The USDA Plant Hardiness Zone Map (PHZM) is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The most recent revision was prepared by the PRISM Climate Group at Oregon State University, and was released by USDA in 2012. Based on the average annual minimum winter temperature over a 30-year period, the map divides the country into zones representing a spread of 10°F (zones 1-13), each subdivided into two half-zones with 5°F spreads (e.g., zone 1a and 1b).
