Illinois · zones 5b–6b
When to plant broccoli in Illinois.
Sow or transplant between Feb 27 to Mar 27 in a typical year, keyed to Illinois's median last frost around Apr 20. A second, fall sowing fits Jul 4 to Aug 13. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Broccoli dates for a typical Illinois season.
1 · Start indoors
Jan 16 to Feb 13
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 27 to Mar 27
Keyed to the ~Apr 20 last frost. Sow 0.5" deep, 18" apart.
Fall round
Jul 4 to Aug 13
Counted back from the ~Oct 18 first frost.
Harvest
~60 days later
~1.2 lb per plant over ~3 weeks.
Illinois spans more than one climate: Zone 6 is the most common zone, but the state runs 5b to 6b. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
In Illinois: watch for
The pests that find broccoli here.
A middle-length season, about 181 frost-free days, means broccoli overlaps cabbage worms & loopers for a stretch each summer. Check plants every week or two and treat early.
Green caterpillars chewing ragged holes; green droppings.
Organic fix: Handpick, cover with netting, spray Bt.
Gray-green clusters tucked into the leaf folds.
Organic fix: Blast with water, then insecticidal soap.
Tiny holes peppering the young leaves.
Organic fix: Row cover from day one; diatomaceous earth.
Black rot
Yellow V-shaped wedges cutting in from the leaf edge, veins blackening inside.
Organic fix: Pull infected plants, water at the base, keep brassicas out of that bed 3 years.
Seedlings wilt, yellow and stall for no reason; white maggots tunnel the roots.
Organic fix: Row cover before the flies are up, when yellow rocket blooms; rotate the bed.
Your season math: Illinois gives broccoli about 181 frost-free days, and it needs roughly 60 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting broccoli in Illinois: questions
When should I plant broccoli in Illinois?+
In a typical Illinois year, sow broccoli or set out transplants between Feb 27 to Mar 27, keyed to the state's median last frost around Apr 20. There's also a fall window: Jul 4 to Aug 13. Your exact dates shift with your ZIP code and microclimate.
When do I start broccoli seeds indoors in Illinois?+
Start seeds inside about 6 weeks before set-out: roughly Jan 16 to Feb 13 for a typical Illinois season.
What USDA zone is Illinois?+
Illinois spans zones 5b–6b, mostly zone 6. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many broccoli plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.