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Plant Hardiness Zones

 

Gardeners in the USA are familiar with the idea of Hardiness Zones and those of us searching the Internet for information about plants will have come across them.

Below is the table which shows the temperature ranges for each zone. The Table below uses the same Zone colours as the United States Depatment of Agriculture (USDA) Maps. There are several Zone Maps Links here.

In 1990 the USDA updated their plant hardiness zones map. It now divides the United States into 11 zones (1-11), based on their average minimum temperature. Each Zone is 10°F 'wide'. Zones 2 to 10 are subdivided into a colder "a" and warmer "b" sub-zone, giving a total of 21 zones or sub-zones. The temperature used by USDA is the average (mean) minimum annual temperature over a period of at least 10 years.

 

Celsius Zone Fahrenheit
Below -45.6 C 1 Below -50 F
-42.8 to -45.5 C 2a -50 to -45 F
-40.0 to -42.7 C 2b -45 to -40 F
-37.3 to -39.9 C 3a -40 to -35 F
-34.5 to -37.2 C 3b -35 to -30 F
-31.7 to -34.4 C 4a -30 to -25 F
-28.9 to -31.6 C 4b -25 to -20 F
-26.2 to -28.8 C 5a -20 to -15 F
-23.4 to -26.1 C 5b -15 to -10 F
-20.6 to -23.3 C 6a -10 to -5 F
-17.8 to -20.5 C 6b -5 to 0 F
-15.0 to -17.7 C 7a 0 to 5 F
-12.3 to -14.9 C 7b 5 to 10 F
-9.5 to -12.2 C 8a  10 to 15 F
-6.7 to -9.4 C 8b 15 to 20 F
-3.9 to -6.6 C 9a 20 to 25 F
-1.2 to -3.8 C 9b 25 to 30 F
1.6 to -1.1 C 10a 30 to 35 F
4.4 to 1.7 C 10b 35 to 40 F
above 4.5 C 11 above 40 F

Links to Zone Maps

United States Zones (at USNA)

European Zone maps (at Gardenweb)

Please note that some Maps do not use the a & b Sub-Zones and Colours vary a lot.
Also not all Zone Maps and Plant Hardiness Information uses the USDA system.
Canada have their own system (see below) and Australia are considering a system with 7 zones 5°C wide.

Canada

The Canadians were not impressed with the USDA's method which only considers minimum average temperatures.
In 1967, Agriculture Canada scientists created a plant hardiness map using Canadian plant survival data and a wider range of climatic variables, including minimum winter temperatures, length of the frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall and maximum wind speed. This was updated in 2000

Canadian Zone Maps (at CANSIS)

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