The USDA today released a new version of it’s hardiness zone map based on climate data from 1976 thru 2005. The map is the first update from the agency since 1990, and incorporates not only the data from over 8,000 monitoring stations, but also integrates GIS technology to improve both user experience and the accuracy of the map, which now partially takes into account variations from terrain, slope,and large bodies of water.
Plant hardiness zones represent the average extreme minimum temperatures at a location. They do not necessarily reflect the coldest a location has gotten or will get, but rather the average lowest winter temperature – the most critical of several factors in determining what plants will survive in a given region.
In a press conference held this morning, USDA officials outlined details of the effort and provided a demonstration of the maps features. While they noted the map is the most sophisticated it has ever been, speakers were quick to mention that the map is a guide and not absolute truth. Each site has its own unique microclimates and unique circumstances that determine its capacity to support various plants. As William Miller, professor of Horticulture at Cornell notes,
“One thing I have learned from growing plants in many locations in the U.S. is that plants can’t read! Experienced gardeners are always pushing the envelope by trying new plants, and especially those that ‘aren’t hardy’ in their area.”
“Aside from global warming or simply more and better data leading to a more accurate map, there is always microclimate variation in any locale and a few feet alteration in planting site, better drainage, locating a plant around a corner, presence of snow cover, mulch, or protection from wind can make a huge difference in winter hardiness.”
Perhaps the most compelling new feature of the site is that users can browse and search an interactive map that shows detailed information for local regions. The old map was simply a static image, not originally design for web use. The site also offers numerous print versions of the map that users can download and use.
The new map comes long overdue, as the 1990 map used data from 1974 to 1984. In 2003 the American Horticultural Society released a draft version of a new map based on data from 1986 to 2002, which showed dramatic northward movement of hardiness zones. USDA pulled this map from circulation and had said they would release an updated map in 2005. Instead, in 2006 the Arbor Day foundation issued a map noting that indeed climate zones had shifted significantly from 1990 to 2006, implying that the climate was warming. The map released today by USDA confirms many of these trends.