Experts and BHG readers answer.
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.