Rethinking the New Plant Hardiness Zone Map

The USDA map released earlier this year is out of date. While the planet average is only 0.8 degrees C warmer, US Winter temperatures are 2 degrees above preindustrial levels.

Nir Krakauer, an assistant professor of civil engineering at the Grove School of Engineering at City College of New York, has overhauled the U.S.D.A.’s hardiness map to better account for recent temperature changes. Unlike the U.S.D.A., which came up with its planting zones by using average annual minimum temperatures from 1975 to 2005, Mr. Krakauer looked at long-term temperature trends, including recent data that shows that winter temperatures are increasing more rapidly than summer temperatures. His results were published this week in Advances in Meteorology.

According to his calculations, about one-third of the country has already shifted half-zones by comparison with the map, and more than one-fifth has shifted a full zone.

“We talk about dangerous climate change as being two degrees centigrade [3.6 degrees Fahrenheit] above pre-industrial values,” Dr. Krakauer explained. “Over all, we are only at about 0.8 degrees warmer today, but these results show that U.S. winter temperatures, at least, have already reached that ominous two-degree mark.”

Dr. Krakauer has also created an online calculator where anyone can plug in a longitude and latitude and see the adjusted temperature change. In New York City, for example, his calculator shows that the minimum winter temperature is 1.5 degrees Celsius warmer (2.7 degrees Fahrenheit warmer) than suggested by the hardiness map. [emphasis mine]