When to plant in Ottawa, IL
USDA Zone 5bHere are the average frost dates, USDA hardiness zone, and a month-by-month planting calendar for Ottawa, Illinois — all computed from Ottawa's nearest NOAA weather station.
In zone 5b, Ottawa gardeners can grow many hardy perennials, while the last and first frost still dictate the annual vegetable calendar.
Frost probability
MARSEILLES LOCK · 1991–2020The date the last spring and first fall frost occur, by threshold and probability. A 90% date is later in spring — and earlier in fall — than a 10% date; the 50% · 32°F row is what most gardeners plan around. These are Ottawa’s own odds, recorded at MARSEILLES LOCK.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | May 22 | May 7 | Apr 22 | Sep 24 | Oct 7 | Oct 20 |
| 32°F | May 13 | Apr 27 | Apr 12 | Oct 3 | Oct 17 | Oct 29 |
| 28°F | May 1 | Apr 15 | Apr 2 | Oct 14 | Oct 27 | Nov 9 |
Download this table as CSV ↓ — every threshold and probability, plus this city’s planting-window dates.
What to plant now
TODAY · JULY 19Full-year planting calendar
Each bar is the exact window to take a planting action in Ottawa, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 24 km · complete 32°F normalsWhen stations disagree by more than a few days, that spread is real microclimate variation — elevation, water, urban heat. Judge which station best matches your own yard.
Ottawa planting FAQ
When is the last spring frost in Ottawa, IL?
Ottawa's average last spring frost falls near April 27 — the 50% mark at 32°F in the 1991–2020 normals. Hold tender transplants until the risk has passed, then plant out.
When is the first fall frost in Ottawa, IL?
The first fall frost in Ottawa typically arrives around October 17 (50% probability at 32°F). Harvest or protect frost-sensitive crops before then.
What hardiness zone is Ottawa in?
Ottawa is in USDA hardiness zone 5b. In zone 5b, winters are cold — pick cold-hardy varieties and protect tender crops.
How long is the growing season in Ottawa?
Ottawa has about 173 frost-free days — a moderate growing season — between the average last spring frost (April 27) and first fall frost (October 17).
When should I plant tomatoes in Ottawa?
For Ottawa, sow tomatoes indoors about March 2–March 16 and move the seedlings out around May 4, after the last spring frost.
Never miss a window in Ottawa
An email when it’s time to start seeds, transplant, and sow — timed to Ottawa’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reachFrost dates recorded at MARSEILLES LOCK, 7 km from the city center · 1991–2020 NOAA climate normals · zone from the USDA/PRISM 2023 map. How we compute this.
BlissGarden. "When to Plant in Ottawa, IL — Frost Dates & Zone 5b." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USC00115372. Retrieved from https://blissgarden.com/illinois/ottawa.