In November, the USDA updated its plant hardiness zone map, which helps growers determine which plants will likely thrive in a location.
The USDA Agriculture Research Service and Oregon State University’s PRISM Climate Group jointly updated the map. The new version is more accurate and contains greater detail, the agency said.
USDA said the new map uses 30-year averages of the lowest annual winter temperature at a specific location to divide the U.S. into 10 zones and five half-zones.
“The map was updated now for numerous factors, including the release of ‘U.S. Climate Normal’ data which are published each decade and provided a current data set through 2020,” Peter Bretting, ARS’ national program leader for crop production and protection, told The Packer. “Because the Plant Hardiness Zone Map takes into account 30 years of data, an updated map would ensure the Plant Hardiness Zone Map is an adequate reflection of contemporary winter low temperatures as compared to the 2012 map (which used 1976-2005 records), due to broader changes of the earth’s climate.”
The half-zones were created based on average extreme minimum winter temperatures to represent the colder and warmer portions of each of the 10 zones. About half the country shifted into the next warmer half-zone, according to the USDA.
“Averaged across the entire country, extreme low winter temperatures increased by approximately 2.5 degrees Fahrenheit, which shifted about half the country into a warmer half-zone (for example, from zone 5b to 6a, or zone 8a to 8b),” Bretting said. “Over time, we expect that future iterations of the map will see zone boundaries continue to move northward, and plant varieties that are marginally hardy might become more adaptable to a site over the time scale of decades.”
The Plant Hardiness Zone Map, available at PlantHardiness.ars.usda.gov, features a geographic information system-based interactive format. It also includes data from 13,412 weather stations.
“The updated map, like its 2012 predecessor, is a risk-management tool that uses historical data on extreme cold temperatures to predict a plant’s likelihood of winter survival,” Bretting said. “Growers must also take into account other factors when selecting plant material, such wind exposure, snow cover and soil moisture; and these factors’ interaction with low temperatures can modify a plant’s ability to withstand low temperatures.”
