America is such an enormous country that the climate varies from one coast to the other by huge amounts. This is why they have developed The Climate Zone map of America which denotes 11 major belts for growing plants and is displayed on most seed packets sold in the States.
This map is 18 years old and does not take account of climate change or the fact that most American gardeners have known for ten years that there has been a gradual shift northward of growing zones for many plants in the US. Examples include the Southern Magnolia, once limited to growing zones from Florida to Virginia, which apparently now thrives as far north as Pensylvania. Or, certain kiwis, previously hardy only as far as Oklahoma, which can now bear fruit in St Louis.
Much of this information originates from a fascinating article in USA Today - www.usatoday.com/weather/climate/2008-04-23-gardening-map_N.htm which explains that possibly one of the main reasons why this map has not been updated to accomodate changing temperatures, is linked to money! It appears that, because so many USA nurseries offer money-back guarantees on plants, many are worried that adjusting the climate maps would encourage customers in cooler areas to possibly risk buying tender plants that would not survive in a freak cold period. The article provides some interesting debate on this issue and makes me realise how much more serious the impact of climate change must be in such a huge country.
The Arbor Day Foundation published an excellent US climate zone map in 2006 based on data from 1991-2005 which includes a comparison to the 1990 version showing the increase northwards in the warm zone for plants and crops http://www.arborday.org/media/map_change.cfm.
The American Horticultural Society also has a draft zone map on its site which is soon to be updated by research being done at Oregon State University which identifies zones in square miles rather than counties as with the 1990 climate zone map. Reproduction restrictions mean that you have to search for this on the AHS site at hhtp:www.ahs.org.
This must be very confusing for American gardeners so the best advice must be to check with neighbours and see what they have been growing succesfully - this is what works for me in the UK!