This month, the U.S. Department of Agriculture (USDA) released a new version of the Plant Hardiness Zone Map, updating this tool for the first time since 2012. USDA’s Plant Hardiness Zone Map helps gardeners determine which plants are most likely to thrive in their garden, given their winter. Jointly developed by USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group, the new map is more accurate and contains greater detail than prior versions.
It is very easy to use, simply enter your zip code. I entered mine and I am still in Zone 7b, no change. But 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 degrees Fahrenheit; however, some locations experienced warming in the range of 0-5 degrees Fahrenheit without moving to another half zone.
You may jump to the conclusion that this is due to climate change, but it is not. Temperature updates to plant hardiness zones are not necessarily reflective of global climate change because of the highly variable nature of the extreme minimum temperature of the year, as well as the use of increasingly sophisticated mapping methods and the inclusion of data from more weather stations.
Also, temperature data from a more recent time period was used. The 2023 map 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. The annual extreme minimum temperature represents the coldest night of the year, which can be highly variable from year to year, depending on local weather patterns.
The 2023 map is based on 30-year averages of the lowest annual winter temperatures at specific locations, is divided into 10-degree Fahrenheit zones and further divided into 5-degree Fahrenheit half-zones. Like the 2012 map, the 2023 web version offers a Geographic Information System (GIS)-based interactive format and is specifically designed to be user-friendly. The 2023 map incorporates data from 13,412 weather stations compared to the 7,983 that were used for the 2012 map. Furthermore, the new map’s rendering for Alaska is now at a much more detailed resolution (down from a 6 ¼ -square-mile area of detail to a ¼ square mile).
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). The designations do not reflect the coldest it has ever been or ever will be at a specific location but is the average lowest winter temperature for the location over a specified time.
As with the 2012 map, the new version has 13 zones across the United States and its territories. Each zone is broken into half zones, designated as “A” and “B.” For example, zone 7 is divided into 7a and 7b half zones.
In addition to the map updates, the Plant Hardiness Zone Map website was expanded in 2023 to include a “Tips for Growers” section, which provides information about USDA ARS research programs of interest to gardeners.
Although a paper version of the 2023 map will not be available for purchase from the government, anyone may download the new map free of charge and print copies as needed.
