Though hardiness zone ratings for plants have been studied and adjusted for many years in response to the earth’s natural climate changes, only recently has research into the impact of excessive warmth on plant growth been initiated. In the mid-90s, the American Horticultural Society began the development of data that tracked the number of days that various regions of the United States experienced temperatures higher than 86 degrees Fahrenheit (30 degrees Celsius). Termed a heat day, it represents the point at which a plant will begin to experience physiological injury because of the temperature. Using a county – specific map, similar to the USDA Hardiness Zone Map, the AHS has divided the nation into 12 heat zones, with Zone 1 having no heat days and Zone 12 averaging 210 heat days. Plants can then be described not only on the basis of the coldest hardiness zone in which they will survive, but the warmest heat zone that they can tolerate without injury.
A few texts on heat zone gardening are beginning to appear, and their number is predicted to increase in the next few years. An increasing number of plants, especially herbaceous species, are coded for their heat tolerances, giving recognition to the role that heat plays in plant survival. A plant that has been rated for both cold hardiness and heat tolerance will be given four numbers. For example, the plant might be identified by its name followed by a 4-7 and 12-1. The first two numbers identify the cold hardiness zones in which it will survive, Zones 4 through 7 and the second two numbers indicate that it is heat tolerant in Zones 12 through 1. However, ratings for cold and heat tolerances will always be generalizations, so it is wise to avoid selecting plants that are at the limits of their tolerance range. Hardiness Zone Maps may be purchased from the USDA and Heat Zone Maps are available from the American Horticultural Society through their website at http://www .ahs. org. Also available for purchase from the AHS is their paperback book, The AHS Great Plant Guide, that assigns hardiness zone ratings to more than 3,000 plants.