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January 9, 2007

Climate-Change Gardening — 'Pushing the zone'


What with spring-like weather beginning the year in parts of the U.S. like mine that usually have had at least some snow by now, gardeners are being forced to rethink their plantings.

Bart Ziegler, in a January 6, 2007 Wall Street Journal article accompanied by the revised Plant Zone Map (top) put out by the National Arbor Day Foundation, addressed the new weather; his story follows.

    Climate-Change Gardening

    Shifting weather alters the equation for spring planting; 'pushing the zone'

    This weekend I'm going to do something I've never done before in January: step outside and garden at my house in upstate New York. Many other people from Missouri to Maine will do the same thing, taking advantage of a freakishly warm winter in the Midwest and East.

    Gardeners have long depended on the hardiness-zone map from the U.S. Department of Agriculture to decide which plants are likely to survive winter where they live. But now, as spring garden catalogs are crowding mailboxes, the old familiar zone numbers are being challenged. For one thing, a conservation group has just released its own revised hardiness map that moves many areas into a warmer zone. Then there's the rising popularity of what experts call "zone pushing" — trying plants that aren't supposed to survive winter in a given region.

    So gardeners who are willing to gamble a little may be able to enjoy plants the USDA map puts off limits. For instance, the stunning new dark-purple rose on the cover of the Wayside Gardens spring catalog, called Night Owl, is rated hardy only to USDA Zone 5, but gardeners in much of Vermont and the upper Midwest, officially in colder Zone 4, might want to give it a try.

    Some of this is contentious, coming as it does at the intersection of gardening and politics. The National Arbor Day Foundation, which published its updated hardiness map two weeks ago, is largely devoted to tree planting. But its map, based on 15 years of data from government weather-monitoring stations, has resonated with people concerned about global warming. There's been controversy since 2003, when the USDA sponsored a revised map, created by the American Horticultural Society, that indicated a similar northward creep of warmer-winter zones. The USDA ultimately rejected the map, explaining in part that it was based on too few years of data — but not until the horticultural society had already released it online and in its magazine. The government says it plans to issue a new map but won't say when.

    But many gardeners don't need a map to tell them winters have been milder. George Arndt of Harvard, Mass., says his peonies have been flowering at least a week earlier in spring — after 30 years of blooming almost the same day each year. Now, Mr. Arndt says he's trying to grow vegetables such as scallions all winter. "The warmer weather definitely opens up new opportunities," he says.

    The horticulture industry for the most part hasn't changed its recommended growing zones; suppliers worry that the plants may not survive if the warming trend reverses. Yet some garden centers are selling plants that aren't rated for their zone — and customers are asking for them. "This is happening everywhere, particularly in the Northeast and Midwest," says Joe Gray, a general manager at Hines Horticulture, one of the country's largest wholesale growers. "Everybody seems to be stretching their plant hardiness zones."

    For instance, Mr. Gray says, some gardeners in the Philadelphia area have been growing camellias, a flowering bush formerly confined to the South. "They can sneak by with the mild winters we've had. The neighbors see them and want them and ask their garden center for them," he says. Mr. Gray adds that Hines doesn't refuse to sell retailers plants not rated for their USDA zone, but it warns them about the risk.

    Alice Longfellow, president of Longfellow's Garden Center in Centertown, Mo., says the warmer climate "has allowed us to have more diversity in what we can sell." According to the updated map, her area now is in Zone 6. That means she can safely sell boxwood. "Fifteen years ago boxwood would burn out in the winter," she says.

    The bottom line: You may want to experiment, but first do some research. Compare the two hardiness maps at arborday.org and consult a local garden center; staff there should know which out-of-zone varieties have been successful. If you decide to try some, plant them in the warmer parts of your yard, such as next to the south side of the house or on the sunny side of a stone wall. Such "microclimates" can be a full growing zone warmer.

    Some plant-industry experts caution about pushing zones. "There is no question that we are experiencing zone creep, which is reflected in people's orders," says Tony Avent, co-owner of mail-order retailer Plant Delights Nursery. But Mr. Avent says gardeners shouldn't take chances unless they're willing to lose plants. "Anyone who has studied historical climate change understands that we swing back and forth from cold to warm."


Compare the 1990 map (below)


to that up top to see what "pushing the zone" means.

Even better is the map below,


with pink or red zones showing the effects of warming nationwide during the 16-year interval between maps.

January 9, 2007 at 12:01 PM | Permalink


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