The U.S. Department of Agriculture (USDA) recently released a new version of its Plant Hardiness Zone Map (PHZM), updating this valuable tool for gardeners and researchers for the first time since 2012. USDA’s Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The new map—jointly developed by USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group—is more accurate and contains greater detail than prior versions.
The new map moves about half of the United States, including much of Colorado, into a warmer growing zone. Large swaths of the Denver metro area moved from zone 5b to 6a, changing the average lowest temperature from 15 degrees below zero to 10 degrees below zero, according to the agency.