What Are Hardiness Zone Maps?

In the mid-1900s, the U.S. Department of Agriculture (USDA) mapped out the entire United States, Mexico and Canada by lowest annual minimum temperature groupings. Each zone represented a 10 degree F. difference. This was invaluable advice for the agriculture industry. Now plants could be rated by hardiness zones, taking the guesswork out of choosing plant varieties. You had a gage, other than experience, for picking plants. The maps have been revised over the years, to reflect changes in climate. When cities and towns were moved from one zone to another, gardeners were left to wonder what would happen to their existing garden plants. While our climate may be shifting, these changes did not occur over night. Plants are adaptable, surviving in many different climates. They also cannot read maps. In 1990, the zones were further divided, with each numbered zone being broken down into an ‘a’, the lower temperature end of the zone, and a ‘b’, the higher. Unfortunately plant breeders have yet to start using these distinctions, so they are mostly useful if a gardener wants to push the envelope a bit. A gardener in zone 6b will be tempted to dabble in 7a plants. Given the variability of climate, it’s a hit and miss situation.

The American Horticulture Society (AHS) introduced a Plant Heat-Zone Map in 1997, intended to supplement the hardiness map. Heat related problems are much harder to quantify. High summer temperatures only told half the story. Plants don’t usually react to a day or two of heat they way they might respond to a frost. However, plants subjected to a two week heat wave could well succumb. Other variables that weren’t considered were things like humidity levels, night time temperatures and rainfall.

Article Source: gardening.about.com