U.S. Frost Dates by Weather Station

NOAA station USC00116526 · Illinois

Frost Dates for Ottawa, IL

NOAA weather station Ottawa 5 SW (USC00116526), Illinois · elevation 525 ft (160 m) · hardiness zone 6a*

  • Lower-risk planting dateafter May 5only 1 spring in 10 still has a frost later
  • Typical last frostApril 2050% chance of a frost after it
  • Typical first fall frostOctober 2250% chance of a frost before it
  • Frost-free season183 daystypical year, 32°F
  • Hardiness zone6acalculated*

The year at a glance

Frost season at Ottawa, ILSpring: a frost after April 6 in 9 years out of 10, after April 20 in half of the years, after May 5 in 1 year out of 10. Fall: the first frost comes before October 7 in 1 year out of 10, before October 22 in half of the years, before November 4 in 9 years out of 10. Typical frost-free season: 183 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 20First frost ~Oct 22Frost season at Ottawa, ILSpring: a frost after April 6 in 9 years out of 10, after April 20 in half of the years, after May 5 in 1 year out of 10. Fall: the first frost comes before October 7 in 1 year out of 10, before October 22 in half of the years, before November 4 in 9 years out of 10. Typical frost-free season: 183 days.JFMAMJJASONDLast frost ~Apr 20First frost ~Oct 22
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 5October 7
50% (typical)April 20October 22
90% (likely)April 6November 4
Frost-free season (32°F, 50% chance)
183 days
Hard frost (28°F), last in spring, 50% chance after
April 8
Hard frost (28°F), first in fall, 50% chance before
November 1

The last 15 years (2006–2020 normals)

Typical last frost April 21 (1 day later than the 30-year normal), typical first fall frost October 24 (2 days later). Frost-free season 186 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at Ottawa: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 5, 20% April 30, 30% April 26, 40% April 23, 50% April 20, 60% April 18, 70% April 14, 80% April 11, 90% April 6. Observed last spring frost in recent years: 2021 April 22, 2022 April 27, 2023 April 27, 2024 April 25, 2025 April 16, 2026 April 20.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 5Apr 20Apr 62021: April 222022: April 272023: April 272024: April 252025: April 162026: April 20Fall at Ottawa: chance of a frost before this dateFall, chance of a frost before the date: 10% October 7, 20% October 13, 30% October 17, 40% October 20, 50% October 22, 60% October 24, 70% October 27, 80% October 30, 90% November 4. Observed first fall frost in recent years: 2021 November 1, 2022 October 18, 2023 October 30, 2024 October 16, 2025 October 24.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Oct 7Oct 22Nov 42021: November 12022: October 182023: October 302024: October 162025: October 24
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there. ◆ marks the actual date in each recent year.

What actually happened in recent years

Observed dates of the last spring and first fall frost (32°F) at this station; ◆ marks them on the charts above
YearLast spring frostFirst fall frost
2021April 22November 1
2022April 27October 18
2023April 27October 30
2024April 25October 16
2025April 16October 24
2026April 20season not over yet

What these dates mean

Planting tender crops on the early spring date, April 6, is a gamble; the record shows a 90 % chance of a light frost occurring later that season in Ottawa, Illinois.

The cautious approach for tender crops uses the later spring date, May 5, which carries only a 10 % chance of a light frost after planting. In the fall, the cautious date is October 7, when there is just a 10 % chance that a light frost has already arrived, providing a safer window to finish harvesting tender varieties.

The 50 % dates—April 20 in spring and October 22 in fall—mark the point where the odds of a light frost are even, so a gardener knows the risk is balanced. A hard frost at 28 °F, represented by the April 8 and November 1 median dates, can damage plants more severely than a light 32 °F frost.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after May 5" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before October 7" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near Ottawa, ILNumbered nearest stations: 1 — Peru, IL — 10 mi; 2 — Paw Paw, IL — 24 mi; 3 — Princeton, IL — 28 mi; 4 — Minonk, IL — 29 mi; 5 — Dwight, IL — 30 mi.INIA25 mi · 40 km1. Peru, IL — 10 mi12. Paw Paw, IL — 24 mi23. Princeton, IL — 28 mi34. Minonk, IL — 29 mi45. Dwight, IL — 30 mi5
  1. Peru, IL 10 mi (17 km)
  2. Paw Paw, IL 24 mi (38 km)
  3. Princeton, IL 28 mi (45 km)
  4. Minonk, IL 29 mi (47 km)
  5. Dwight, IL 30 mi (48 km)

Sources

* Hardiness zone 6a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 41.3283, -88.9106.