You have to take this with a grain of salt though. If you live in a frost pocket or have various micro-climates in your garden the plants can perform differently. Our house is situated in a frost pocket and despite the USDA zone map which shows us in a zone 7a location and has a 0°-5° Fahrenheit low. We typically end up with temperatures below 0°F once or twice a season.
National USDA Plant Hardiness Zone Map(Click on the Map to see a larger picture)
Tennessee USDA Plant Hardiness Zone Map(Click on the map to see a larger picture)
It's interesting that there is so much similarity between the USDA zone map and the Arbor Day map from a few years ago! You can look at your state or area by visiting the USDA Plant Hardiness Zone Map.