USDA hardiness zone lookup
O'Fallon, IL — USDA Zone 6b
O'Fallon, Illinois · 187-day growing season
Frost dates and growing season in O'Fallon
| USDA hardiness zone | Zone 6b |
|---|---|
| Average last spring frost | April 14 |
| Average first fall frost | October 18 |
| Growing season length | ~187 days |
| Temperature range (F) | -10 to 0°F |
| Temperature range (C) | -23 to -18°C |
All of O'Fallon's mapped ZIP codes fall in the same hardiness band, Zone 6b.
These are 50%-probability averages modeled from O'Fallon's USDA hardiness zone and regional climate normals — not a single-station reading. In a typical year the last spring frost will have passed by April 14, but a colder-than-average year can run 1-2 weeks later. Plant tender crops (tomatoes, peppers, basil) once both soil and night temperatures are consistently warm — a thermometer beats the calendar.
Growing season in O'Fallon
O'Fallon, Illinois sits in USDA Zone 6b, with roughly 187 frost-free days between an average last spring frost around April 14 and a first fall frost around October 18. That is a long season — succession-sow through summer and run a full fall crop; heat-sensitive greens still need spring/autumn timing.
What grows in O'Fallon
O'Fallon falls in USDA Zone 6b, so the same hardiness constraints apply as the full Zone 6 guide. Vegetables, herbs, and fruit trees rated to Zone 6b (or hardier) will overwinter here in a typical year.
- Tomatoes (all types)
- Peppers, eggplant
- Squash, melons, cucumbers
- Beans, peas
- Sweet corn
- Apples, pears, peaches, plums
- Cherries, blueberries
- Asparagus, rhubarb
- Garlic (fall-planted)
- Strawberries
What to plant in O'Fallon this week
O'Fallon is in high summer — most spring plantings are in. Keep an eye on watering and start planning your fall crop. Cool-season seedlings (broccoli, cabbage, lettuce) can be started indoors for a fall transplant.
- When to plant tomatoes in zone 6
- When to plant peppers in zone 6
- When to plant bush beans in zone 6
- When to plant cucumbers in zone 6
- When to plant basil in zone 6
Full planting calendar for O'Fallon
Crop-by-crop sowing, transplant, and harvest dates calibrated to zone 6 averages:
- When to plant tomatoes in zone 6
- When to plant peppers in zone 6
- When to plant basil in zone 6
- When to plant garlic in zone 6
- When to plant lettuce in zone 6
- When to plant bush beans in zone 6
- When to plant cucumbers in zone 6
- When to plant summer squash in zone 6
- When to plant peas in zone 6
- When to plant carrots in zone 6
ZIP codes in O'Fallon
Drill down to the precise frost window and planting calendar for a specific ZIP in O'Fallon:
Local microclimate notes
Zone tables give you the average — but O'Fallongardens vary. South-facing walls and paved areas can run a full half-zone warmer than the published rating. Low-lying spots, frost pockets, and shaded north sides can run colder. If you've gardened here a few seasons, your own frost record — the last time you actually got frost damage — beats any national average.
Source and methodology
Hardiness zone from the USDA Plant Hardiness Zone Map (2023 revision). Frost-date and growing-season figures are modeled from O'Fallon's USDA hardiness zone and regional NOAA 1991-2020 climate normals — zone-level estimates, not a per-station record, so treat them as planning guidance and confirm against your own local frost history. Crop recommendations draw on US Cooperative Extension references, curated by the Growli editorial team. Last reviewed June 2026.
Other cities in Illinois
- Addison, IL — USDA Zone 6a
- Aurora, IL — USDA Zone 6a
- Carbondale, IL — USDA Zone 7a
- Champaign, IL — USDA Zone 6a
- Chicago, IL — USDA Zone 6a
- Elgin, IL — USDA Zone 5b
- McHenry, IL — USDA Zone 5b
- Naperville, IL — USDA Zone 6a
- Oak Lawn, IL — USDA Zone 6a
- Oak Park, IL — USDA Zone 6a
- Peoria, IL — USDA Zone 5b
- Rockford, IL — USDA Zone 5b
- All of Illinois by zone