The U.S. Department of Agriculture (USDA) released a new version of its Plant Hardiness Zone Map (PHZM) last Wednesday, updating this valuable tool for the first time since 2012. 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.
When compared to the 2012 map, the 2023 version reveals that about half of the country shifted to the next warmer half zone, and the other half of the country remained in the same half zone. That shift to the next warmer half zone means those areas warmed somewhere in the range of 0˚- 5˚ F; however, some locations experienced warming in the range of 0˚- 5˚ F without moving to another half zone.
These national differences in zonal boundaries are mostly a result of incorporating temperature data from a more recent time period. The 2023 map is based on 30-year averages of the lowest annual Winter temperatures at specific locations, is divided into 10˚ F zones and further divided into 5˚ F half-zones. It includes data measured at weather stations from 1991 to 2020. Notably, the 2023 map for Alaska is “warmer” than the 2012 version. That’s mainly because the new map uses more data representing the state’s mountain regions where, during winter, warm air overlies cold air that settles into low-elevation valleys, creating warmer temperatures.
It’s available online at https://planthardiness.ars.usda.gov/.