RHS and USDA Hardiness Zones

RHS and USDA hardiness zones explained! The British Royal Horticultural Society (RHS) and United States Department of Agriculture (USDA) hardiness zones are systems classifying geographical regions based on the average minimum temperatures experienced over a ten-year period. These zones are crucial for gardeners to make informed decisions about what plants can be grown successfully in their local climate.

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Royal Horticultural Society Climate Hardiness Zones

RHS hardiness zones range from H7 to H1a, with H7 being the hardiest and H1a plants needing temperatures above 15°C / 59°F.

RHS ZoneTemperatureHardiness Notes
H1Colder than 15°C / 59°F.Requires a heated greenhouse.
H1aWarmer than 15°C / 59°F.Tropical plants for indoors and heated greenhouses.
H1b10°C / 50°F to 15°C / 59°F.Subtropical plants for indoors and heated greenhouses.
H1c5°C / 41°F to 10°C / 50°F.Warm temperate plants that can go outdoors in summer.
H21°C / 34°F to 5°C / 41°F.Plants that need a frost-free greenhouse in winter.
H3−5°C / 23°F to 1°C / 34°F.Hardy outside in some regions or situations, or which – while usually grown outside in summer – need frost protection in winter (e.g. dahlias).
H4−10°C / 14°F to −5°C / 23°F.Plants hardy outside in most of the UK in an average winter.
H5−15°C / 5°F to −10°C / 14°F.Plants hardy outside in most of the UK in severe winters.
H6−20°C / −4°F to −15°C / 5°F.Plants hardy outside in the UK and northern Europe.
H7Below −20°C / −4°F.Plants hardy outside in the severest European climates.

USDA Climate Hardiness Zones

USDA hardiness zones range from 1 to 13, with zone 1 being the coldest and zone 13 the warmest (above 15°C / 59°F).

USDA ZoneTemperatureHardiness Notes
1a-51.1°C / -60°F to -48.3°C / -55°F.
1b-48.3°C / -55°F to -45.6°C / -50°F.
2a-45.6°C / -50°F to -42.8°C / -45°F.
2b-42.8°C / -45°F to -40°C / -40°F.
3a-40°C / -40°F to -37.2°C / -35°F.
3b-37.2°C / -35°F to -34.4°C / -30°F.
4a-34.4°C / -30°F to -31.7°C / -25°F.
4b-31.7°C / -25°F to -28.9°C / -20°F.
5a-28.9°C / -20°F to -26.1°C / -15°F.
5b-26.1°C / -15°F to -23.3°C / -10°F.
6a-23.3°C / -10°F to -20.6°C / -5°F.
6b-20.6°C / -5°F to -17.8°C / 0°F.
7a-17.8°C / 0°F to -15°C / 5°F.
7b-15°C / 5°F to -12.2°C / 10°F.
8a-12.2°C / 10°F to -9.4°C / 15°F.
8b-9.4°C / 15°F to -8.7°C / 20°F.
9a-6.7°C / 20°F to -3.9°C / 25°F.
9b-3.9°C / 25°F to -1.1°C / 30°F.
10a-1.1°C / 30°F to 1.7°C / 35°F.
10b1.7°C / 35°F to 4.4°C / 40°F.
11a4.4°C / 40°F to 7.2°C / 45°F.
11b7.2°C / 45°F to 10°C / 50°F.
12a10°C / 50°F to 12.8°C / 55°F.
12b12.8°C / 55°F to 15.6°C / 60°F.
13a15.6°C / 60°F to 18.3°C / 65°F.
13b18.3°C / 65°F to 21.1°C / 70°F.

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