What a zone measures
The map is based on the average of each location's lowest winter temperature each year, over 30 years (1991 to 2020). It is not the coldest temperature ever recorded. Each zone is 10°F wide and is split into two 5°F half zones, "a" (colder) and "b" (warmer).
The zone chart
| Zone | Average coldest winter night | ZIP codes |
|---|---|---|
| 1a | -60 to -55°F (-51 to -48°C) | 5 |
| 1b | -55 to -50°F (-48 to -46°C) | 11 |
| 2a | -50 to -45°F (-46 to -43°C) | 27 |
| 2b | -45 to -40°F (-43 to -40°C) | 17 |
| 3a | -40 to -35°F (-40 to -37°C) | 23 |
| 3b | -35 to -30°F (-37 to -34°C) | 230 |
| 4a | -30 to -25°F (-34 to -32°C) | 814 |
| 4b | -25 to -20°F (-32 to -29°C) | 1,284 |
| 5a | -20 to -15°F (-29 to -26°C) | 2,426 |
| 5b | -15 to -10°F (-26 to -23°C) | 2,880 |
| 6a | -10 to -5°F (-23 to -21°C) | 5,187 |
| 6b | -5 to 0°F (-21 to -18°C) | 5,271 |
| 7a | 0 to 5°F (-18 to -15°C) | 4,877 |
| 7b | 5 to 10°F (-15 to -12°C) | 3,855 |
| 8a | 10 to 15°F (-12 to -9°C) | 3,430 |
| 8b | 15 to 20°F (-9 to -7°C) | 3,379 |
| 9a | 20 to 25°F (-7 to -4°C) | 2,199 |
| 9b | 25 to 30°F (-4 to -1°C) | 2,085 |
| 10a | 30 to 35°F (-1 to 2°C) | 1,234 |
| 10b | 35 to 40°F (2 to 4°C) | 752 |
| 11a | 40 to 45°F (4 to 7°C) | 205 |
| 11b | 45 to 50°F (7 to 10°C) | 16 |
| 12a | 50 to 55°F (10 to 13°C) | 30 |
| 12b | 55 to 60°F (13 to 16°C) | 137 |
| 13a | 60 to 65°F (16 to 18°C) | 102 |
| 13b | 65 to 70°F (18 to 21°C) | 26 |
What changed in 2023
The USDA and Oregon State University's PRISM Climate Group released the new map on November 15, 2023, the first update since 2012. It used 13,412 weather stations, up from 7,983. About half the country moved to the next warmer half zone; the other half stayed in the same half zone. The USDA says the change mostly reflects the newer 30-year period plus better mapping methods and more stations, and warns it should not be read as reliable evidence of climate change on its own, because annual extreme lows vary a lot from year to year.
How to use your zone
Plant labels often give a zone range, such as "zones 5 to 9". If your zone falls inside the range, the plant should survive a typical winter where you live. Winter cold is a crucial factor in plant survival, but it is not the only one: summer heat, rainfall, soil and shelter matter too.
Find yours with the zone finder.
Sources: USDA Agricultural Research Service, "USDA Unveils Updated Plant Hardiness Zone Map" (Nov 15, 2023); PRISM Climate Group, Oregon State University: 2023 USDA Plant Hardiness Zone Map ZIP code listings.
