From wiki vis USDA
A hardiness zone is a geographically-defined zone in which a specific category of plant life is capable of growing, as defined by temperature hardiness, or ability to withstand the minimum temperatures of the zone. The zones were first developed by the United States Department of Agriculture (USDA), and have subsequently been adopted elsewhere. They are categorized according to the mean of the lowest temperature recorded each winter, termed the "average annual minimum temperature". Thus if five successive winters reach respective minima of −14 °C, −12 °C, −8 °C, −16 °C, and −13 °C, the mean coldest temperature is −12.6 °C, placing the site in zone 7.
Benefits and criticisms
The hardiness zones are effective in that, for many situations, extremes of winter cold are a major determining factor in whether a plant species can be cultivated outdoors at a particular location. However, it does have a number of drawbacks, most significantly in not incorporating summer heat levels into the zone determination. Thus sites which may have the same mean winter minima, but markedly different summer temperatures, will still be accorded the same hardiness zone. An extreme example is the Shetland Islands and southern Alabama, which are both on the boundary of zones 8 and 9 and share the same winter minima, but very little else in their climates; in summer, the continental climate of Alabama is about 20 degrees Celsius hotter than the oceanic climate of Shetland, and there are very few plants that can be grown at both locations. Due to its maritime climate, the UK is in AHS Heat Zone 2 (having 1 to 8 days hotter than 30 degrees Celsius), whereas Alabama is in Zones 7 to 9 (61 to 150 days hotter than 30 degrees Celsius).
Another problem is that the hardiness zones do not take into account the reliability of the snow cover. Snow acts like an insulator against extreme cold temperatures, protecting the root system of hibernating plants. If the snow cover is reliable (then present during the coldest days), the actual temperature to which the roots are exposed is not as low as the hardiness zone number would indicate. As an example, Quebec City in Canada is located in zone 4 but can rely on an important snow cover every year, making it possible to cultivate plants normally rated for zones 5 or 6, whereas in Montreal, located in zone 5, it is sometimes difficult to cultivate plants adapted to the zone because of the unreliable snow cover.
Low temperature is not the only determinant of plant survival - other factors such as high summer temperature, humidity, soil temperature, etc. may be equally important. Also, many plants will survive in a locality but won't flower if the day length is inappropriate or if they require vernalisation (a particular duration of low temperature). The low temperature statistic is only appropriate for woody perennial species, and even then its use is limited. With annuals the time of planting can often be adjusted to allow growth beyond their normal geographical range.
