Plant Zone Map Made Obsolete By Rising Temperature
September 14, 2012

Plant Zone Map Rendered Obsolete Thanks to Rising Temperature

April Flowers for - Your Universe Online

Every packet of plant seeds that you buy for your yard or garden has a map on the back showing you what regions to plant the seeds in during what time of the year. The U.S. Department of Agriculture (USDA) creates that little map and the planting guidelines, and a new set was just recently released. According to Professor Nir Krakauer, assistant professor of civil engineering at the City College of New York's Grove School of Engineering, the new map and instructions are already obsolete.

In a recently published article, Professor Krakauer describes a new method that he has developed to map cold-weather zones in the United States that takes rapidly rising temperatures into account. Analyzing recent weather data, he overhauled the Department of Agriculture´s latest plant zone map that was released in January.

The new USDA Plant Hardiness Zone Map, which predicts which trees and perennials can survive the winter in a given region, was a long time coming. The last map came out in 1990, and temperature boundaries have shifted significantly northward since then. Krakauer's calculations, however, show that the zones have moved even further north than the new map suggests.

“Over one-third of the country has already shifted half-zones compared to the current release, and over one-fifth has shifted full zones,” Professor Krakauer wrote this summer in the journal Advances in Meteorology.

What does this mean in concrete terms? Fig trees, once challenged by frosty temperatures above North Carolina, are already weathering NYC winters thanks to changing temperatures and the insulating effect of the metropolis. Camellias, once happiest south of Ohio, may now be able to shrug off Detroit winters.

The USDA map divides the country into zones based on their annual minimum temperatures — frigid dips that determine which plants perish overnight or live to flower another day. Full zones have a minimum temperature range of 10 degrees Fahrenheit and half zones have a 5-degree range.

Krakauer's calculations exposed a weakness in the agency's model, however. The USDA averaged annual minimum temperatures over a 30-year span, from 1976 to 2005, but winters have warmed significantly over that period. Zones are now averaging about 2 degrees Fahrenheit warmer than the USDA's 30-year average.

“What is happening is that the winter is warming faster than the summer. Since [my] hardiness temperatures are based on minimum temperatures each year, they are changing faster than the average temperatures,” Professor Krakauer explained. He found that these lowest yearly temperatures warmed roughly two and a half times faster than the average temperatures.

His analysis also reveals that the country's temperatures are changing unevenly, with more warming occurring over the eastern interior of the country and less in the Southwest.

Professor Krakauer´s technique will allow gardeners and farmers to reassess more frequently which plants they can expect to survive the next year´s winter. “The idea is that you could use this method to keep updating the zone map year by year instead of waiting for the official map — just keep adding new data and recalculate.”

He noted that similar analyses could distinguish long-lasting climate trends — in wind or rainfall, for example — from year-to-year weather variations to distinguish between what some are calling the recent “weird weather" and the natural variations in global weather.