On these hot, dry days, your plants may look as though they're pleading for water. You used to be able to just haul out the hose and soak every wilted thing. Now, though, in many parts of the United States and more and more places around the world, water is limited. Snow cover has diminished in shorter winters, reservoir levels are sinking, and many parched Western states keep adding more water consumers.
Just this month the U.S. Department of Agriculture declared the state of Alabama a drought disaster. Alabama's governor called for prayer (some rain came, but not enough, said the spoilsport climatologists), but there are other ways of coping. Home gardeners might take a lead from farmers who can't so easily switch a field from corn to cactus or soybeans to sedum. They're looking at improving their soil's ability to hold water and their crops' ability to take it in. Gardeners can plan ahead by making their soil water-retentive, planting drought-tough foliage, and nurturing perennials with plenty of moisture before the water runs out.
Over the past decade, we've been advised by garden writers and garden centers and environmentalists that the way to go in a hotter and drier world is to buy and use drought-tolerant plants. When home gardeners began including more drought-resistant plants, it was certainly a good change from green lawns in Phoenix and hydrangeas in El Paso, Texas. Those resinous water-storing plants harmonize with what's left of the surrounding landscape. But some drought-resistant combinations have become tiresome clichés. Look at the planting around almost any North American airport. You'll see Russian sage, yellow "Stella d'Oro" daylilies, sedum "Autumn Joy," and prairie grasses.
The gray leaves of Russian sage, the deep roots of prairie grasses, and the succulent water-holding leaves of sedums are evolutionary adaptations to arid environments. For the nonarid parts of the country, though, drought is a relative and temporary condition. Meanwhile, the yucca and sage you plant today could be drowning in the next rain. I'm haunted by the words of the meteorologist who predicted a future of "more rainfall, in fewer events." Where are the gentle showers of yesteryear?
What's more, you might not want your yard to look like the Denver Botanic Gardens, lovely in their dryish way though they are. You may have plants that need moisture at the roots—a rose that grew from a cutting in a graveyard, or a rhododendron planted by the people who lived in your house 30 years ago, or an apple tree left over from when your suburb was an orchard, or, let's face it, a Japanese maple that cost $175.
A good response to longer periods of dryness broken by more violent rainstorms is to make your soil drought-resistant. What you want is a way for your valuable plants to survive a temporary water deficit, without having to use a lot of water and perhaps pay a fine to your municipality. So, make sure that the water from a deluge doesn't run off. Make the water percolate down to plants' roots. Don't till the soil; bare plowed soil loses water to evaporation. Leave organic material lying on the soil surface or plant groundcover (a cover crop like clover or alfalfa in the case of farmers). Midwestern farmers are now leaving corn plants up after harvest to catch the snow and protect the soil. Encourage worms, whose tunnels, about the diameter of a pencil, direct water down to root level.
Plants are, in a sense, cannibals. They thrive when they have partially decomposed plant material—little bits of bark or crumbly leaves—to consume. (Though they'll also happily take up the minerals in decomposed animal material.) Little bits of bark or crumbly leaves work like sponges, holding moisture in the soil. Humus, the name for that decomposing stuff, is sort of like exercise, which can make fat people thinner and thin people more rounded. It improves both sandy soils and clay soils, increasing the water-holding capacity of sand and water penetration in clay. Some of the boosters of arid-region plants have insufficiently stressed that in a garden with heavy clay soil that doesn't drain well, or where water fails to percolate, drought-tolerant plants will suffer in times of average rain.
The cheering news is that perennials on the whole are drought-resistant once their roots have developed well; they may flower less in dry conditions, but they're in it for the long haul. Annuals panic and go to seed, hoping their offspring will find moister conditions next year.
Perennials, even the prairie and desert ones, do have to be watered thoroughly when they're planted. Last weekend I saw a woman filling the back of her station wagon with about a dozen achillea (aka yarrow, often first on the list of drought-tolerant plants) with lovely terra-cotta colored flowers.
"So, I don't have to water these at all, right?" she called in parting to the nurseryman, who kind of nodded as he moved on to another customer. I suppose it's a good thing that I restrained myself from running after her car, yelling that newly planted perennials don't have a big root system, that they need a lot of water at planting time and attentive watering through the first year until their roots have matured and spread out. It's a complex message to get across while appearing to be a deranged person running down the road.
The achilleas at the home of the woman at the nursery may well be wilting right now, their nice ferny gray leaves sagging limply; she may be considering calling the nursery and asking for replacements. Roots have a hard time making contact with dry soil; watering at planting makes the soil stick to the roots and gets rid of air pockets where a root might dangle. Here is a gift to Scrabble players, a word likely to be mocked and challenged—turgor. To exhibit turgor means to be in a state of distension. From the Latin turgidus, swollen, inflated. (From which we have derived the idea of turgid prose—inflated and, thus, pompous.)
When the roots encounter a dry place, a hormonal message travels to the leaves to close their pores to slow down water loss. The pores, called stomata, are usually on the underside of the leaf. Squash and cucumber plants, which have pores on both sides, are extremely sensitive to lack of water; veteran vegetable growers use them as the canaries in the coal mine. Plants owe their capacity to be erect to water pressure; with less water in the system, they grow limp.
Which takes us to a much bigger picture. Scientists are working to make food crops that aren't adapted to arid places better at surviving drought by making their roots more efficient. A team of scientists headed by Roberto A. Gaxiola at the University of Connecticut has discovered a way to manipulate plant genes to increase root proliferation. Many naturally drought-resistant plants, especially the grasses of our great prairies, develop deep and dense root systems. It's a new idea; roots haven't previously been targeted in genetic engineering. Deeper, wider roots can spread out to more territory in search of water. The point, Gaxiola said, is to help agriculture in arid regions—Pakistan, Africa, China, and his native Mexico, not to mention Alabama.
In the past we've coddled our crop plants, giving them lots of fertilizer and water, things we used to think were unlimited. Gaxiola is aware that not everyone is on board with the manipulation of plant genes: "We are the witches of our time. People who don't understand the science would like to burn us."
Instead of being burned by drought, home gardeners, track the conditions in your area here and here, and check out these suggestions of plants that can thrive in gardens wanting for water. There's nothing wrong with praying for rain, but consider how nature keeps the soil moist—plenty of mulch, no bare earth.