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In the Garden | In the (plant-hardiness) zone | Gardening

The USDA Plant Hardiness Zones have long been a standard in communicating plant suitability across the U.S. Everyone from gardeners and growers to scientists and crop insurance adjusters use these designations to make plant decisions every day. However, the geographic range of these zones is shifting as the effects of our changing climate are being realized.

Back in November, the USDA released an updated map, reflecting the addition of more recent weather data. The shift was a bit of a surprise given the relatively short time frame it reflects.

“The magnitude of change in the new maps really surprised me since it just reflects the last 30 years,” says Illinois State Climatologist Dr. Trent Ford.

The updated maps were released Nov. 15 and are the first update since 2012. They represent data from 1991-2020 collected by 13,412 weather stations across the U.S. In contrast, the 2012 map was built from just 7,983 weather stations with a data set spanning 1976-2005. The larger number of stations included in the most recent map partly reflects the growth of private weather stations that can produce scientific quality weather information.

The Plant Hardiness Zones detailed in the new map define 13 distinct zones across the U.S. and its territories. These zones are delineated based on the average annual lowest minimum temperature. More simply put, the zones represent the lowest temperature each year, on average, for a geographic area. Zones are expressed as 10-degree Fahrenheit zones, noting an average extreme winter low within their specific 10-degree range communicated by the zone number. The numerical zones are further divided into 5-degree half zones indicated by letters “a” and “b.”

When looking at the new maps across the country, about half of the country shifted about a half zone warmer, whereas the other half of the country

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