The USDA Plant Hardiness Zone map has been updated by the Natioanl Arbor Day Foundation and is now available on its web site, www.arborday.org.
According to the newsletter HortIdeas, “An official revision of the hardiness zone map has been in the works for years, but its release apparently was sidetracked by Bush administration concerns that it might be biased (you guessed it, toward support for the notion of global warming).”
The NADF prepared its own revision of the map in 2002 and posted it in 2004 and has again updated it. According to the Arbor Day Foundation, the map “is based on the most recent 15 years’ data available from more than 5,000 National Climatic Data Center cooperative stations across the United States…. The need to reflect climate change was one of the reasons given by the USDA for the [last officially published] 1990 revision of its original Hardiness Zone Map, first introduced in 1960. The NADF used the updated versions of the same sources of data as had been utilized by the U.S.D.A….”
HortIdeas notes that the new NADF map shows general warming trends. For example, most of Ohio is Zone 5 on the 1990 USDA map, but most is now in Zone 6 — reflecting an increase in the average winter minimum temperatures of about 10°F. (HortIdeas, 750 Black Lick Rd., Gravel Switch, KY 40328; gwill@mis.net)