USDA Zone Maps
The U.S. Department of Agriculture (USDA) on January 25, 2012, 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. This new map was jointly developed by the USDA's Agricultural Research Service (ARS) and the Oregon State University's (OSU) PRISM Climate Group. About 80 million American growers and gardeners, as well as those who breed plants and seeds, are the largest users of the USDA Plant Hardiness Zone Map.
For the first time, the new map offers a Geographic Information System (GIS)-based 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, the increases in accuracy and detail that this map represents will be extremely useful for gardeners and researchers." ~ Dr. Catherine Woteki, USDA Under Secretary for Research, Education and Economics
The 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.
Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.
Some of the changes in the zones, however, are a result of new, more sophisticated methods for mapping zones between weather stations. These include algorithms that considered for the first time such factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops. Also, the new map used temperature data from many more stations than did the 1990 map. These advances greatly improved the accuracy and detail of the map, especially in mountainous regions of the western United States. In some cases, advances resulted in changes to cooler, rather than warmer, zones.