On January 25, 2012 the USDA released an updated Plant Hardiness Zone Map. The new map was jointly developed by USDA’s Agricultural Research Service and Oregon State University's PRISM Climate Group. It is available online by clicking on the link below.
This new map offers a GIS-based interactive format designed to be Internet-friendly. The map website also incorporates a “find your zone by ZIP code” function. Static images of national, regional and state maps have been included as well.
Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time.
The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees Fahrenheit) and 13 (60-70 degrees Fahrenheit). Each zone is a 10-degree Fahrenheit band, further divided into 5-degree Fahrenheit zones “A” and “B.”
Using data measured at weather stations during the 30-year period 1976-2005, zone boundaries on the new map have shifted in many areas—generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the U.S.