On Wednesday, the U.S. Department of Agriculture released its new [Plant Hardiness Zone Map], the national standard by which gardeners can determine which plants are most likely to survive the coldest winter temperatures at a certain location.
This map is developed in conjunction with Oregon State University and replaces the earlier version which was released in January 2012. The map is based on 30-year averages (1991 to 2020) for the lowest annual winter temperatures within specified locations. The 2012 edition was based on averages from 1976 to 2005.
Plant hardiness zone designations represent what’s known as the “average annual extreme minimum temperature” at a given location during a particular time period (30 years, in this instance). Put another way, the designations do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.
According to Christopher Daly, director of the PRISM Climate Group and the map’s lead author, “”Overall, the 2023 map is about 2.5 degrees warmer than the 2012 map across the conterminous United States. This translated into about half of the country shifting to a warmer 5-degree half zone, and half remaining in the same half zone.”
USDA instructions on how to use the map can be found here:
A full page Hawaii map download can be found here: https://planthardiness.ars.usda.gov/system/files/HI300_HS.png