I think Rick's right. It's probably best to consult several sources of information, and go with the average.
Too many factors determine ultimate survival- type of soil (clay, silt, sandy?), actual extremes of temp during any season in a particular year (an extremely cold winter in New England?, an extremely hot summer in Fl?, a prolonged drought in the southwest, etc).
I think of hardiness zones as a guestimate, and assume that my mileage may vary.
Karen
