The U.S. Department of Agriculture has released an updated Plant Hardiness Zone Map that is more accurate and timelier than previous versions, according to Peter K. Bretting, Crop Production and Protection National Program Leader at the USDA.
Plant hardiness zone designations represent what’s known as the “average annual extreme minimum temperature” at a given location.
The scientist leading the updating of the map was Christopher Daly, founding director of Oregon State University’s PRISM Climate Group.
The new plant hardiness map incorporates data from 13,412 weather stations, compared to the 7,983 that were used for the 2012 edition. “That’s a 68 percent increase” in data, Daly said.
The map is based on 30-year averages (1991–2020) for the lowest annual winter temperatures within specified locations. The 2012 edition was based on averages from 1976 to 2005.
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.”
The latest map includes information from the latest digital mapping technology called PRISM, which helps capture data from mountains and valleys.
“The map is based on the plant hardiness statistic over a 30-year period, means that we have 30 numbers — one for each year,” he said. “And if you look at difference between 1976-2005 average used in the 2012 version of the map, and 1991-2020 average used in this version, the average temperatures increased an average of 2.5 degrees,” he said.
“Overall, the 2023 map is about 2.5 degrees [Fahrenheit] warmer than the 2012 map across the conterminous United States,” Daly said. “This translated into about half of the country shifting to a warmer 5-degree half zone, and half remaining in the same half zone. The central plains and Midwest generally warmed the most, with the southwestern U.S. warming very little.”
“If you’re on the edge of a zone it doesn’t take many degrees to move your zone,” Daly said, 1–5 F.
“The changes in Oregon weren’t very dramatic,” Daly said. “The Willamette Valley had a half zone shift around the Portland area. Depending on whether or not you’re on the edge of zone, it could have pushed you into another zone. The area between Salem and Portland had a half-zone change.”
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 and others who grow and breed plants. The website can be reached by clicking here.
