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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.

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.

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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 as 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|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.

 

Other factors that affect plant survival but are 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.

 

 

An alternative system for describing plant hardiness is to use indicator plants (the USDA also publishes a list of these to go with their map). Common plants with known limits to their range are used. For example, many people will know whether lemons will grow in their locality. If you then say Geraldton Wax will grow more or less where lemons will grow you have defined the range of Geraldton Wax with some accuracy (whether or not it will flower is another problem). Unfortunately no two plant species have exactly the same requirements and there are even within-species differences. 'Meyer' lemons, for example, are more cold tolerant than 'Eureka'. You can really only define core areas and they are often fairly arbitrary.

==United Kingdom and Ireland Hardiness Zones==

==United Kingdom and Ireland Hardiness Zones==