National Soil Temperature Map
When looking at zones for plant growing, you should also look at soil temps, the warmer the soil general indicates the possibility to growing plants and trees outside your zone. I live in extreme SW Missouri, depending on source 6b-7a, but our soil temperatures are far warmer than those of the east coast that are in USDA zone 7.
Just another variable to the magical zone formulations.
Here is a link that might be useful: Soil Temp Link