When to Plant Onion in Illinois
Start Seeds Indoors
Wednesday, February 18, 2026
Transplant outdoors: Wednesday, April 29, 2026
State
Illinois
USDA Zones
5a, 5b, 6a, 6b, 7a
Avg Last Frost
Wednesday, April 15, 2026
Cities Covered
1286
Illinois Planting Schedule by Zone
Illinois spans 5 zones — find yours below for the most accurate dates.
| Zone | Last Frost | Start Indoors | Transplant |
|---|---|---|---|
| 5a | Sunday, May 10, 2026 | Sunday, March 15, 2026 | Sunday, May 24, 2026 |
| 5b | Saturday, April 25, 2026 | Saturday, February 28, 2026 | Saturday, May 9, 2026 |
| 6a | Monday, April 20, 2026 | Monday, February 23, 2026 | Monday, May 4, 2026 |
| 6b | Wednesday, April 15, 2026 | Wednesday, February 18, 2026 | Wednesday, April 29, 2026 |
| 7a | Friday, April 10, 2026 | Friday, February 13, 2026 | Friday, April 24, 2026 |
Dates based on average last frost. Check your local forecast before planting.
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Growing Onion in Illinois: Complete Guide
Onion (Allium cepa) is a warm-season crop that needs a head start indoors before Illinois's last frost. Most Illinois gardeners begin seeds indoors around Wednesday, February 18, 2026 and move transplants outside around Wednesday, April 29, 2026, after the risk of frost has passed.
Because Illinois stretches across 5 USDA hardiness zones (5a, 5b, 6a, 6b, 7a), planting windows can shift by as much as six to eight weeks from one end of the state to the other. Gardeners in the warmer southern zones can start earlier, while those in cooler northern zones should wait until the zone table above shows their last frost has passed.
With 100 days to maturity, Onion needs the indoor head start that Illinois's growing season requires. Starting indoors ensures you have strong transplants ready the moment outdoor conditions are right.
Onion requires full sun (6+ hours) and moderate moisture. Most of Illinois receives adequate sunlight during the growing season, though gardeners in humid eastern regions should watch for fungal issues and ensure good air circulation.
For the most precise planting dates tailored to your exact location, enter your ZIP code on our planting calendar or use the zone table above to find the schedule for your area of Illinois.
Growing Onion in Illinois: Local Tips
Climate-specific advice for Illinois gardeners.
Long-day varieties, planted early
Plant sets or transplants as early as the soil can be worked (March in most of the Midwest); onions need the full season to size up before the long summer days trigger bulbing.
Frequently Asked Questions
When should I plant Onion in Illinois?
In Illinois, start Onion seeds indoors around Wednesday, February 18, 2026 and transplant outdoors around Wednesday, April 29, 2026 after your last frost. Illinois spans zones 5a–7a, so dates shift by 1–4 weeks depending on where you live.
What is the last frost date in Illinois?
Illinois spans 5 USDA zones (5a, 5b, 6a, 6b, 7a), so last frost dates vary by region. Apple River, Baileyville, Cedarville (5a) sees frost until around Sunday, May 10, 2026, while Alton, Cottage Hills, East Alton (7a) is frost-free much earlier.
How long does Onion take to grow in Illinois?
Onion takes about 100 days from transplant (or direct sow) to harvest. Starting seeds indoors in Illinois gives you a head start so plants are ready to go out after the last frost.
What USDA zones does Illinois cover for Onion?
Illinois includes zones 5a, 5b, 6a, 6b, 7a. Onion grows well across all of these zones with adjusted timing — use the zone table above to find the right dates for your part of the state.
Find Precise Dates for Your City
Select a city for exact planting dates based on local frost records.