The Plant Hardiness Zone Map is new for 2012 and represents the first overhaul of the zones in 20 years. (They are a half zone warmer now.)
In order to assist gardening enthusiasts determine an ideal planting schedule, the USDA created a Plant Hardiness Zone Map. The map separates the country into zones depending on average minimum winter temperature. The zone map covers all 48 contiguous states, Hawaii, Alaska and Puerto Rico. Areas are clustered into zones in 10-degree increments.
The zones are based on measurements taken over a 30-year period. With this in mind, if your zone changes from one map to the next, it doesn’t mean you must change your green thumb’s habits. Gardening results may not exactly follow map’s zones. Microclimates (small pockets that are warmer or cooler than the surrounding area) can cause different results than the map may suggest.
Hardiness zone maps have been developed in the US since the 1920s and 1930s. The USDA’s first publication of such a map was in 1960.