Charlotte gardeners comment on hardiness map controversy

Charlotte gardeners comment on hardiness map controversy

by Ellen Goff

For most people, gardening is a leisure activity, one that can be creative, relaxing and superbly therapeutic. Yet none of these words can be used to describe the rancor over the 2006 Arbor Day Foundation Hardiness Zone Map and the revised U.S. Department of Agriculture's Plant Hardiness Map due out this year. Frankly, it's gotten so complicated it's a wonder the casual home gardener can sort it all out.

The USDA hasn't published a map since 1990, while the Arbor Day Foundation issued its map in response to gardeners' requests for up-to-date information. It based the zones on the most recent 15 years' data available and, according to the foundation, is proof positive of global warming.

For the first USDA map, released in 1960, isothermal zones were drawn across the country ranging from a cold northern Zone 1 to a warm subtropical Zone 9, each defined by an average of the lowest winter temperatures during the previous 40 years. Plants were assigned to zones where they could best acclimate according to hardiness.

In 1990, National Arboretum Director Marc Cathey developed a new USDA hardiness map based on temperature data from 1974 to 1986. Cathey coordinated with the American Horticulture Society in 2003 to update the USDA map with data from 1986 to 2002.

However, the USDA rejected this map for reasons that remain unclear. Critics believed the new map was dumped because it illustrated a dramatic climate change.

Tony Avent, who owns Raleigh's Plant Delights Nursery and serves on the advisory team assisting the USDA in preparing the latest map, says there's no question that the climate has warmed in the last 10 years. "We've got the data," he says. "[But] the earlier maps didn't account for the 40-year temperature cycle — 20 years of warming followed by 20 of cooling.

"The 1990 map data included a cooling period. The 2003 data included heat extremes through the 1990s. The new map will include a 30-year temperature data set and many more temperature recording stations. But when it comes out, people are going to say the changes are the result of global warming. That's not the whole story."

Arbor Day spokesperson Mark Derowitsch simply says: "Our map is consistent with the consensus of climate scientists that global warming is underway."

Robin Armstrong Glover, Daniel Stowe Botanical Garden curator, says that regardless of your position, "the map is a helpful tool for gardeners, as long as people understand what the designations mean — it's a guideline, not exact."

Ellen Goff is a master gardener and environmental advocate. Aside from writing about and photographing plants, Ellen tends to a 3-acre landscape she shares with her husband, cat and border collie on the shores of Lake Wylie, S.C.


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