The Potluck

New plant hardiness maps reflect a winter warming trend

By: - November 16, 2023 2:17 pm

A sphinx moth visits a sunflower in a northern Minnesota garden (Christopher Ingraham/Minnesota Reformer)

The U.S. Department of Agriculture this week updated its official map of plant hardiness zones for the first time in more than a decade. 

The map is a familiar resource for gardeners and plant growers who use it as an indicator of what types of plants can be successfully grown outdoors in a given locale. It’s also used by horticultural researchers and the USDA regulators who set certain crop insurance standards. 

The map is “based on 30-year averages of the lowest annual winter temperatures at specific locations,” the USDA explains, and is “divided into 10-degree Fahrenheit zones and further divided into 5-degree Fahrenheit half-zones.” 

Zone 4a, for instance, corresponds to an average annual extreme low temperature between -25 and -30 Fahrenheit, while 4b can expect temperatures between -20 and -25. Those extremes are often the factor determining whether an overwintering plant can survive until the spring.

While it’s not primarily a tool for measuring climate change, the 2023 update reflects an ongoing winter warming trend. Roughly half the country has moved up a half zone since the last release in 2012, indicating a warming of up to 5 degrees Fahrenheit. Some of that apparent shift is due to measurement improvements. 

The change is especially prevalent in Minnesota, as this side-by-side comparison of the 2012 and 2023 maps shows.

Zone 5 has crept into much of the southernmost part of the state, which had previously been categorized as Zone 4. The core of the Twin Cities metro area is now squarely within Zone 5 as well.

In the northern part of the state, warmer temperatures are marching north and east. In 2012 much of the northern border region was classified as Zone 3a, but that’s no longer the case.

Comparisons with older versions of the map present even more stark contrasts, although they’re less reliable due to measurement improvements over the decades. In the 1990 map, for instance, the northernmost edge of Zone 5 remains roughly 100 miles below Minnesota’s southern border, and parts of the north are listed as Zone 2.

All told, the latest map suggests winter extreme lows in Minnesota are now between 5 and 10 degrees warmer than assumed back in the 1980s and 1990s. The implications for the environment and native ecosystems verge on the catastrophic, but gardeners may at least comfort themselves with a wider selection of perennials than in years past. 

Fall is a great time to plan your Minnesota cactus garden.

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Christopher Ingraham
Christopher Ingraham

Christopher Ingraham covers greater Minnesota and reports on data-driven stories across the state. He's the author of the book "If You Lived Here You'd Be Home By Now," about his family's journey from the Baltimore suburbs to rural northwest Minnesota. He was previously a data reporter for the Washington Post.