Palm tree in the snow

Palm tree in the snow

You have probably noticed the mentions USDA zone 7, 8, 9… on gardening websites and wondered what they are about and how to use them.

They are climate zones based on the average minimum temperature.

These zones can be used to indicate plant rusticity, i.e. the minimal temperature a plant can survive.

Origin:

The USDA (United States Department of Agriculture) has divided the country in climate zones based on the average minimum temperature.

There are 12 zones, zone 11 being the warmest and zone 0 the coldest.

Later the scale was refined using temperature differences of about 2,7 °C, subdividing each zone into an A and B zone:

Zone From: To:
0a -53.9°C (-65°F)
Ob -51.1°C (-60°F) -53.9°C (-65°F)
1a -48.3°C (-55°F) -51.1°C (-60°F)
1b -45.6°C (-50°F) -48.3°C (-55°F)
2a -42.8°C (-45°F) -45.6°C (-50°F)
2b -40°C (-40°F) -42.8°C (-45°F)
3a -37.2°C (-35°F) -40°C (-40°F)
3b -34.4°C (-30°F) -37.2°C (-35°F)
4a -31.7°C (-25°F) -34.4°C (-30°F)
4b -28.9°C (-20°F) -31.7°C (-25°F)
5a 26.1°C (-15°F) -28.9°C (-20°F)
5b -23.3°C (-10°F) -26.1°C (-15°F)
6a -20.6°C (-5°F) -23.3°C (-10°F)
6b -17.8°C (0°F) -20.6°C (-5°F)
7a -15°C (5°F) -17.8°C (0°F)
7b -12.2°C (10°F) 15°C (5°F)
8a -9.4°C (15°F) -12.2°C (10°F)
8b -6.7°C (20°F) -9.4°C (15°F)
9a -3.9°C (25°F) -6.7°C (20°F)
9b -1.1°C (30°F) -3.9°C (25°F)
10a -1.1°C (30°F) +1.7°C (35°F)
10b 1.7°C (35°F) +4.4°C (40°F)
11a +4.4°C (40°F) +7.2°C (45°F)
11b +7.2°C (45°F) +10°C (50°F)
12a +10°C (50°F) +12.8°C (55°F)
12b +12.8°C (55°F)

Inconveniences:

  • the USDA scale takes only the minimum temperatures into account but not the maximum temperatures, which can vary significantly within a given USDA zone, from tropical heat to moderate or even quite cold.
  • the scale also ignores other factors such as cold protection from snow, pollution or modern culture techniques.
  • the scale is based on statistical averages. In a harsh winter, the lowest temperature might be a lot lower than the average.
  • de scale provides information for a region, it does not take into account the specificities of your garden. A walled in urban garden exposed to the South will always be warmer than a countryside garden that’s open to the wind.

Utility:

Nevertheless, the USDA scale gives a fair indication about how resistant to cold resistance plants are.

If a plant is said to be USDA zone 10 or 11, it will never survive a UK or USA winter.

If it is said to be USDA zone 5, chances are it wil survive.

Find your USDA zone:

This website allows to find your USDA zone anywhere in the world: Plantmaps.com

Image sources

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