The USDA Plant Hardiness Zone Map and the UK Hardiness Zone Map serve a similar purpose – to show the minimum temperatures that a specific plant can withstand in a given area.
The USDA map takes into account the average annual minimum winter temperature, while the UK map takes into consideration the minimum winter temperature and the frequency and duration of cold temperatures.
Both maps are helpful in guiding gardeners in determining which plants are suitable for their climate. But, it’s important to remember that these maps should not be the sole factor in choosing plants, as other elements like soil type, humidity, and wind exposure also play a role.
| USDA Zone | UK equivalent | Average Minimum Winter Temp. (°C) |
|---|---|---|
| 1 | n/a | below -46 |
| 2 | n/a | -46 to -40 |
| 3 | n/a | -40 to -34 |
| 4 | n/a | -34 to -29 |
| 5 | H4 | -29 to -23 |
| 6 | H4 to H3 | -23 to -18 |
| 7 | H3 to H2 | -18 to -12 |
| 8 | H2 | -12 to -6 |
| 9 | H2 to H1 | -6 to -1 |
| 10 | H1 | -1 to +4 |
| 11 | H | +4 to 9 |
The USDA map assigns zones, ranging from 1 to 13, based on temperature ranges. On the other hand, the UK map uses the letters H to designate hardiness, with H1 being the hardiest (able to survive temperatures down to -15°C) and H7 being the least hardy (able to survive temperatures down to 0°C).
How this relates in reality to UK gardening!
| Hardiness | Minimum temperature range |
|---|---|
| Hardy | Tolerates -15°C (5°F) and below |
| Frost-hardy | Survives down to about -5°C (23°F) |
| Half-hardy | Survives down to 0°C (32°F) |
| Semi-tender | Damaged by temperatures below 5°C (41°F) unless protected |
| Tender | Must be overwintered indoors at 7°C (45°F) or higher, depending on provenance |
