What are heat zones?
The American Horticulture Society (AHS) in conjunction with the USDA has expanded on the USDA Hardiness Zone designations to develop an AHS Plant Heat-Zone map. The USDA map tracks average winter cold. The AHS map measures the number of days that the high temperature reaches 86°F or higher at a given location. The reason this temperature is critical is that above 86°F, photosynthesis begins to shut down in most plants. That puts the plants under stress. Other plants operate with a different type of physiology and can take the heat better. Similar to the USDA Hardiness Zone map, the AHS Plant Heat-Zone numbering system ranges from Zone 1 as the coolest to Zone 12 as the warmest. However, when a plant grows in a range of heat zones, the numbers are reversed. For example, a plant may be adapted to Heat Zones 9-4, meaning that it can tolerate 120-150 days (Heat Zone 9) to 14-30 days (Heat Zone 4) above 86°F annually.
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