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The USDA map, although enormously popular and widely used, has its limitations. For example, Zone 7 in Maryland is a world away from Zone 7 in Oregon, or north Texas, or the foothills of California’s Sierra Nevada mountains. In many parts of the country, heat rather than cold dictates which plants are able to survive from one year to the next.

Thus, in 1997 (after years of study and research), the American Horticultural Society released its own map, the AHS Heat-Zone Map. This has proven quite useful to gardeners in the South and West. You can search online for this map or it is available http://solanomg.ucanr.edu/files/245158.pdf.

The AHS map has 12 zones. Relatively cooler Zone 1 is defined as having only one day of 86°F weather per year; sweltering Zone 12 has 210 days of such heat or more.

Research has shown that 86°F (30°C) is the temperature at which many plants — that is, their cells, or plant tissue — start to experience damage from heat. That’s why that point became the baseline for laying out the heat zones.

U.S. gardeners in areas where the main source of plant stress isn’t winter cold but summer heat prefer this system. Nurseries in hot areas are starting to refer to these zones more and more. If the heat-zone information isn’t supplied for a plant you’re interested in, look in newer regional reference books and plant catalogs and websites.