1. USDA Hardiness Zones Overview
The USDA divides the US into 13 main zones (1–13), each further split into “a” and “b” (e.g., 5a, 5b), based on 10°F increments in minimum winter temperature. These zones help gardeners understand cold tolerance but are also useful for estimating frost-free days.
Zone | Min Temp (°F) | Min Temp (°C) | Typical Frost-Free Days | Typical Growing Season |
1a | -60 to -55 | -51 to -48 | <60 | Very short, late June–August |
1b | -55 to -50 | -48 to -46 | <60 | Very short, late June–August |
2a | -50 to -45 | -46 to -43 | 60–90 | Late June–September |
2b | -45 to -40 | -43 to -40 | 60–90 | Late June–September |
3a | -40 to -35 | -40 to -37 | 90–120 | Early June–September |
3b | -35 to -30 | -37 to -34 | 90–120 | Early June–September |
4a | -30 to -25 | -34 to -32 | 120–150 | May–October |
4b | -25 to -20 | -32 to -29 | 120–150 | May–October |
5a | -20 to -15 | -29 to -26 | 150–180 | April–October |
5b | -15 to -10 | -26 to -23 | 150–180 | April–October |
6a | -10 to -5 | -23 to -21 | 180–210 | March–November |
6b | -5 to 0 | -21 to -18 | 180–210 | March–November |
7a | 0 to 5 | -18 to -15 | 210–240 | March–November |
7b | 5 to 10 | -15 to -12 | 210–240 | March–November |
8a | 10 to 15 | -12 to -9 | 240–270 | March–December |
8b | 15 to 20 | -9 to -7 | 240–270 | March–December |
9a | 20 to 25 | -7 to -4 | 270–300 | February–December |
9b | 25 to 30 | -4 to -1 | 270–300 | February–December |
10a | 30 to 35 | -1 to 2 | 300–330 | February–January |
10b | 35 to 40 | 2 to 4 | 300–330 | February–January |
11a | 40 to 45 | 4 to 7 | >330 | Year-round |
11b | 45 to 50 | 7 to 10 | >330 | Year-round |
12a | 50 to 55 | 10 to 13 | Year-round | Year-round |
12b | 55 to 60 | 13 to 16 | Year-round | Year-round |
13a | 60 to 65 | 16 to 18 | Year-round | Year-round |
13b | 65 to 70 | 18 to 21 | Year-round | Year-round |
2. Key Notes on Growing Season
- The growing season is usually measured as the frost-free period between the last spring frost and the first fall frost.
- Northern zones (1–4) have short seasons (60–150 days).
- Mid-zones (5–7) have moderate seasons (150–240 days).
- Southern zones (8–13) have long seasons (240+ days), often allowing multiple crop cycles or year-round growing.
3. Seasonal Start & End (Generalized)
- Zone 1–3: Growing season begins late May to early June, ends August–September.
- Zone 4–5: Starts April–May, ends September–October.
- Zone 6–7: Starts March–April, ends October–November.
- Zone 8–9: Starts February–March, ends December.
- Zone 10–13: Can grow crops year-round, minimal frost risk.
4. Practical Tip
- Gardeners often consult their local extension service or use the National Gardening Association frost date tool for exact planting dates.
- Zones only measure cold tolerance, not heat, humidity, rainfall, or soil — so check those before selecting crops.
If you want, I can draw a full US map color-coded by zone and include exact frost start/end dates for all major zones, which makes planning crops much easier.
Do you want me to make that map?
Usda