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The United States
Department of Agriculture Plant Hardiness Zone Map divides North America into 11
zones based on average minimum winter temperatures, with Zone 1 being the
coldest and Zone 11 the warmest. Each zone is further divided into sections that
represent five-degree differences within each ten-degree zone.
This map should only be
used as a general guideline, since the lines of separation between zones are not
as clear-cut as they appear. Plants recommended for one zone might do well in
the southern part of the adjoining colder zone, as well as in the neighboring
warmer zone. Factors such as altitude, exposure to wind, proximity to a large
body of water, and amount of available sunlight also contribute to a plant's
winter hardiness. Because snow cover insulates plants, winters with little or no
snow tend to be more damaging to marginally hardy varieties. Also note that the
indicated temperatures are average minimums -- some winters will be colder and
others warmer.
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