When to plant basil 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). Basil needs 30–60 days to mature, which fits comfortably within Illinois's growing season.
When to plant basil in Illinois: the short answer
For a spring planting, plant out Apr 27 – May 14, with seeds started indoors Mar 9 – Mar 23. Wait until the soil is at least 65°F and frost is over.
Succession sow again in Illinois: May 25 · Jun 15 · Jul 6 · Jul 27 · Aug 17
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 basil in Illinois, step by step
- Start seeds indoors (Mar 9 – Mar 23). Sow seeds ⅛ inch deep in seed-starting mix under bright light. They take 5-10 days to sprout, fastest in soil around 80°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. It needs at least six to eight hours of direct sun a day and loose soil at least 10 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Apr 27 – May 14). Space plants about 10 inches apart in rows 12 inches apart. It is killed by frost, so nothing goes outside until the danger has passed.
- 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: aphids, Japanese beetle, slugs. Diseases and disorders: basil downy mildew, fusarium wilt, damping off.
- Harvest (from about Jun 3 – Jul 3). Basil takes 30-60 days to reach harvest. Cold soil stalls it - wait for warm nights. Pinch above a leaf pair every couple of weeks and remove flower buds to keep leaves coming. Downy mildew-resistant varieties are worth seeking out.
Figures come from our crop data; dates use Illinois’s average frost dates. More in the full basil growing guide.
Basil 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 basil dates shift with them.
When to plant basil in nearby states
Questions gardeners in Illinois ask
When is the best time to plant basil in Illinois?
Plant basil transplants outdoors in Illinois between April 27 and May 14. That timing is set by Illinois's average last spring frost of April 20.
When should I start basil seeds indoors in Illinois?
Start seeds indoors between March 9 and March 23, about 5 weeks before the last frost, so the seedlings are ready to plant out from April 27.
What is the latest I can plant basil in Illinois?
About September 14. Basil needs at least 30 days to mature, and frost around October 14 ends the season, so later plantings risk being cut short.
When will basil planted in Illinois be ready to harvest?
Planted on time, basil is usually ready from about June 3 to July 3, 30-60 days after planting.
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.