To compensate for the shortcomings of the USDA cold hardiness zones, the American Horticultural Society introduced in 1997 another climate zone system called the AHS Heat zones.
They compiled US Weather Bureau data for several years and calculated the average number of days per year in each location where the high temperature exceeded 86 degrees Fahrenheit. The theory is that above that temperature, plants begin to suffer cell damage, and some plants are better adapted than others to grow under high temperature conditions.
In 1998, the book "Heat Zone Gardening" was published, written by Dr. H. Marc Cathey. In this book there are listings of plants with cultural information and ratings both of the USDA cold hardiness zones and the AHS Heat zone ranges for over 500 hundred plants.
My Tallahassee gardens are located in AHS heat zone 9 and bordering heat zone 10, according to the heat zone map. I have compared the ratings of plants in Dr. Cathey's book against my own experiences with plants that I have grown. Some of the ratings are consistent with my results, but many plants seem to be rated incorrectly, judging from my results with them. There are also some glaring errors in the plant ratings. For example, the book lists common Liriope muscari (border grass) with a southern limit of heat zone 8, which falls about the Atlanta area. As any gardener south of Atlanta will verify, Liriope thrives all along the Gulf coast.
There are some serious shortcomings with the new AHS heat zone system. The first problem is that the heat zones do not take any other climatic conditions into account. The warm parts of the dry southwest are in the same zone as the humid southeast with its summer rain showers. The second problem is that the heat zone ratings have been around such a short time that there has not been much data compiled on the heat tolerance of plants, as applied to the AHS scale.
The American Horticultural Society encouraged growers and mail order companies to start using the new system on their labeling and in their catalogs, but I have not seen much acceptance in the trade. Perhaps the system will catch on eventually, and the ratings of plants will improve.
NEWS FLASH: The latest issue of Fine Gardening Magazine (Issue# 69, October '99, page 4) has a searing letter to the editor from Tony Avent, owner of Plant Delights Nursery. Tony really blasts the Heat Zone Map as being little value. "Let's allow this useless document to die a quick death before the public is hopelessly confused, and hope someone steps up to the plate with a useful map."
Dr. H. Marc Cathey replies that it took 50 years for the USDA Hardiness Map, that most gardeners need some guidance, and the heat zone map is the best available so far.
There is another climate zone system available to southern gardeners, that has impressed me as an excellent alternative. Turn to the next page to learn all about it.
