What is a 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.
Hardiness zones are given a number ranging from 1 to 11 to group together the range of average temperatures.
UK Hardiness Zones
Due to the moderating effect of the Gulf Stream on the Irish and UK’s temperate maritime climate, the UK, and Ireland even more so, have rather milder winters than their northerly position suggests.
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.
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