Understanding Plant Heat Zone Designations

Phyllis Heuerman
Frederick County Master Gardener Program

Most gardeners are familiar with the U.S. Department of Agriculture's Plant [Cold] Hardiness Zone Map. By using the map to find the zone you live in, you can determine which plants will survive the winter in your garden. That map was first developed in 1960. Today nearly all reference books, nursery catalogs and gardening magazines describe plants using the USDA hardiness zones. Most of Frederick County, Maryland, is in hardiness zone 6.

Until recently, gardeners have had to rely solely on the cold hardiness zone ratings to determine a plant's regional hardiness. Cold hardiness, however, is only one factor in a plant's chances of survival. Although heat damage is not as obvious, it can be as crippling to plants as cold. Heat damage is usually characterized by a slow decline. Plants exposed to inappropriate heat may stop blooming or flowers may wither; leaves may turn pale, blue-gray or brown. Heat stressed plants become more susceptible to pests and disease.

The American Horticultural Society (AHS) has developed a heat zone map to help take some of the guess work out of knowing which plants will thrive or at least tolerate the heat in an area. The map was first published in 1997. The AHS Heat Zone Map parallels the USDA Hardiness Zone Map in that it has 12 different zones. Each zone represents a range of summer heat. These ranges are defined by the average number of days annually above 86 degrees, Fahrenheit. That is the temperature at which plants begin to suffer physiological damage from heat. (People too, I think!) Our Frederick County heat zone is 6, just as our hardiness zone is 6.

Since heat zone ratings are fairly new, they are not listed as regularly in references, catalogs and plant labels as hardiness zones. However, as time progresses, you will see heat zone designations joining the hardiness zone designations. Each plant will have four numbers. For example, a chrysanthemum will have the designation 4-10, 12-1. The first two numbers refer to hardiness zones and the second two numbers refer to heat zones. So, a chrysanthemum is cold hardy in zones 4 through 10, and heat tolerant in heat zones 12 through 1. That makes it a good plant for our area.

You can obtain more information on heat zones and a copy of the map at the American Horticultural Society website www.ahs.org. AHS has also published a reference book, The AHS Great Plant Guide, which lists over 3,000 plants and their heat zone and hardiness zone ratings. This book is available in paperback at a reasonable price in retail and on-line bookstores.

Remember that heat zones and hardiness zones are just guides. Many factors affect a plant's survival. For example many plants that thrive in dry heat suffer in the humidity in this area. The AHS did not take into consideration the humidity of an area when it developed heat zones; so some plants that are listed as being within our heat zone may suffer in mid summer.

Also, the availability of water affects how well plants live in heat. A plant that is tolerant of the heat in our area may not be drought tolerant. Mulch your plants to conserve water and provide water to the plant roots instead of overhead.

Another factor that affects a plant's cold and heat tolerance is where it is planted within your yard. Plants placed near asphalt or brick walls, are in a warmer microclimate than those planted on an open hill.

Hey, nobody said gardening is easy! The AHS Heat Zone Map is just another tool to help you cope with the challenges of Mother Nature.

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