The AHS Heat-Zone Map
By Dr. H. Marc Cathey, President Emeritus, American Horticultural Society
The following article is reprinted by permission of the American Horticultural Society. It was written for the Quill & Trowel Newsletter, a publication for professional garden writers. The Kanawha Valley is in Zone 7 of the Heat-Zone Map, in the same area as eastern Virginia and northern North Carolina.
Garden writers are all familiar with the USDA Hardiness Zone Map, first published in 1960. In the early 1980s, a group of writers who were concerned that the first version was no longer valid, came to the U.S. National Arboretum with a request that it be updated. As arboretum director, I undertook that project, and the latest version of the map was published in 1990. It has 11 zones showing average annual minimum temperatures, with a 10-degree difference in each zone. If you see a reference work using a map with only 10 zones, you know that the zones are out of date!
Over the years, many of you, and the gardeners you communicate with, have noted that cold hardiness is not the only factor affecting plant growth, and of course, you are absolutely right. Every plant has a tolerance range of various environmental factors that affect its growth. Several of these factors, such as soil minerals and the availability of water, can be controlled to some extent by the gardener. Others, such as temperature, are largely beyond the backyard gardener’s control, but do have predictable yearly averages.
With any of these factors, the best advice to any gardener can be summarized by the slogan “right plant, right place.” But while planting a thirsty plant in a dry location will mean more work and more demand on natural resources, selecting a plant that thrives in a narrow temperature range to include in a garden that is outside that range will spell the death of the plant.
The effects of cold on a plant are very apparent and immediate. When a plant’s lower limit of temperature is reached, ice forms in its cells and tears them apart beyond hope of repair, so that the plant dies quickly. Over the years your audience has learned to ask first when buying a plant, “Is it hardy in my zone?”
Today, when the possibility of global warming has become a concern, your next step will be to teach them the effects of heat damage. Heat damage to plants may first appear in many different parts of the plant: Flow buds may wither, leaves may droop or become more appealing to insects, chlorophyll may disappear, or roots may stop growing. These subtle signs may continue reappearing for several summers, until desiccation reaches a high enough level that the enzymes that control growth are deactivated. Death from heat damage is slow and lingering.
The American Horticultural Society Heat-Zone Map, which was published as part of the AHS 75th anniversary celebration, should enable gardeners to choose plants that won’t succumb to heat damage. The heat map divides the country into 12 zones, based on the number of days each year that the daily high temperatures reach or exceed 86 degrees, the temperature at which plants begin to sustain physiological damage. The data used to create the map was gathered from 4,745 weather stations and analyzed by the Meteorological Evaluation Services, Inc., in Amityville, NY, which was also involved in developing the hardiness map.
I’ve done many interviews with the media since the release of the map, and here are some of the questions your colleagues have been asking:
What are some examples of plants that are heat tolerant? Unfortunately, scientists have not done a lot of research on heat tolerance, and nurseries have not selected many plants specifically for that trait. That is one thing we at AHS hope will result from our publication of the Heat Map. The “Whitespire” birch will grow farther south than other birches, and the “Bloodgood” Japanese maple is less likely to be scorched by summer sun. There are more examples of plants that won’t tolerate heat – white spruce, rhododendrons, Pieris japonica.
How can people get a copy of the map? A two-by-three foot version of the map, printed on plastic,
can be obtained from the American Horticultural Society, 7931 East Boulevard Drive, Alexandria, VA 22308. Garden writers can get more information about the map, and a transparency to use in articles, by calling our public relations director, Mary Ann Patterson, at (800) 777-7931, extension 21.
Why do I have to pay for the AHS map when I didn’t have to pay for the USDA map? The USDA hardiness map was underwritten by the U.S. government, so in a sense, you and all other taxpayers did pay for it. Although the compilation of data for the AHS map was underwritten by a number of corporate sponsors, the printing of the map and other related expenses were borne by the American Horticultural Society, a nonprofit organization that must recoup those costs.
What good is the map unless all of our plants are coded? I have been assigning heat-zone codes to hundreds of plants. Some of the first compilations of these codes are now in the Monrovia catalog and in a book from Time-Life, Heat Zone Gardening. As you know, we are still making changes in the hardiness codes, and that map has been out for almost 40 years! But with the vastly superior technology and means of communications we have today, we expect to have thousands of plants coded in the next three years. As you know, gardeners who are skillful at using microclimates are likely to find many exceptions to plant codes. When enough exceptions are found, these new findings will gradually work their way into our reference books. Only through the input of gardeners all across the nation, through the sort of communications tools that garden writers provide, can we produce a coding system that helps everyone choose the right plant for the right place.
FOR MORE INFORMATION or to order a full-color map, contact the AHS at 1-800-777-7931; Website: www.ahs.org.