The USDA has released the long-awaited, updated version of its hardiness zone map, giving some areas of the country new zone designations. The new 2012 map is based on temperature data from the thirty year period 1976- 2005, so more recent temperature trends are incorporated into its zone listings. It is also based on data from even more weather stations than the previous map, enhancing its accuracy.
The hardiness zone map is divided into zones based on the average annual extreme minimum temperature in a particular area -- in other words, the coldest temperature, on average, that occurs over the winter. There are now 13 zones, (two new ones have just been added) further subdivided into ″a″ and ″b″ categories. Each zone represents a temperature range of ten degrees; each ″a″ or ″b″ subdivision five degrees. The lower the number, the colder the zone; and ″a″ is colder than ″b.″
While a few areas were moved to slightly cooler zones, the majority of the changes on the new map reflect a shift to warmer zones, with the zone boundary generally moving up by a half zone, or a five degree change. While many of these changes are due to changes in climate patterns, some are a result of the increased regional accuracy of the map to better reflect the influence of local geography, like changes in elevation or nearness to a body of water, on temperature extremes.
One of the handiest aspects of the new map is its interactive feature that lets you find your zone by typing in your zip code on the map website. You can also zoom in on the map of your state to see the zone boundaries in detail and download and print maps of various sizes and resolutions.
Does this new zone map show that the climate is warming? Taken by itself, the map is not designed to answer this question. Remember that what the map shows is the average lowest yearly temperature over the last thirty years, which has indeed moved higher in many areas in recent decades. But it doesn't provide information about winter temperatures apart from these extremes, or about summer temperatures or changes in any other climatic parameters. Figuring out if and how much the climate is warming is based on overall climate trends that take lots of factors in account, of which the average of extreme winter lows shown by the zone map is just one.
To see the new map and find out if your hardiness zone has changed, go to: USDA Hardiness Zone Map.