New USDA Plant Hardiness Map Reflects Changing US Climate

For about a decade, the US Dept. of Agriculture has been working to update its map of plant hardiness zones. Those zones guide many growers and groups on what the average cold temperature has been — and thus what plants will thrive — in any given area.

The effort has stalled numerous times, amid allegations of political interference by people who don't want to see documented evidence of climate shifts. In addition, there have been disagreements over how long a time period to incorporate to calculate an average low temperature, technical complications related to Web hosting for a highly detailed, interactive map, and challenges in crafting a Web site that can handle very high online use from an immense potential audience. That audience includes about 80 million gardeners, farmers, nurseries, landscape architects, agencies and organizations dealing with crop insurance, researchers and organizations trying to cope with invasive plants and pests, state and local governments recommending appropriate local trees and other plants, and others.

But the wait is over. On Jan. 25, 2012, USDA announced the launch of the new map. It's based on data from 1976-2005, and is the first official revision since the 1990 update (which was based on data from 1974-1986, and which updated its 1965 predecessor).

• Plant Hardiness Zone Map; 1990 map, for comparison (at bottom); Kim Kaplan, 301-504-1637.

The interactive map is much more detailed than its predecessor. Based on temperature data from about 8,000 stations, it contains 26 zones, each with a 5-degree range, compared to the 20 zones on the old map (18 of which were in 5-degree increments, plus the warmest and coldest zones covering the remainder of the spectrum).

You can enter a zip code and quickly identify its zone. For more detail, on the interactive map you can get an average low temperature calculated to a tenth of a degree for any place you click. However, such seeming precision needs to be kept in context, since there still are microclimates at very small scales, and, by definition, the temperature in any location may get much colder than the average, or may not get as cold as the average, adversely affecting certain plants with either condition.

There are substantial zone shifts in some areas, with much of the country now labeled a full 5-degree-zone warmer than in 1990. Many people familiar with plant materials have been saying for years that such shifts on paper have been needed to reflect the migration of plant conditions on the ground.

At the same time, the new map sometimes indicates a cooler zone. That occurs in some areas because the new algorithm for the calculations accounts for microclimate influences such as elevation, nearby large bodies of water, or relative terrain location such as ridge top or valley bottom.

In addition to overall shifts in location for a zone, there now are many more "islands" of zones within a warmer or colder zone, thanks to the added detail available with the new methodology and data.

For your audience members with broadband service, you might mention they have full access to all the detailed information. For those with limited online access, there are maps at the state, regional, and national level that aren't interactive and have a relatively small file size, and can be downloaded and manipulated in a few ways.

For your media coverage, there are very high-resolution maps in numerous formats.

For more background on plant hardiness maps, historical perspective on the many efforts made in the past decade to create this map, and a number of organizations to contact, see the TipSheet of May 10, 2006.