USDA Plant Hardiness Zone Map

The intent of the hardiness zone map has always been as a guide to gardeners. In a general way, it allows us to standardize how we think and talk about plant hardiness. So it’s also useful to other plant growers and researchers, and it serves as a basis for the USDA Risk Management Agency’s crop insurance standards program. This most recent version was jointly developed by USDA’s Agricultural Research Service and Oregon State University’s PRISM Climate Group. It’s available at http://planthardiness.ars.usda.gov/PHZMWeb/# for access.

The map is characterized by a series of colorful bands that cross the country from east to west. These represent temperature “zones” that differ in10° F increments; each zone is further subdivided into two 5° F temperature increments (designated “a” and “b”, with “a” being cooler than “b”). These incremental temperature zones represent average annual minimum temperatures. They do not indicate the coldest temperature ever recorded or that will ever occur in an area.