22 Years Later, USDA Releases New and Improved Map

By Nicole Heller

Twenty-two years after its last revision, the U. S. Department of Agriculture finally released an update to the Plant Hardiness Zone map, which serves as a kind of detailed road map for gardeners, farmers, foresters, nurseries, or just about anyone who works with plants.

The Plant Hardiness Zone Map, a useful tool for gardeners and researchers, was updated for the first time since 1990. Credit: U.S. Department of Agriculture.

Last published in 1990, the new map illustrates the effects of global warming across the country, and in a less abstract vein, how increases in temperature are impacting what people can plant, harvest or grow in their backyards.

The new color-coded map, which is online, interactive, and downloadable — will surely be a welcome update to the estimated 80 million home gardeners in the U.S., in addition to those individuals and industries that rely on the data for their business. The map is used to set national crop insurance standards and predict locations at risk for weeds and troublesome invasive plants that can cause large economic and ecological impacts. The map divides the U.S. into 26 zones — each in a 5-degree Fahrenheit temperature range — to reflect growing conditions.

What the data shows should come as no surprise. As the planet has warmed, so has the U.S., and some plants and trees may now have longer growing seasons and be able to survive farther north in regions of the country.

USDA spokesperson, Kim Kaplan was quick to say that the map was not hard evidence for climate change and she cautioned gardeners that “just because your zone has changed, what is thriving in your garden today, will thrive tomorrow. Twenty years from now is another question. But don’t go ripping things up and re-planting just because of this new map.”

Changes in the new map are not due to climate changes alone as they also reflect the methodology used to create it. Plant hardiness zones are based on the annual extreme minimum temperature in a location over a particular period of time. The colder the winters, the lower the zone.

The new map was based on temperatures observed from 1976 to 2005, whereas the old map used 1974 to 1986, cooler years than those in 1990s and 2000s, when the hottest years on record have occurred.

What the new map does that the previous one did not is use an algorithm developed at University of Oregon, PRISM group, that takes into account many physical variables to estimate climate in the places where there is no data. So changes in the location of zones between the 1990 and 2012 map in part reflect more accurate climate estimates beyond actual changes in climate. This new method is particularly more accurate in mountainous regions, and so while most regions shifted to warmer zones, some regions have shifted toward cooler zones, because elevation was not properly taken into account.

Attempts to update the Hardiness Zone map have been done before, but this is first to be officially adopted by the USDA. An updated map was released by the National Arbor Day Foundation in 2006 based on climate data from 1990-2005. And the American Horticultural Society released a map update in 2003 based on data from 1987-2002. These maps showed many areas shifting one to two zones. According to Kaplan, the earlier edition was not adopted or reviewed by the USDA because it was not produced in ways that were web-friendly or useful for mapping and research. Further, those updates were not taking advantage of advances in methods to estimate climate, like the PRISM work used in the current map.