Answer: The USDA Zone map was created in 1960 and updated in 1990. These zones are based on average winter minimum temperatures from 125,000 weather stations across North America. Zones are very important when choosing plants for your landscape, but they are only one of the factors to consider. Consider there are "zone 9" in California and Florida, and then think of how vastly different climate can be in those two states. Also, climate can very somewhat in each individual person's landscape. You can have "microclimates" in any given neighborhood. This can be areas where frost tends to settle and make the area cooler than the zone specification dictates, or, areas where it tends to be warmer (up against a home or other structure, near machinery, dryer vents, etc.). I'm going to go out on a limb and say that just about all gardeners I know of (and that definitely includes myself) have "pushed" the zones a time or two in their past. I am in zone 6 and I have planted things that are hardy only to zone 7. This has worked out for me and it has failed. I would say it has failed more than it has worked because weather is so incredibly upredictable. My recommendation is to go ahead and try something as long as the plant isn't too expensive and you won't be heart broken if you lose it. You can also try to provide winter protection to plants you are pushing the envelope with - just in case.
Will your zone change? Perhaps. As you are aware of, the US, especially the eastern US has experienced unbelievably warm winters these past few years. If we are to believe many scientists worldwide, the entire earth is warming up at a quick clip due to the "greenhouse effect". I am inclined to believe this given the peculiar weather patterns I have had to deal with in my own garden.
For more information about zone maps and climate change, check out this articles in the National Gardening Magazine, on-line:
Although it's not on line yet, Susan Chamberlin's article "Hard Rain Falling" from the December 1998 issue, should be up soon. Hope this helps!
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