Plant hardiness maps trigger global warming debate

Plant hardiness maps trigger global warming debate

by Jo Ellen Myers Sharp

Weather experts, researchers, plant growers, farmers and gardeners have been conducting meetings for more than four years trying to develop the latest issue of the hardiness zone map for the U.S. Department of Agriculture. The USDA introduced its hardiness zone map in 1960, and the current version was updated in 1990. When a new map expected to be issued this year, horticulture experts expect most Midwestern gardeners will find themselves in a half- to full-zone warmer than where they are now.

For gardeners, the USDA Plant Hardiness Zone Map is an initial guide for plant selection with zones frequently referenced on plant tags and descriptions. Each USDA hardiness zone covers a 10-degree Fahrenheit range that reflects the lowest average annual winter temperatures for an area. The zones are subdivided into "a" and "b" subzones that represent a 5-degree difference within each 10-degree zone. With the new map, gardeners in Zone 5, with the lowest temperatures ranging from -10 to -20 F may be redesignated to Zone 6, where the lowest temperatures range from 0 to -10 F.

But a new map won't come without debate. A 2006 National Arbor Day Foundation hardiness zone map stirred controversy because it described many climate zones, including much of Illinois, Indiana and Ohio, shifting to warmer designations. Citing a consensus of climate scientists, the foundation attributed the changes to global warming. Similar to the Arbor Day map, the American Horticulture Society's proposed update to USDA hardiness zones in 2003 showed a trend towards warmer climates. The USDA rejected the proposal.

"Our fear is that consumers will buy plants rated for warmer zones and the plants will die," says Tony Avent, owner of Plants Delight Nursery in Raleigh, N.C., and a USDA map study group member. But Dave Tyznik, owner of Planter's Palette garden center in Winfield, Ill., says zone changes shouldn't make a big difference because the gardener's climate will remain the same, just carry a new rating.

Even with map changes, gardeners shouldn't assume they can successfully grow plants rated for warmer zones. Steven Still, executive director of the Perennial Plant Association, an international organization of nurseries, plant breeders and educators says the changes may encourage gardeners to experiment with plants designated hardy for warmer zones. Last year in Columbus, Ohio, Still planted 12 Crocosmia 'Lucifers,' a red, iris-like perennial rated hardy for Zone 6, but marginally hardy for the area's Zone 5 rating. He says only three survived the winter.

Avent says the map is only the first thing gardeners should consider when selecting plants. Other considerations, such as summer temperatures, moisture needs, humidity tolerance and required sunlight, are also important.


Jo Ellen Meyers Sharp, freelance writer, author, speaker and photographer, is an Advanced Master Gardener and a regional director of the Garden Writers Association. A self-proclaimed trial-and-error gardener, she also enjoys spending time with her dog, Penn, and cat, Cowgirl.


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