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What is a Hardiness Zone?

Strictly speaking, USDA* Hardiness Zones (climate zones, growing zones) only apply to the USA, but an appreciation of their definition is helpful in interpreting nursery catalogues and other data, even for those living elsewhere. Similar Hardiness Zone maps are available for other countries. Note that humidity, winter rain and wind chill factor makes a huge difference to the ability of many succulent plants and cacti to survive cold.
The eleven USDA Zones are defined by 'average annual minimum temperatures' as follows:

Zone 1:below -46°C  below -50°F
Zone 2: -46°Cto -40°C -50 to -40°F
Zone 3: -40°C to-34°C  -40 to -30°F
Zone 4: -34°C to-29°C -30 to -20°F
Zone 5: -29°Cto-23°C -20 to -10°F
Zone 6: -23°Cto-18°C -10 to    0°F
Zone 7: -18°Cto-12°C    0 to  10°F
Zone 8: -12°Cto-7°C  10 to  20°F
Zone 9:   -7°Cto-1°C  20 to  30°F
Zone 10:   -1°Cto 4°C  30 to  40°F
Zone 11:above    4°C  above 40°F
Maps of: USA Hardiness ZonesEuropean Hardiness Zones
*USDA = United States Department of Agriculture