University of Illinois Extension

New USDA Plant Hardiness Zones

A tool long used by gardeners, landscapers and anyone in the green industry is the USDA Plant Hardiness Zone Map (PHZM). Using this map helps determine what plants should survive local winter temperatures, said a University of Illinois Extension horticulture educator.

"About 80 million American gardeners as well as those in the green industry are the largest user of this zone information," said Martha Smith.
"Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location. It does not designate the coldest temperatures ever experienced but simply the average lowest winter temperature over a specified time. There is still a 50% chance that winter temperatures could drop lower than the designated zone for a particular area, and a 50% chance that winter temperatures will be higher."

As with the old map the new 2012 map is divided into zones. Each zone is a 10-degree Fahrenheit band, further divided into A and B 5-degree Fahrenheit zones. The new map includes 13 zones, two more than the old 1990 map. In many areas these zones have shifted and reflect a one 5-degree Fahrenheit half-zone warmer change. Why was this done?

"It was based on using temperature data from a longer more recent time period," she said. "The 1990 map was based on temperature data from only a 13-year period of 1974 through 1986. The new map uses data measured at weather stations during a 30-year period of 1976 through 2005.

"More sophisticated methods for mapping zones between weather stations were also used. For the first time, factors such as changes in elevation, nearness to large bodies of water, and the natural change of the land such as valley bottoms and ridge tops were figured in. These additional details greatly improved the accuracy and detail of the map. In some cases, zones dropped cooler rather than warmer."

Another difference is the 2012 map does not include Canada or Mexico. This new map focused on creating the highest-quality PHZM for the United States and Puerto Rico.

As an example, the Quad Cities area and Peoria used to be in USDA hardiness zone 5A with average annual cold temperatures between -15 to -20° F. The new map now lists these counties in zone 5B, which reflects a 5-degree shift warmer with average annual cold temperatures between -10 to -15°F.

"So, what does this new map mean as far as global warming? USDA addresses this issue on their website with the following statement: 'Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming,' " Smith explained.

The new map is available at http://planthardiness.ars.usda.gov/PHZMWeb/. For the first time, the map is available as an interactive GIS-based map, for which a broadband Internet connection is recommended, and as static images for those with slower Internet access. It is user friendly with an easy search using your zip code to find your 2012 plant hardiness zone.

"As a 30-year-plus gardening veteran, I find change is good but not always easy to embrace," said Smith. "It will take time to not immediately look for my old zone. I wonder when my flowering dogwood will realize it is 5-degrees warmer in my garden and bloom dependably every spring?"