USDA Resists Updating Plant Hardiness Zones to Reflect Warming Trend

The U.S. Department of Agriculture (USDA) awarded a grant to the American Horticultural Society (AHS) to update the map of plant hardiness zones, reflecting the average annual low temperatures across the country. More than three years after AHS completed the revised map, however, USDA has yet to publish it. The map is based on 16 years of data (compared to 13 years used in the previous version, published in 1990), but USDA now claims that a 30-year interval would yield a more reliable map. The AHS map shows how warming climates have pushed the zones slightly northward, causing critics to believe that the Bush administration has refused to publish the map for political, not scientific, reasons. Meanwhile, the National Arbor Day Foundation has published a new hardiness map, which is available online at www.arborday.org/media/zonechanges.cfm.