Last week, the USDA Agricultural Research Service released the updated version of its Plant Hardiness Zone Map, which was last updated in 1990. The new version is based on climactic data collected between 1976 and 2005.
The most interesting change to the map is that just over half of all hardiness zones were bumped up at least a half a zone higher in this version than they were in the previous one, and many zones were bumped up by a full zone. Warm-climate gardeners may find themselves in one of the two new zones (zones 12 and 13) created by the USDA to reflect very warm zones in which the average low seldom falls below 50 degrees Fahrenheit. Via the USDA web site:
"Compared with the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new PHZM is generally one half-zone warmer than the previous PHZM throughout much of the United States, as a result of a more recent averaging period (1974–1986 vs. 1976–2005). However, some of the changes in the zones are the results of the new, more sophisticated mapping methods and greater numbers of station observations used in this map, which has greatly improved accuracy, especially in mountainous regions."
The new map is also more user-friendly. For example, you can now search by ZIP-code to find out exactly what zone your garden is in, and, as mentioned above, the map more accurately depicts anomalies such as the increased warmth that comes from living near a major metropolitan area.
What It Means for Gardeners
Most of what's on the new map isn't really news to avid gardeners. Many of us push our zone in our never-ending quest to grow MORE, or to grow something different from the rest of the neighborhood. In addition to hardiness zone, it pays to keep in mind that every garden contains its own microclimates. It's a good guideline to follow when ordering plants or seeds, but you never really know if something will thrive in your garden until you try it. If your zone has changed, you won't need to do anything drastic; keep growing what has been growing well for you. If you've been considering growing something that wasn't previously thought hardy in your zone, you may want to give it a try.
Check out the updated Plant Hardiness Zone Map here.