The U.S. Department of Agriculture (USDA) has released the new version of its Plant Hardiness Zone Map (PHZM), updating a useful tool for gardeners and researchers for the first time since 1990 with greater accuracy and detail. The new map is available online at www.planthardiness.ars.usda.gov. ARS is the chief intramural scientific research agency of USDA.
For the first time, the new map offers an interactive format and is specifically designed to be Internet-friendly. The map website also incorporates a "find your zone by ZIP code" function.
"This is the most sophisticated Plant Hardiness Zone Map yet for the United States," said Catherine Woteki, USDA Under Secretary for Research, Education and Economics. "The increases in accuracy and detail that this map represents will be extremely useful for gardeners and researchers."
The Illinois State climatologist has some interesting observations regarding the shift in minimum winter temperatures. You can read it here.
Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.