Learn more about Hardiness zone
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).
 European hardiness zones
 United Kingdom and Ireland
This means that the hardiness zones relevant to the UK are quite high, from 7 to 10, as shown below.
- 7. In Scotland the Grampians, Highlands and locally in the Southern Uplands and in England the Pennines.
- 8. Most of England, Wales and Scotland, and parts of central Ireland.
- 9. Most of western and southern England and Wales, western Scotland, also a very narrow coastal fringe on the east coast of Scotland and northeast England (within 5 km of the North Sea), London, and most of Ireland.
- 10. Very low lying coastal areas of the southwest of Ireland, and the Isles of Scilly.
 North American hardiness zones
The temperatures are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. The map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape. A draft of the latest version also introduces zone 11 to represent areas that have average annual minimum temperatures above 40°F (4.4°C) and that are therefore essentially frost free.
In 2003, a preliminary draft of a new map was produced by the American Horticultural Society (AHS), using temperature data collected from July 1986 to March 2002. This was a period of unusually warm winters, especially in the eastern U.S.A., and thus the 2003 map places many areas a zone higher (warmer) than the 1990 map. The draft map also shows a finer level of detail, for example reflecting urban heat islands by showing the downtown areas of several cities (e.g., Baltimore, Maryland and Washington, DC) as a full zone warmer than the outlying areas. The 2003 draft map has been widely criticized and its current status is uncertain; as of April 2006 the AHS website still presents the 1990 map as the current one.
In addition, the National Arbor Day Foundation has recently completed an extensive updating of U.S. Hardiness Zones in 2004. The new 2004 Arborday.org Hardiness Zones map is based on the most recent 15 year’s data available from more than 5,000 National Climatic Data Center cooperative stations across the United States.
Once the Foundation analyzed the new data, hardiness zones were revised, generally reflecting warmer recent temperatures in many parts of the country. The Arbor Day Foundation used the updated versions of the same sources of data as had been utilized by the U.S. Department of Agriculture in the creation of its hardiness zone maps. The 2004 map appears to validate the data used in the 2003 draft completed by the AHS. http://www.arborday.org/media/zones.cfm
 U.S. Cities
Here are the USDA plant hardiness zones for major U.S. cities (based on the draft 2003 map):
 See also
 External links
- Hardiness Zone maps for several continents
- USDA Plant Hardiness Zone Map
- Hardiness Zones in Canada
- Sunset Magazine Garden Climate Zone Map
- American Horticultural Society Heat Zone Mapda:Hårdførhedszone