Plant hardiness zone

A geographic area defined based on average annual minimum winter temperatures, developed by the USDA Agricultural Research Service to guide movement of perennials based on cold-tolerance. (http://planthardiness.ars.usda.gov/PHZMWeb/). Each zone extends across the country and spans a range of 5° F. These zones do not represent the absolute lowest temperatures that may be reached. Neither do these zones represent the growing season length which is also biologically important, but the two traits are often correlated. The hardiness zones are updated periodically as new weather data become available. These zones have been used to guide the transfer of landscape plants for more than 50 years. They are the basis of some provisional seed zones.