When to Plant Onions in Illinois
Published: April 21, 2026
Illinois gardeners in Zone 5b have a 28-week frost-free window for onion (April 7 to October 22). Cool-season crops like onion do well here because the short, cool summers match the plant's temperature preferences naturally. This guide gives exact dates for Illinois based on your local frost calendar.
Onion Planting Dates for Illinois
| Start seeds indoors | January 27–February 3 |
| Last frost (average) | April 7 |
| Transplant outdoors | March 10–20 |
| Direct sow outdoors | March 10–20 |
| Expect first harvest | June 8 – July 8 |
| First fall frost (average) | October 22 |
Dates above assume Zone 5b, the most common zone in Illinois. Gardeners in cooler Chicago (Zone 5b) plant 1–2 weeks later; warmer Peoria (Zone 6a) can plant 1–2 weeks earlier.
⚠ CRITICAL: Choose variety based on your latitude. Short-day varieties for south of 35°N (Texas, Florida, California). Long-day varieties for north of 35°N (Ohio, Minnesota, New York). Intermediate varieties work in the middle band.
Growing Onions in Illinois's Climate
Illinois's cold Zone 5b climate is close to ideal for onion. The cool springs and moderate summers match the plant's natural temperature preferences, producing crisp, sweet, slow-bolting harvests that hot-climate gardeners cannot grow.
The fall onion crop in Illinois is often better than the spring crop. Plants mature as temperatures cool in September and October, concentrating sugars and tightening heads or roots. Light frost can improve flavor; hard frost signals harvest time.
Common onion pests to watch for in Illinois include Onion Thrips and Onion Maggot. The first line of defense is companion planting: Carrot and Tomato planted nearby discourage several of these pests by confusing host-plant identification or repelling adults before they can lay eggs. Weekly garden walks in the early morning catch problems when they're still manageable.
Onion Garden Calendar for Illinois
| Month | Task | Notes |
|---|---|---|
| January | Start seeds indoors | Use grow lights; warm soil to 70–75°F for best germination |
| February | — | Dormant season |
| March | Transplant outdoors | Soil temp must be 35°F+; nighttime lows above 50°F |
| April | Harden off seedlings | Set seedlings outdoors 1–2 hours daily, increasing gradually |
| May ← | Watch for bolting | Harvest promptly; spring heat arrives quickly |
| June | First harvest window opens | Onion begins producing 90–120 days after transplant |
| July | — | Dormant season |
| August | — | Dormant season |
| September | — | Dormant season |
| October | First frost approaches | Harvest remaining onion; cover plants on frost nights |
| November | — | Dormant season |
| December | — | Dormant season |
Onion Growing Tips for Illinois Gardeners
- •Match variety to latitude: short-day south of 35°N, long-day north of 35°N, intermediate in between.
- •Growing from sets (small bulbs) is fastest; growing from transplants gives the most variety options.
- •Illinois's short growing season means start seeds indoors under grow lights — don't rely on windowsill light which produces leggy, weak seedlings that struggle outdoors.
- •Cover young transplants with floating row covers for the first 2–3 weeks outside. The 3–5°F of protection reduces frost damage risk and accelerates early growth.
- •Reduce watering as tops begin to fall over — dry conditions encourage tight, firm bulbs that store longer.
Companion Planting for Onions in Illinois
In Illinois's cold climate, planting onion with Carrot and Tomato helps deter pests and improve soil. Avoid planting near Bean and Pea, which inhibit onion growth through root chemistry or shared pest pressure. See the full onion companion planting guide for the complete list.
Onions Planting Dates by City in Illinois
Top cities in Illinois — select for city-level frost dates and planting calendars.
Frequently Asked Questions
When should I plant onion in Illinois?
In Illinois (Zone 5b), start onion seeds indoors around January 27–February 3 and transplant outdoors around March 10–20 — 4 weeks before the state's average last frost of April 7.
What is the last frost date in Illinois?
Illinois's average last frost date is April 7 in the state's most common Zone 5b. Frost dates vary by city — higher elevations and northern counties run 1 to 3 weeks later than the state average. See the city list below for city-level frost dates.
Can I grow onion year-round in Illinois?
onion is a cool-season crop in Illinois, with spring and fall windows. Summer heat bolts the plants before they can produce a harvestable crop. Succession planting every 2–3 weeks stretches the harvest window.
What onion varieties grow best in Illinois?
For Illinois's Zone 5b, Long-day varieties like Walla Walla, Copra, and Yellow Sweet Spanish bulb under the 14+ hour days north of 35°N.
When do I start onion seeds indoors in Illinois?
Start onion seeds indoors in Illinois around January 27–February 3, which is 10 weeks before the state's average last frost of April 7. Use grow lights and keep soil temperatures at 70–75°F for fastest germination.