"Feeling Warmth, Subtropical Plants Move North"

ATLANTA — 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.”

...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.”