A warming climate is evident in the latest winter plant hardiness map

CHAMPAIGN — Changes in the new 2023 U.S. Department of Agriculture’s plant hardiness map are strong evidence of global warming, according to Illinois State Climatologist Trent Ford at the Illinois State Water Survey.

Plant hardiness zones represent the best estimate of the average annual extreme low temperature. The newest hardiness zone map is based on weather data from 1991 to 2020, and like earlier versions, is divided into 13 zones across the United States.

The most obvious change in Illinois in the past 10 years is a northward progression of zones 6a, 6b, and 7a, indicating average annual extreme minimum temperatures from -10 to 5 degrees are creeping upward. The average annual extreme low temperature of -10 degrees migrated 60 to 70 miles north, from around Springfield 10 years ago to around Peoria in the newest map.

An extreme minimum temperature of 0 degrees migrated 40 to 50 miles north from Cairo to the St. Louis Metro East area in the 2023 map. For the first time, a minimum temperature of 5 to 10 degrees appears in the southern tip of Illinois.

Winter nighttime low temperatures have increased at a faster rate than daytime high temperatures, Ford said. Also, the most extreme cold winter nighttime temperatures have increased faster than the milder winter temperatures. These changes have noticeably decreased the frequency and intensity of extreme cold and the risk of cold exposure for plants.

There are benefits of less extreme winter cold, including a reduced risk of cold injuries to perennial crops and natural ecosystems. However, warmer winters may be more conducive to certain species of non-native, invasive plants, pests, and plant and animal diseases and vectors. Recent examples of impacts from changing environment, habitat, and climate in Illinois, including growing incidences of ticks and the crop disease tar spot, have been seen.

The transition to a warmer winter climate in Illinois comes with large year-to-year variability, which makes conservation and agricultural management more demanding. Significant damage to Illinois peaches from extreme cold in an otherwise mild winter last year is an example of this variability and its problems.

“Overall, winter climate change in Illinois is messy and will continue to challenge farmers, gardeners, conservation professionals, and anyone else working hard to grow plants in the prairie state,” Ford said.

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