The U.S. Department of Agriculture has released a new Plant Hardiness Zone Map (PHZM). The map is the first updated in over twenty years and incorporates greater accuracy and detail since the last map from 1990. The new map was developed by the USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group and is available in various digital image formats and as an interactive online mapping application. The online mapping was built using Esri technology and has a transparency slider which allows users to see the relationship between the underlying topography and localized differences in plant hardiness zones. The USDA Plant Hardiness GIS data is available in shapefile and raster grid formats from Climate Source for a fee.
The map is divided into 13 zones which represent a spread of 10 degrees Fahrenheit for each zone. Each zone is divided into an A and B zone bands of 5 degrees fahrenheit each. The zones represent the average annual minimum winter temperatures. The new maps uses temperature measurements over a 30-year period 1976-2005. The previous 1990 map only averaged temperature over a 13-year period of 1974-1986 and some temperature shifts have been observed:
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.
While it could be interpreted that the changes are due to climate change, the announcement on the map’s publication notes that some of the changes are the result of more detailed and accurate information:
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.