-sprayed coastlines and frigid mountaintops — plants’ protective adaptations often appear silvery. In some cases, their green surfaces are covered with fine, light-colored hairs, which reflect sunlight to shade the leaf and cool by trapping moisture. Silver ponyfoot (Dichondra argentea), a tender perennial, is an incandescent groundcover in warmer parts of Texas to New Mexico. Feathery-looking sand sage (Artemisia filifolia) withstands the sandy soil and extreme heat, cold and wind of dunes, yet tolerates decent garden soil.
Other silvery plants have a waxy or mealy coating. Rub leaves of rattlesnake master (Eryngium yuccifolium), a prairie plant, or desert-dwelling Agave americana and the coating comes right off. The stems of candelilla (Euphorbia antisyphilitica) even resemble frosted candlesticks. Silvery blues blend into dry landscapes and add pizzazz to greener gardens. For intensely blue grass selections, try little bluestem cultivars (Schizachyrium scoparium ‘The Blues’ or ‘Standing Ovation’) and switchgrass cultivars (Panicum virgatum ‘Heavy Metal’ or ‘Prairie Sky’).
Plants produce an array of potent protective chemicals. The pungent, evocative scents of sagebrush, sun-warmed junipers or pines, and creosote bush stem from volatile chemicals or resins that limit water loss, reduce ultraviolet light penetration, and deter animals and insects. Many plants respond to insect feeding by releasing chemicals we can’t smell that attract insect predators or signal nearby plants to make protective compounds. Some chemical defenses can be downright deadly, such as the cardiac glycosides in the white sap of milkweed, which deter all but the monarch caterpillar from munching. Consider any plant that leaks milky white sap guilty until proven innocent. Gloveless gardeners have suffered contact dermatitis worse than poison ivy from many Euphorbia members. Blue star (Amsonia spp.) sap is relatively benign by comparison, though deer and insects stay away.
Plants that are allelopathic release compounds that hinder most neighbors. Juglone, the chemical released by black walnut trees (Juglans nigra), is toxic or detrimental to many plants and some animals, notably horses. Oaks, junipers, sassafras, elderberries and sumacs are all somewhat allelopathic. These plants can be useful if you prefer a minimalist understory. Sunflower seeds have hulls that exude chemicals that stifle other seeds’ germination, something you may have noticed with hulls beneath your bird feeder.
All plants hold water in their cells, but succulents have evolved to excel at this. Dr. James Mauseth, botany professor in the Department of Integrative Biology at The University of Texas at Austin, says, “Some succulents are close to 99 percent water; when they die there’s almost nothing left.” Hoarding water in tissues allows for photosynthesis during drought. With cacti such as prickly pear (Opuntia spp.), their flat, succulent pads are actually stems that function like leaves. Barrel cactus, saguaro and other ribbed or accordion-pleated species expand as they absorb water and contract as it’s consumed. Their ridges shade portions of the plant during sunny hours.
As is true of being succulent, protective spines, prickles and thorns evolved many times over. These dead, sharp projections protect cacti, roses, brambles, euphorbias and countless other plants. Mauseth says, “Spines are more productive when dead,” noting that they don’t lose water, attract hungry herbivores or require internal plant resources. In addition to discouraging animals from eating water-filled stems, spines insulate the plant, act as windbreakers and direct rainwater to roots. “Cacti can’t afford to squander water through transpiration, so they make a thick layer of spines. [Mexican native] Mammilaria plumosa, for instance, is so covered in featherlike spines you can’t see the greenness beneath. It’s growing in full sun, but the plant itself is completely in shade.”
Agaves, yuccas and red false yucca (Hesperaloe parviflora) hoard water in large leaves covered with waxy secretions and reinforced with fibers that add strength and flexibility. Plus, they’re indigestible. Among succulents’ other leaf defenses are sharp, dead points, usually found at the ends of leaves but sometimes forming toothed leaf margins as well.
Dormancy — Timing Is Everything
The old saying, “Everything in the desert stings, sticks or stinks” suggests how the balance tips between investing energy in growth or protection in resource-poor environments. Some plants just give up and go dormant during times of drought or excessive heat or cold. Plants with bulbs, corms, rhizomes or tubers store their resources underground. Deciduous plants usually drop their leaves in fall but also may defoliate during extended drought. Paloverde’s green bark carries on photosynthesis when leaves can’t keep it up.
Evolution didn’t only happen eons ago to help weather the weather. Super-weeds that have become unaffected by herbicides are among the indications that evolution continues all around us. Last year was the hottest ever recorded globally, and climate change models predict that much of the U.S. will become hotter and drier. If plants can adapt to changing circumstances, surely gardeners can too. Let’s look to nature to learn how to create water-saving landscapes full of diverse native plants already programmed for survival.
Karen Bussolini is a garden photographer, writer and speaker, an eco-friendly garden coach and coauthor of “Elegant Silvers: Striking Plants for Every Garden.”