By: Teo Spengler
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.
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.
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) are in zone 1, while those with the most (more than 210) heat days are in zone 12.
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.
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Read more about USDA Planting Zones
Like the USDA Hardiness Zone Map, the AHS Heat Zone Map should only be used as a guide. The map is based solely on temperature and does not take into account unusual weather patterns, areas with sparse populations, humidity, or variances in nighttime temperatures-a significant factor in determining how far south a plant will grow. The overall accuracy of the map is based on the assumption that plants are getting adequate moisture and does not factor in drought conditions.
Although the AHS Heat Zone Map is not as widely used as USDA Hardiness Zone Map, it is showing up in more and more catalogs and an increasing number of plant labels-over 15,000 different plants have been assigned heat zones since the map was first released. If included, the heat zone numbers usually appear directly after the hardiness numbers on the label. The highest number (hottest zone) is listed first-the exact opposite of how the USDA Hardiness Zones numbers are presented (lowest zone to highest).
The AHS website cites several examples of how this works. In one example, they state that the cold tolerant English wallflower may be listed as 5-8, 6-1 (5-8 being its hardiness zones and 6-1 its heat zones). This means the plant is not suitable for over-wintering below Zone 5 and will not perform well in summer heat above Zone 6. Unfortunately, the hardiness zone and heat zone ratings overlap a bit, which makes things a bit confusing.
Many factors influence which plants will thrive in a given location. Heat, cold, and elevation are just a few that can have a big impact. If you’re trying to figure out which plants to use in your garden, check out the gardening maps below that designate plant zones based on various important factors.
The Arbor Day Foundation developed this U.S. Hardiness Zone Map based upon data from 5,000 National Climatic Data Center cooperative stations across the continental United States.
Search the U.S. Department of Agriculture (USDA) plant hardiness zone map by zip code. This interactive map divides regions based on average minimum temperatures.
Sunset’s regional climate zone maps are designed to tell you where a plant will thrive year-round.
Founded in 1922, the American Horticultural Society is a non-profit, 501(c)(3) organization dedicated to sharing with all Americans the critical role of plants, gardens, and green spaces in creating healthy, livable communities and a sustainable planet.