U.S.A.

For gardeners sad to see the summer drawing to a close, there’s some comfort to be drawn from the fall planting season for perennials, trees and shrubs, which is just around the corner. What’s more, there’s the novelty of this year’s updated Plant Hardiness Zone Map, released early this year.

The previous version of the map was issued in 2003, but the agency fielded so much criticism over the ways in which it incorporated climate change into the equation — too little and too much — that the map was withdrawn, and professional and amateur planters were left with guidelines dating back to 1990.

The 2012 version shows that planting zones have been shifting northward as winters become more mild. But a researcher contends that this long-awaited map is already outdated.

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. (Each zone has a minimum temperature range of 10 degrees Fahrenheit, and half zones have a 5-degree range.)

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.

“It’s difficult to give specific examples of plants you can grow somewhere that you couldn’t before, because plant varieties are also changing so much,” Mr. Krakauer said, “But generally speaking, you can now grow varieties 100 miles further north than you could about 30 years ago.”

“I’m a gardener, so my results interest me on a personal level, but I was also really struck by just how much temperatures have changed,” he said.

“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.”