When to plant onions in Illinois
Illinois’s average last spring frost is April 20, and the first fall frost typically arrives around October 14 — about 177 frost-free days (USDA zones 5a, 5b, 6a, 6b, 7a). Onion needs 90–120 days to mature, which fits comfortably within Illinois's growing season.
When to plant onions in Illinois: the short answer
For a spring planting, plant out Mar 23 – Apr 9, with seeds started indoors Feb 2 – Feb 16. Seeds germinate in soil from 40 to 90°F.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow onions in Illinois, step by step
- Start seeds indoors (Feb 2 – Feb 16). Sow seeds ½ inch deep in seed-starting mix under bright light. They take 7-14 days to sprout, fastest in soil around 70°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 12 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Mar 23 – Apr 9). Space plants about 4 inches apart in rows 12 inches apart. It handles frost well, so it can go out before the last frost date. As a cool-season crop it does best before summer heat arrives in Illinois.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: onion maggot, onion thrips. Diseases and disorders: downy mildew, white rot, neck rot, purple blotch.
- Harvest (from about Jun 28 – Jul 28). Onion takes 90-120 days to reach harvest. Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.
Figures come from our crop data; dates use Illinois’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in Illinois
Illinois spans USDA zones 5a, 5b, 6a, 6b, 7a. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the onion dates shift with them.
When to plant onions in nearby states
Questions gardeners in Illinois ask
When is the best time to plant onions in Illinois?
Plant onion transplants outdoors in Illinois between March 23 and April 9. That timing is set by Illinois's average last spring frost of April 20.
When should I start onion seeds indoors in Illinois?
Start seeds indoors between February 2 and February 16, about 10 weeks before the last frost, so the seedlings are ready to plant out from March 23.
When will onions planted in Illinois be ready to harvest?
Planted on time, onions are usually ready from about June 28 to July 28, 90-120 days after transplanting.
When is the last frost in Illinois?
On average around April 20, with the first fall frost around October 14 - about 177 frost-free days. Your town may differ by two weeks or more.