Global warming's impact on your garden.

Global warming's impact on your garden.

Global warming's impact on your garden.

All things green.
Feb. 28 2007 12:20 PM

The Greenhouse Effect

Global warming's impact on your garden.

A garden at the Denver Botanic Gardens
The hardly-any-water-at-all garden at the Denver Botanic Gardens

Lilac bushes in New England have pretty steadily been blooming earlier every year for the past 30 years. One way some gardeners have begun to think about shorter winters is to say, "Hey, great, I live in Ohio [or wherever, north of the Mason-Dixon Line] and now I can grow some camellias, maybe a fig tree." This is, frankly, nuts. Be careful what you wish for. Kudzu is creeping north, poison ivy is growing more toxic on its diet of extra carbon dioxide, and allergy season lasts longer.

Another reaction is to say, "I will make the world greener by planting a tree in my front yard that will inhale carbon dioxide and slow global warming." This is not nuts, and it's better than nothing, especially if you are planting a shade tree on the south side of your house and thus cutting your air conditioning use. But in the face of the enormity of global temperature change, it's only a little bit better than nothing.

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There are two very specific and more sensible ways to prepare than by cheerily planting the flora of Charleston, S.C., in Cleveland. First, insulate the green things you have from the shock of drought to come by making the soil they live in better at holding on to moisture. Next, choose new plants that can tolerate drought and a wide range of temperatures.

The institutions that guide gardeners have themselves started to adjust. The Arbor Day Foundation recently released a hardiness zone map. Their members complained that the widely used official government source—the U.S. Department of Agriculture map—didn't reflect how much the country was warming up.

The Arbor Day Foundation map, using the USDA Hardiness Zone Map of 1990 as a starting point, tracked some dramatic changes from then to 2006. Our heartland—Illinois, Indiana, Ohio, and even Michigan's mitten—has shifted from Zone 5 to a warmer Zone 6. (Higher temperatures mean that most U.S. gardeners will be facing longer summer stretches without water.)

The USDA, slower off the mark, will release its own update of the 1990 map sometime this year. When asked if the forthcoming new map was a response to global warming, USDA spokesperson Kim Kaplan said, "Not specifically." She gave as more compelling reasons the fact that the Government Printing Office is out of copies of the last version and that the old version wasn't Internet-friendly.

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Both maps divide the United States into zones by shared average low temperatures. Bands of different colors run from a very cold Zone 1 (Fairbanks, Alaska) to a tropical Zone 11 (Honolulu). The maps are a general guide; you may have something like a sunny wall where cold-averse plants can grow. Or your garden may be in a valley that's colder than the surrounding hillsides.

For those who acknowledge that warming is definitely here, the highest priority is to protect trees, which shelter other plants from drying wind and sun, as well as absorb carbon dioxide. The single best drought-survival help you can give those beneficial trees and your other plants is to cover any bare earth, from which water evaporates quickly, with mulch. Spread shredded bark or bark chips or compost about 3 inches deep on top of the soil (take care not to push mulch right up against plant stems or tree trunks where it can cause rot).

When you're planting new things or moving old plants, improve the moisture-holding capacity of your soil even more by digging in organic matter—the same bits of bark or well-rotted cow manure, or compost.

Compost—dark, earthy, decomposed organic matter—has the immediate and obvious effect of making your soil moister, and it's also a welcoming home for the earthworms and microorganisms that make nutrients available to your plants.

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A less obvious effect of making and using compost is to keep the atmosphere healthier. When the stuff rots in a municipal landfill, instead of decomposing in a nicely aerated compost heap, it exudes methane—a greenhouse gas that traps heat on Earth at a greater rate than carbon dioxide does.

Selecting plants for drought tolerance doesn't mean that your yard will be all yucca and cactus. Lots of favorite annuals bloom well with low water—cosmos, petunias, verbena, marigolds, and zinnias. (Remember that even drought-tolerant plants need to be watered thoroughly when they're first planted.)

Perennials from Mediterranean climates have evolved to thrive without summer rain. Pinch the leaf of a plant native to Provence or Greece (and similar climates in coastal California, South Africa, and Chile), and you'll see these plants are resinous and fragrant. These include useful and beautiful plants like lavender, rosemary, sage, catmint, oregano, and thyme. A less familiar herb—agastache, also known as hyssop—is a real garden star; it flowers nonstop through the summer, beloved by bees. The red agastaches attract hummingbirds.

The Denver Botanic Garden, which has long been landscaping with natives adapted to Colorado's semi-arid climate, has clear advice and useful plant lists. The catalog of High Country Gardens in Santa Fe, N.M., has particularly interesting water-smart planting designs including "The Inferno Strip Garden"—for hot, narrow spaces.

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Any botanical garden will have good advice on the native plants that can stand up to hot summers; planting natives will make life easier for your local birds and beneficial insects.

Like everything involved with interacting with the natural world, water-wise gardening gets a little complicated. Climate watchers who are warning us of longer droughts also are predicting heavier rains in winter, coming in fewer events—downpours, buckets. The plants from the Mediterranean or the Colorado high plains do not do well if their roots are drowned. Happily, and also paradoxically, a good cure for waterlogged soil is to add organic material. It's an apparent paradox because that's the stuff that holds on to water, but the organic stuff also keeps the soil aerated and keeps water from collecting in fatal puddles.

You can hedge your bets in this chancy new world by choosing trees and shrubs that do well across many temperature zones. Among the most adaptable: oakleaf hydrangeas, amelanchiers, many of the deciduous magnolias, and a lot of the pines. It may not be precisely right, I suddenly realize, to use the word adaptable. People are adaptable; we can change our behavior. (Not long ago no one used car seatbelts, and everybody smoked.)

It's more accurate to say that some plants and trees have evolved to tolerate or survive or withstand a range of conditions. There are some clever little weeds that can shift strategies quickly, but for the most part it takes generations for trees to adapt to new conditions, which makes them terribly vulnerable.

The tree losers in the coming warming, sadly, are sugar maples and white birches, which thrive in a niche and are unwilling to adapt. Their populations are dwindling in the warming Northeast, land of the precocious lilacs.

Gardeners tend to be the most adaptable of human beings. In fact, working in a garden is an experience that trains you to be flexible and to find consolations where you can. So the poppies never came up and deer ate the roses? Well, the irises looked great, and the lilacs were fabulous.