The Plant Heat Zone Map of the American Horticultural Society is an attempt to quantify more accurately the range of adaptation of plants.  In the past, if one relied only on the hardiness zone map to determine where a particular species would thrive, errors occurred.  According to the Hardiness Zone Map, both San Antonio and Seattle are in Zone 8.  Yet, we know  of many plants which, though hardy to Zone 8, will not thrive in both cities.

     The hardiness zones are based on average low temperatures in winter.  Though that is a very important indication of survival, it is not a complete one.  More and more books, horticultural magazines and wholesale nurseries are beginning to note both the Hardiness Zone as well as the Heat Zone rating for plants.

     Please use this map to help you select the right plant for your particular site.  For example, if your landscape is in northern Bexar County, it is in Hardiness Zone 8b and Heat Zone 9.  Southern Bexar county is also in Hardiness Zone 8b, but in Heat Zone 10.