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The amount of heat that plants are exposed to in summer is equally critical. For this reason, the American Horticultural Society (AHS) Heat Zone Map was developed in 1997. AHS President Emeritus Dr. H. Marc Cathey supervised the development of the map, using data collected from the National Climatic Data Center and the National Weather Service. The map divides the U.S. into 12 heat zones according to their average annual number of "heat days." A heat day is defined as a day in which temperatures reach or exceed 86 degrees Fahrenheit. AHS Heat Zone 1 averages less than one heat day per year, while Zone 12 averages more than 210 heat days.
Like hardiness zones, the heat zones for a particular plant are given as a range. The first number indicates the hottest zone in which it will grow successfully; the second represents the zone with the minimum amount of summer heat necessary for it to complete its annual growth cycle.
As for cold hardiness, heat tolerance in plants involves more than just temperature. Summer rainfall—and the lack of it—limits the successful cultivation of many plants. High humidity rings the death knell for many plants that thrive in drier conditions with similar heat. Some plants are able to thrive in warmer zones if nights are cool. Qualities of the soil—its fertility, acidity or alkalinity, and drainage—also influence the summer survival equation. These factors should also be taken into account when selecting plants.
Although temperature is not the only determinant involved in a plant's ability to thrive in summer conditions, it is an important factor, and one that has been extensively assessed for the use of gardeners.
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