All About Heat Zones: Why Temperature Extremes Matter for Plant Survival

The American Horticultural Society's Heat Zone Map parallels the USDA Plant Hardiness Zone Map. Instead of cold extremes, it's based on the number of “heat days” in an area. Download the AHS Heat Zone Map (PDF).

For a successful garden, you must know your site's growing conditions—especially winter cold. Many plants adapted to warmer climates die in extreme cold. Most cold-climate gardeners know the USDA's 11 zones indicating average minimum winter temperatures: Zone 1 sees -50°F; Zone 11 rarely below 40°F. Most catalogs, websites, and garden centers now provide Hardiness Zone ratings. If you garden in Zone 5, plants rated Zones 1–5 should survive your winter. Find your zone with our Zone Finder.

But cold hardiness isn't the only survival factor. Too much heat can be just as damaging. Cold-intolerant plants simply die; heat stress signs are subtler—plants may stop blooming, leaves turn pale, and they become more pest-susceptible.

In the late 1990s, the AHS developed the Heat Zone Map based on “heat days”—days above 86°F, when many plants suffer physiological damage and shut down. Heat Zones range from 1 (no heat days) to 12 (210+ heat days).

Heat Zone ratings are relatively new, so not all sources indicate them yet. But increasingly you'll see dual ratings like 5-10, 11-1. The first pair is cold Hardiness Zone (survives winter in zones 5–10). The second is Heat Zone (heat-tolerant in zones 11 through 1).

When planting, Heat Zones and Hardiness Zones are vital guidelines. For best success, also consider rainfall, humidity, sunlight, and soil pH. Read H. Marc Cathey's article on heat zones for more on environmental factors affecting plant health.