AHS Heat Zones
Climate zone maps show where various permanent landscape plants can adapt. If you want a shrub, perennial, or tree to survive and grow year after year, the plant must tolerate year-round conditions in your area such as the lowest and highest temperatures and the amount and distribution of rainfall.
Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. Zone maps provide this critical climate information.
The AHS Plant Heat-Zone Map
The significance of winter's lowest temperatures decreases as we shift from places where winter freezes may kill many plants to areas where freezes merely mean frost on lawns and windshields. Obviously, winter lows in the high 20s are much less damaging than lower temperatures. But on the other hand, areas with mild winter temperatures often have soaring summer temperatures. Gardeners have discovered that summer heat limits plant survival just as surely as winter cold.
That's why the American Horticultural Society (AHS) published a map that takes heat into account. Called The Heat Map, this 12-zone map indicates the average number of days each year when given regions experience temperatures of 86F or higher. According to the AHS, that's the temperature at which many common plants begin to suffer physiological damage. The zones range from 1 (one day or less at 86F or warmer) through 12 (210 days or more per year at 86F or warmer).