New USDA Hardiness Zone Map Shows Warming Trend

Enlarge Image Image Gallery Gardeners across America are making adjustments as warmer, more erratic weather takes hold. iStockphoto.com/Elena Elisseeva

The U.S. Department of Agriculture recently unveiled a new Plant Hardiness Zone Map that shows most U.S. growing regions have warmed some 4 to 5 degrees since the last map was created in 1990. For many gardeners and farmers, that temperature difference has meant an earlier start and a later finish to the growing season, but it also has come with some unpredictable weather.

The new map is already garnering positive reviews in the growing community for being more user-friendly and more accurate than previous maps. For the first time, growers can go online and click a region on the map or enter a zip code to get a closer look at local plant hardiness zones. The device demonstrates a more-nuanced approach to hardiness zoning, one that takes elevation and distance from the sea more into account. And it confirms the hunches that millions of gardeners and farmers have had about what can grow, says Chris Daly, an engineering professor at Oregon State University and director of the PRISM Climate Group, which created the map in conjunction with the USDA.

“People have been engaging in what has been known as ‘zone-denial’ for such a long time,” Daly says. “The map has really been more of a confirmation of what they know.”

The USDA Plant Hardiness Zone Map has come a long way in recent decades. Typically published every 30 years, it averages a region’s coldest annual temperature. But the data, collected from weather stations, often has been too incomplete to create more than a broad picture of temperatures in a region, Daly says. Regional temperature variations created by heightened elevation or ocean breezes had to be ignored by mapmakers for a lack of data.

“These climate maps used to be hand-drawn,” he says. “You’d get something different every time you did it.”

The 1990 map switched methodology by only taking in 12 years of data. It’s unclear why the USDA elected to do this. Daly says he was only able to find a few vague paragraphs about methodology decisions on the 1990 map. One theory is that the mapmakers decided that a nationwide weather fluctuation trending toward cooler temperatures in the 1960s and ’70s might have distorted the results too much to make an accurate map with 30 years of data, says Jan Curtis, a climatologist for the National Water and Climate Center.

While there are more weather stations available to mapmakers, there still aren’t enough to paint an accurate picture of plant hardiness zones, says Curtis, who helped evaluate the PRISM model.

“You’d need millions of stations across the nation. Obviously, we didn’t have that,” he says.

Instead, researchers combined the real data with computer models that can predict what effect elevation and proximity to the coast can have on temperatures. The new map shows much more zone diversity than previous maps. For example, while most of Ohio is solidly in Zone 6a, most of Ohio’s Knox County is now in the colder 5b Zone. The new map is more than 25 times more detailed, Curtis says.