The USDA Plant Hardiness Zones have long been a standard in communicating the suitability of plants throughout the United States to everyone from gardeners and growers to scientists and insurance adjusters. cultures use these designations to make plant decisions every day. However, the geographic location of these areas is changing as the effects of our changing climate are realized.
Back in November, the USDA released an updated map, reflecting the addition of more recent weather data. The change came as a bit of a surprise given the relatively short time frame it reflects.
“The magnitude of the change in the new maps really surprised me since it only reflects the last 30 years,” says Dr. Trent Ford, Illinois State Climatologist.
The updated maps were released on November 15 and are the first update since 2012. They represent data from 1991 to 2020 collected from 13,412 weather stations in the U.S. In contrast, the 2012 map was constructed from only 7,983 weather stations with a data set covering 1976. – 2005. The larger number of stations included in the most recent map partly reflects the growth of privately owned weather stations that can produce scientific-quality meteorological information.
The Plant Hardiness Zones detailed in the new map define 13 distinct zones in the United States and its territories. These zones are delineated based on the average annual minimum minimum temperature. More simply, the zones represent the lowest annual temperature, on average, for a geographic area. The zones are expressed as zones of 10 degrees Fahrenheit, noting a minimum average winter extreme in their specific interval of 10 degrees communicated by the zone number. The numerical zones are also divided into zones of 5 degrees, indicated by the letters “a” and “b”.
Looking at the new maps across the country, about half of the country has changed to a warmer zone, while the other half of the country has remained almost the same. In areas that have changed, it reflects up to a 5 degree shift towards warmer temperatures. However, USDA notes, “Some locations experienced warming in the 0-5 degree Fahrenheit range without moving into another half zone.” Keep in mind that each zone represents 10 degrees, so a 5 degree change in warmer temperatures may not necessarily drive a zone change for a given area.
“In Illinois, the most obvious change between the new hardiness zone map and the 2012 map is a northward progression of zones 6a-6b and 7a,” says Dr. Ford. “The boundary between zones 5b and 6a, which represent an annual mean extreme minimum temperature of -10°F, has migrated 60 to 70 miles north, from around Springfield in the 2012 map to around Peoria in the map of 2023”.
On the 2012 map, central Illinois is divided from north to south by the dividing line between zones 5b and 6a. On the new map, most of central Illinois is now in Zone 6a, with Zone 6b moving north to take in parts of Coles, Moultrie and Shelby counties.
Gardeners use hardiness zones to determine how cold it can get in their area and to assess which plants will be able to tolerate the expected extreme low temperatures. Plant hardiness zones are widely communicated, including on everything from plant tags in nurseries to guides or online resources.
“One of the most consistent changes in Illinois’ climate that is directly related to human-caused global warming is the warm winters,” explains Dr. Ford. “In fact, the average temperature of the climatological winter (from December to February) has increased faster than any other season in the last century.
Warm winters can reduce stress on plant populations, helping some landscape plants to thrive, but they also bring negative impacts, such as expanding populations of invasive species from the south, introducing a new pest pressure on crops or ecological competition in natural areas.
“The USDA hardiness zone maps represent average winter extreme temperatures, but do not explicitly account for year-to-year variability, and the transition to a warmer winter climate in Illinois comes with great year-to-year variability per year,” notes Dr. Ford.
Most landscape plants are hardy in a variety of zones, so interpreting relative hardiness and vigor in your particular area may take some critical thinking. It is safe to say that a plant known to be hardy from zones 4-8 will thrive in Zone 6, while a plant that is only hardy in Zone 6-8 may struggle if an extreme winter temperature drops below average expected due to year-to-year variability.
Dr. Ford concludes by saying, “Overall, winter climate change in Illinois is messy, and will continue to challenge farmers, gardeners, conservation professionals, and all those who work hard to grow plants in the state. of the prairie”.
More information
Are you interested in collecting weather data? Consider participating in CoCoRaHS, which provides training and materials needed to collect data from home. More information is available at go.illinois.edu/cocorahs.
