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Feeling Warmth, Subtropical Plants Move North

By Shaila Dewan (The New York Times) - May 3, 2007

Like a true belle, this city flounces into bloom when the weather turns, its redbuds, azaleas and forsythia emerging like so much lace on a bodice.

But in recent years, plants that thrive in even warmer weather have begun crashing the ball. At the Habersham Gardens nursery, where well-heeled homeowners choose their spring seedlings, a spiky-leafed, sultry coastal oleander has been thriving in a giant urn.

“We never expected it to come back every year,” said Cheryl Aldrich, the assistant manager, guiding a visitor on a tour of plants that would once have needed coddling to survive here: eucalyptus, angel trumpets, the Froot Loop-hued Miss Huff lantana. “We’ve been able to overwinter plants you didn’t have a prayer with before.”

Forget the jokes about beachfront property. If global warming has any upside, it would seem to be for gardeners, who make up three-quarters of the population and spend $34 billion a year, according to the National Gardening Association. Many experts agree that climate change, which by some estimates has already nudged up large swaths of the country by one or more plant-hardiness zones, has meant a longer growing season and a more robust selection. There are palm trees in Knoxville and subtropical camellias in Pennsylvania.

But horticulturists warn that it is shortsighted to view this as good news. Warmer temperatures help pests as well as plants, and studies have shown that weeds and invasive species receive a greater boost from higher levels of carbon dioxide, a heat-trapping gas, than desirable plants do. Poison ivy becomes more toxic, ragweed dumps more pollen, and kudzu, the fast-growing vine that has swallowed whole woodlands in the South, is creeping northward.

Already, some states are facing the possibility that the cherished local flora that has helped define their identities — the Ohio buckeye, the Kansas sunflower or the Mississippi magnolia — may begin to disappear within their borders and move north.

By the end of the century, the climate will no longer be favorable for the official state tree or flower in 28 states, according to “The Gardener’s Guide to Global Warming,” a report released last month by the National Wildlife Federation.

By the time of the annual Atlanta Dogwood Festival last month, the pale dogwood blooms had come and gone. Tara Dillard, a landscape designer and garden writer, said she now steers clients away from longtime favorites. “I’m writing a column about rhododendrons right now,” Ms. Dillard said. “And I think my conclusion is going to have to be not to plant rhododendrons. We have heated out of the rhododendron zone.”

In this warmer age, she said, “You might be planting some stuff you don’t like, like hollies.” But she brooks no objections from her clients. “I don’t care if you don’t like them,” she tells them. “I used to not like them either.”

David W. Wolfe, a professor of plant ecology at Cornell University, who spoke at a recent symposium at the New York Botanical Garden called “Gardening in a Changing Climate,” confirmed that in many places bellwether plants and animals were beginning to disappear. “There is clear evidence that the living world is responding to this change already,” Dr. Wolfe said.

Groups that cater to gardeners have hastened to keep up. In December, the National Arbor Day Foundation released an updated version of the United States Department of Agriculture’s Hardiness Zone Map, which shows the lowest winter temperatures in different parts of the country and is used by gardeners to determine which plants can survive in their yards.

Using data from the National Oceanic and Atmospheric Administration, the Arbor Day map indicates that many bands of the country are a full zone warmer, and a few spots are two zones warmer, than they were in 1990, when the map was last updated.

Atlanta, which was in Zone 7 in 1990, is now in Zone 8, along with the rest of northern Georgia. That means that areas in the northern half of the state where the average low temperature was zero to 10 degrees Fahrenheit are now in a zone where the average low is 10 to 20 degrees. A scientific consensus has concluded that this warming trend has largely been caused by the human production of heat-trapping gases.

The Agriculture Department is in the process of redoing the map itself. But critics have taken issue with the department’s decision to use 30 years of temperature data, saying it will result in cooler averages and fail to reflect the warming climate. The 1990 U.S.D.A. map used 13 years of data; the Arbor Day map used 15 years ending in 2004.

Cameron P. Wake, a climatologist at the Climate Change Research Center of the University of New Hampshire, said a 30-year period would include several cycles of multiyear effects like El Niño, with an underlying assumption that climate is stable and varies around a mean. Warming, on the other hand, “is not variability, it’s a long term trend,” Dr. Wake said. “I would say the U.S.D.A. doesn’t want to acknowledge there’s been change.”

Kim Kaplan, a spokeswoman for the department’s Agricultural Research Service, defended the decision to use 30 years, saying the longer time period would strike a balance between weather, which can vary greatly from year to year, and climate. The new map, which Ms. Kaplan said would be released in “the near future,” will also account for elevation, slope and wind exposure for the first time.

“It will be more precise and more reflective of real-world conditions than we’ve ever been able to do before,” she said.

But frustration from tree planters who needed an accurate guide immediately prompted the Arbor Day Foundation not to wait on the Agriculture Department, said Woodrow L. Nelson, the chief spokesman for the foundation.

Still, landscapers did not respond to the warming trend with alarm, Mr. Nelson said. “It was actually much more of a positive thing,” he said. “It’s kind of like a tree planter in Pennsylvania having some success with flowering dogwoods, where that hadn’t been a possibility 20 years ago.”

Gardeners have always had a penchant for pushing the limits, and some are banking on a combination of their own expertise and climate change to keep their plants alive. John Denti, the greenhouse manager at the University of North Carolina Botanical Gardens in Charlotte, is tending 50 varieties of palm trees in his yard. Some grow easily in the region; others are responding well to Mr. Denti’s special techniques, like wrapping them in strands of Christmas tree lights to keep them warm; and still others are gambles.

“I surely believe in global warming, so I’m planting a lot of marginal stuff and hoping it gets warm enough,” Mr. Denti said.

Some experts said global warming was affecting gardeners in another way, by raising awareness. In the Atlanta area, where in recent years watering has been restricted, nurseries and landscapers note a growing interest in drought-resistant plants and xeriscaping — landscaping that requires minimal water.

Nationally, the use of products like organic fertilizer, which requires less energy to produce than conventional fertilizer — and thus results in fewer emissions of heat-trapping gases — is ballooning, with some manufacturers reporting a doubling in demand each year.

Gardening and do-it-yourself magazines have begun to popularize rain gardens, which collect rainwater in barrels or shallow basins that are part of the landscaping. And mainstream publications like Martha Stewart Living and Better Homes and Gardens have advocated cutting back on gasoline-powered lawnmowers and blowers in favor of greener machines like rechargeable or push mowers, which come in sleek new lightweight designs.

Environmentally gentle gardening choices go hand in hand with hybrid cars, compact fluorescent bulbs and “An Inconvenient Truth,” the Oscar-winning documentary with Al Gore, said Mary Pat Matheson, the executive director of the Atlanta Botanical Garden. “Only in the last year has it even been accepted that it’s really happening,” Ms. Matheson said. “Awareness is starting to turn into action.”

In many instances, consumers are a step ahead of the market. Elaine Morgan, 64, said she was researching drought-resistant plants and “steppables,” low ground cover which requires less maintenance and water than grass but can stand to be trod upon, for her home in Duluth, an Atlanta suburb. Ms. Morgan said she was using more organic products and fewer pesticides.

But she was having trouble finding a lawn care company that would meet her environmental standards. Picking her way through the lavender varieties at Habersham with a critical eye, she said, “I’ve gone through six landscapers.”