Illinois · zones 5b–6b
When to plant basil in Illinois.
Sow or transplant between May 11 to Jul 27 in a typical year, keyed to Illinois's median last frost around Apr 20. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Basil dates for a typical Illinois season.
1 · Start indoors
Mar 30 to Jun 15
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
May 11 to Jul 27
Keyed to the ~Apr 20 last frost. Sow 0.25" deep, 8" apart.
Harvest
~40 days later
~0.5 lb per plant over ~12 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 basil here.
A middle-length season, about 181 frost-free days, means basil overlaps aphids for a stretch each summer. Check plants every week or two and treat early.
Fine webbing and pale stippling in hot, dry spells.
Organic fix: Spray the undersides with water, then insecticidal soap or neem.
Yellowing with gray fuzz underneath, or white powder on top.
Organic fix: Space for airflow, water at the base, spray neem.
Metallic beetles chewing basil and mint leaves to lace.
Organic fix: Handpick into soapy water in the morning; neem.
Your season math: Illinois gives basil about 181 frost-free days, and it needs roughly 40 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting basil in Illinois: questions
When should I plant basil in Illinois?+
In a typical Illinois year, sow basil or set out transplants between May 11 to Jul 27, keyed to the state's median last frost around Apr 20. Your exact dates shift with your ZIP code and microclimate.
When do I start basil seeds indoors in Illinois?+
Start seeds inside about 6 weeks before set-out: roughly Mar 30 to Jun 15 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 basil 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.