A hardiness zone is a geographically defined area in which a specific category of plant life is capable of growing, as defined by climatic conditions, including its ability to withstand the minimum temperatures of the zone. For example, a plant that is described as "hardy to zone 10" means that the plant can withstand a minimum temperature of 30°F. A more resilient plant that is "hardy to zone 9" can tolerate a minimum temperature of 19°F. First developed by the United States Department of Agriculture (USDA), the use of the zones has been adopted by other countries.
Benefits and Drawbacks
The hardiness zones are informative: the extremes of winter cold are a major determinant of whether a plant species can be cultivated outdoors at a particular location; however, the USDA hardiness zones have a number of drawbacks if used without supplementary information. The zones do not incorporate summer heat levels into the zone determination; thus, sites which may have the same mean winter minima, but markedly different summer temperatures, will be accorded the same hardiness zone.
Another issue is that the hardiness zones do not take into account the reliability of the snow cover. Snow acts as an insulator against extreme cold, protecting the root systems of hibernating plants. If the snow cover is reliable, the actual temperature to which the roots are exposed will not be as low as the hardiness zone number indicates.
Other factors that affect plant survival, though not considered in hardiness zones, are soil moisture, humidity, the number of days of frost, and the risk of a rare catastrophic cold snap. Some risk evaluation – the probability of getting a particularly severe low temperature – often would be more useful than just the average conditions.
Lastly, many plants may survive in a locality but will not flower if the day length is insufficient or if they require vernalization. With annuals, the time of planting can often be adjusted to allow growth beyond their normal geographical range.
An alternative means of describing plant hardiness is to use "indicator plants" (the USDA also publishes a list of these to go with the hardiness zone map). In this method, common plants with known limits to their range are used. Gardening books are available that provide more information on climate zones. For example, Sunset Books (associated with Sunset magazine) publishes a series that breaks up climate zones more finely than the USDA zones. They identify 45 distinct zones in the US, incorporating ranges of temperatures in all seasons, precipitation, wind patterns, elevation, and length and structure of the growing season.
United States Hardiness Zones
Most of the warmer zones (zones 8, 9, 10, and 11) are located in the southern half of the United States. The low latitude and often stable weather in the far southwestern states, California, and Florida is responsible for the few episodes of severe cold relative to normal in those areas. The more northerly and central portions of the USA are the cooler zones (zones 7, 6, 5, and 4). The central and northerly portions of the mainland often have much less consistent range of temperatures in winter due to being more continental, and thus the zone map has its limitations in these areas.
The USDA first issued its standardized hardiness zone map in 1960, and revised it in 1965. The newest maps were issued in 2012 by the USD, and show a warming of most zones across the USA. The 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.
United States residents can view the specific hardiness zone of their city by visiting the Plant Maps website. Residents of Georgia can visit the List of 2012 Hardiness Zones for Cities in Georgia. For those of you living in our hometown of Marietta, you'll find were in zone 7 just shy of being in zone 8.