The 2012 version of the USDA Plant Hardiness Zone Map 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 But a researcher contends that this long-awaited map is already outdated, says a recent article in the New York Times.
Nir Krakauer, an assistant professor of civil engineering at the Grove School of Engineering at City College of New York, has overhauled the USDA’s hardiness map to better account for recent temperature changes. Unlike the USDA, which came up with its planting zones by using average annual minimum temperatures from 1975 to 2005, 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 in September 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.