One of the most misunderstood concepts for "budding" gardeners is the zone designations found in books and catalogs. Typically these publications will show a zone range for a given plant, but will not explain the significance of the zones. A new gardener in the deep South will see a plant listed for zones 5 to 8 and will falsely assume that since they are in zone 8, the plant should thrive in their garden. In fact, all the zone ratings mean is that the plant will be cold hardy within the zones mentioned. The higher number means very little in most catalogs, unless the mail order company is specifically considering the other climate factors such as heat and humidity, and is writing their book or catalog specifically for the southeastern United States.
These zones are the U. S. Department of Agriculture cold hardiness zones, and are based on the average minimum low temperatures in a given area.
For example, Tallahassee is in zone 8 (specifically zone 8B), which means the average minimum winter temperature ranges between 15 and 20 degrees Fahrenheit. Some plants will not survive that low a temperature and therefor are not rated for zone 8. To learn more about USDA hardiness zones, and to find your city's zone, go to the U. S. D. A. hardiness zone map page.
The cold hardiness zones are based on years of reports of official low temperature by the U. S. Weather Bureau, and as most gardeners know there are "micro-climates" which can differ substantially. Last year I tracked the temperature at my home throughout the winter with a good quality thermometer that stores the low overnight temperatures until it is reset. I then compared my readings against the official temperatures taken at the airport. (Click to see the chart.) Whenever a cold front moved through, and the official readings dropped below freezing, the temperatures at my house near downtown Tallahassee, near the top of a hill, ran 8 degrees warmer than the official temperature. My effective zone is probably zone 9 instead of zone 8.
There are two relatively new climate zone concepts, the American Horticultural Society "HEAT" zones and the Southern Living zones. To learn more about these, turn to the next page.
