The U.S. Department of Agriculture released an updated version of its Plant Hardiness Zone Map in November, with data provided by Oregon State University's PRISM (Parameter-elevation Regressions on Independent Slopes Model) Climate Group and the USDA's Agricultural Research Service. This map answers the age-old questions for gardeners, landscapers, plant nurseries, and others: What plant zone do I live in and which plants will survive the winter in my zone?
The map has gone through several iterations since it was first published in 1927 by the Arnold Arboretum at Harvard University. That map showed only eight hardiness zones. The USDA first published a PHZM in 1960 with weather data collected from 450 locations, but this map created confusion because it and the still-in-use Arnold Arboretum map used different criteria to create the zones. It wasn't until 1990, when the USDA updated its map with weather data compiled from over 4,500 weather stations that it became the standard over the Arnold Arboretum's map. In 2007, PRISM was awarded the contract to work with the USDA to produce an update of the map.
"We did the 2012 map, as well as the 2023 one," said Christopher Daly, OSU professor of engineering, director of the PRISM Climate Group and lead author of the map. "Before that, the maps were not available in digital form and there were some differences in the way zones were defined and what temperature ranges they encompassed, so it was hard to compare one map to the next."
The 2023 PHZM is based on weather data showing the average annual extreme minimum temperatures derived from a 30-year period (1991-2020) and includes data from 13,412 weather stations, compared to 7,983 for the 2012 version.
"There are always questions about zones changing and the answer is, 'Is that a change in climate,' which it could be, but it could also be we had more data in one area and did a better job of modeling an area," said Daly. "PRISM is a digital modeling system that I started over 30 years ago and have been refining ever since then."
A complex algorithm integrates the data, allowing for contributing factors such as the weather station's elevation and proximity to water, coastlines, mountain tops or valley bottoms.
"PRISM produces a grid cell which is about a half mile on each side and there are tens of millions of cells across the country," said Daly. The cells then incorporate data from weather stations and the 1991-2020 climate normals to model for the extreme annual minimum temperature over the year.
"It's not telling you when to plant in the spring or how hot the summers are," said Daly. "It's telling you on average what's the coldest night of the year that the perennial plant has to live through."
Thirteen climatic zones each with a spread of 10F (zones 1-13) represent a range of -60F to 70F. Each zone is further subdivided into two half-zones with a 5F spread, such as zone 6a and 6b for the Bend area. One feature of the map is to search for plant zones by Zip code.
The PHZM is a guide, not a definitive source for plant survival. Winter acclimation, temperature drops, winter precipitation, cultivars, fertilizer use and microclimates are just some of the factors that affect plant survivability in the winter that the PHZM can't account for, as well as snow cover, summer heat, humidity, soil moisture and spring frosts. "The idea of duration is really important," said Daly. "Some plants will survive a zero temperature for a few hours but not a few days." The map doesn't tell how cold or hot it can be, but what is that average minimum.
"Overall, the 2023 map is about 2.5 degrees 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." Predicting climate change with this map may take a bit of caution, although one would expect the zones to go upward as the climate changes, because of numerous factors outside of the data set.
Additionally, the PHZM targets mainly horticultural plants. "Native plants have pretty much sorted themselves out, such as the plants around Bend have to be pretty cold hardy or they wouldn't have adapted to that area," added Daly.
Besides growers and gardeners, other entities, such as the USDA's Risk Management Agency, which administers the Federal Crop Insurance Program, use this map for different insurance plans and setting policies for nursery losses. Researchers also utilize the map for invasive weeds and insects. The digital version of the PHZM is available online and has several features gardeners and growers will find useful. "Tips for Growers" section provides links and information to USDA research programs and other topics gardeners will find interesting and hopefully help their gardens grow and thrive.
