Illinois · zones 5b–6b
When to plant sweet potatoes in Illinois.
Sow or transplant between May 27 to Jun 16 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
Sweet Potatoes dates for a typical Illinois season.
1 · Start indoors
Apr 15 to May 5
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
May 27 to Jun 16
Keyed to the ~Apr 20 last frost. Sow 3" deep, 14" apart.
Harvest
~110 days later
~2.5 lb per plant over ~1 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 sweet potatoes here.
A middle-length season, about 181 frost-free days, means sweet potato overlaps wireworms & white grubs for a stretch each summer. Check plants every week or two and treat early.
Wireworms & white grubs
Shallow holes and grooves chewed into the roots, found only at digging time.
Organic fix: Avoid planting into ground that was in sod last year; rotate away from grassy beds.
Clusters on the tender growing tips of new vines.
Organic fix: Hose them off in the morning, or use insecticidal soap.
Black rot
Dry, firm dark patches on the roots, usually found at harvest or in storage, with a sweet fruity smell.
Organic fix: Start from clean slips cut well above the soil line, never save roots from a bad crop, and rotate 3 years.
Your season math: Illinois gives sweet potatoes about 181 frost-free days, and it needs roughly 110 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting sweet potatoes in Illinois: questions
When should I plant sweet potatoes in Illinois?+
In a typical Illinois year, sow sweet potatoes or set out transplants between May 27 to Jun 16, 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 sweet potatoes seeds indoors in Illinois?+
Start seeds inside about 6 weeks before set-out: roughly Apr 15 to May 5 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 sweet potatoes 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.