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PLANT HARDINESS ZONE MAP OF THE US DEPTARTMENT OF AGRICULTURE
Hardiness is most commonly understood as the ability of a plant to endure low temperatures. More appropriately, it should be considered as the ability of a plant to thrive in a variety of physical conditions.
The U. S. Department of Agriculture's Plant Hardiness Zone Map provides a recommended range in which a plant will grow well. The map identifies eleven zones according to climate: the colder the winter temperature, the lower the zone number. Zone ratings are designed to indicate that a plant will not merely survive in a zone, but thrive. The zones are approximate, however, so some gardeners experiment with growing plants rated hardy for one zone north or one zone south of their location. Other factors — such as high temperatures, rainfall, altitude, soil and drainage, available light, and air quality — should also be considered when choosing plants for your area.
PLANT HEAT ZONE MAP OF THE AMERICAN HORTICULTURAL SOCIETY
In 1997, Dr. H. Marc Cathey, President Emeritus of the American Horticultural Society (AHS), working with the Meteorological Evaluation Service Co. Inc., produced the AHS Plant Heat Zone Map, thus completing the circle of information available to establish a plant's likelihood of survival from extremes in temperature. The 12-zone map indicates the average number of days each year that a particular region experiences "heat days" - those days with temperatures over 86 degrees and the point at which plants experience damage to cellular proteins. The zones range from Zone 1 (with no heat days) to Zone 12 (210 or more heat days).
To view the AHS Plant Heat-Zone Map, follow this link: http://www.ahs.org/publications/heat zone_map.htm