USDA plant hardiness zones explained
The USDA Plant Hardiness Zone Map divides the United States into 13 zones, each split into "a" and "b", based on the average extreme minimum winter temperature measured from 1991 to 2020. Each zone is 10°F wide; each half-zone is 5°F.
How to use your zone
Plant tags list the coldest zone a plant can survive. If you live in zone 6b, a perennial "hardy to zone 5" will survive your winters; one hardy only to zone 8 will not, unless you protect it or grow it as an annual.
Zones do not tell you when to plant vegetables: for that you need your average frost dates.
Zone temperature table
| Zone | Avg. extreme winter low | Cities 10k+ on this site |
|---|---|---|
| 2a | -50°F to -45°F | 1 |
| 2b | -45°F to -40°F | 2 |
| 3b | -35°F to -30°F | 2 |
| 4a | -30°F to -25°F | 15 |
| 4b | -25°F to -20°F | 52 |
| 5a | -20°F to -15°F | 126 |
| 5b | -15°F to -10°F | 213 |
| 6a | -10°F to -5°F | 443 |
| 6b | -5°F to 0°F | 558 |
| 7a | 0°F to 5°F | 523 |
| 7b | 5°F to 10°F | 556 |
| 8a | 10°F to 15°F | 303 |
| 8b | 15°F to 20°F | 370 |
| 9a | 20°F to 25°F | 232 |
| 9b | 25°F to 30°F | 357 |
| 10a | 30°F to 35°F | 295 |
| 10b | 35°F to 40°F | 164 |
| 11a | 40°F to 45°F | 75 |
| 11b | 45°F to 50°F | 2 |
| 12a | 50°F to 55°F | 10 |
| 12b | 55°F to 60°F | 31 |
| 13a | 60°F to 65°F | 1 |
Source: USDA Agricultural Research Service and PRISM Climate Group, Oregon State University, 2023 Plant Hardiness Zone Map.