Heat Zone Map Info – What Do Heat Zones Mean Anyway

Temperature Thermometer
(Image credit: Detry26)

Weather temperatures are among the most important factors in determining whether a plant thrives or dies in a particular setting. Almost all gardeners have the habit of checking a plant’s cold hardiness zone range before installing it in the backyard, but what about its heat tolerance? There is now a heat zone map that can help you make sure your new plant will survive summers in your area too.

What do heat zones mean? Read on for an explanation, including tips, on how to use heat zones when selecting plants.

Heat Zone Map Info

For decades gardeners have used cold hardiness zone maps to figure out whether a particular plant can survive winter weather in their backyard. The USDA put together the map dividing the country into twelve cold hardiness zones based on the coldest recorded winter temperatures in a region.

Zone 1 has the coldest average winter temperatures, while zone 12 has the least cold average winter temperatures. However, USDA hardiness zones do not take summer heat into account. That means that while a particular plant’s hardiness range may tell you that it will survive your region’s winter temperatures, it does not address its heat tolerance. That’s why the heat zones were developed.

What Do Heat Zones Mean?

Heat zones are the high-temperature equivalent of cold hardiness zones. The American Horticultural Society (AHS) developed a "Plant Heat Zone Map" that also divides the country into twelve numbered zones.

So, what are heat zones? The map’s twelve zones are based on the average number of “heat days” per year - days that temperatures rise above 86 F. (30 C.). The area with the least heat days (less than one) is in zone 1, while those with the most (more than 210) heat days are in zone 12.

How to Use Heat Zones

When selecting an outdoor plant, gardeners check to see if it grows in their hardiness zone. To facilitate this, plants are often sold with information about the range of hardiness zones they can survive. For example, a tropical plant might be described as thriving in USDA plant hardiness zones 10-12.

If you are wondering how to use heat zones, look for heat zone information on the plant label or ask at the garden store. Many nurseries are assigning plants heat zones as well as hardiness zones. Remember that the first number in the heat range represents the hottest area the plant can tolerate, while the second number is the lowest heat it can tolerate.

If both types of growing zone information are listed, the first range of numbers is usually hardiness zones while the second will be heat zones. You’ll need to know where your area falls on both the hardiness and heat zone maps to make this work for you. Select plants that can tolerate your winter cold as well as your summer heat.

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.