The National Arbor Day Foundation Plant hardiness zones based on 2006 climate, contrasted with 1990 USDA update. The New York Times reports that "[t]he Agriculture Department is in the process of redoing the map itself. But critics have taken issue with the department’s decision to use 30 years of temperature data, saying it will result in cooler averages and fail to reflect the warming climate. The 1990 U.S.D.A. map used 13 years of data; the Arbor Day map used 15 years ending in 2004."
Posted by Susan at 4.5.07