U.S. Frost Dates by Weather Station

NOAA station USC00116998 · Illinois

Frost Dates for Princeton, IL

NOAA weather station Princeton (USC00116998), Illinois · elevation 697 ft (212 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 7only 1 spring in 10 still has a frost later
  • Typical last frostApril 2250% chance of a frost after it
  • Typical first fall frostOctober 1750% chance of a frost before it
  • Frost-free season177 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Princeton, ILSpring: a frost after April 9 in 9 years out of 10, after April 22 in half of the years, after May 7 in 1 year out of 10. Fall: the first frost comes before October 3 in 1 year out of 10, before October 17 in half of the years, before October 29 in 9 years out of 10. Typical frost-free season: 177 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 22First frost ~Oct 17Frost season at Princeton, ILSpring: a frost after April 9 in 9 years out of 10, after April 22 in half of the years, after May 7 in 1 year out of 10. Fall: the first frost comes before October 3 in 1 year out of 10, before October 17 in half of the years, before October 29 in 9 years out of 10. Typical frost-free season: 177 days.JFMAMJJASONDLast frost ~Apr 22First frost ~Oct 17
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 7October 3
50% (typical)April 22October 17
90% (likely)April 9October 29
Frost-free season (32°F, 50% chance)
177 days
Hard frost (28°F), last in spring, 50% chance after
April 12
Hard frost (28°F), first in fall, 50% chance before
October 27

The last 15 years (2006–2020 normals)

Typical last frost April 22 (same as the 30-year normal), typical first fall frost October 20 (3 days later). Frost-free season 179 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 Princeton: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 7, 20% May 1, 30% April 29, 40% April 26, 50% April 22, 60% April 20, 70% April 17, 80% April 14, 90% April 9. Observed last spring frost in recent years: 2021 April 22, 2022 April 28, 2023 April 27.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 7Apr 22Apr 92021: April 222022: April 282023: April 27Fall at Princeton: chance of a frost before this dateFall, chance of a frost before the date: 10% October 3, 20% October 8, 30% October 12, 40% October 15, 50% October 17, 60% October 20, 70% October 22, 80% October 25, 90% October 29. Observed first fall frost in recent years: 2021 November 2, 2022 October 18, 2023 October 30.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Oct 3Oct 17Oct 292021: November 22022: October 182023: October 30
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 2
2022April 28October 18
2023April 27October 30
2024no datano data
2025no datano data
2026no dataseason not over yet

What these dates mean

Planting tender crops on April 9 in Princeton is a gamble; the record shows a 90% chance that a frost will occur later that day. The cautious approach waits until May 7, when only a 10% chance of a later frost remains. In autumn, the cautious date to finish harvesting tender crops is October 3, when only a 10% chance of an earlier frost exists.

The 50% light-frost dates, April 22 in spring and October 17 in fall, mark the point at which the odds of a frost are even; they give a midpoint reference for risk. Likewise, the 50% hard-frost dates are April 12 and October 27, indicating when a 28 °F frost is as likely as not. Gardeners can use these dates to gauge where they sit between the low-risk and high-risk windows.

A hard frost at 28 °F can injure plants that would survive a light frost at 32 °F, so it represents a greater danger.

How to read the probabilities

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

  • Spring: "10% after May 7" 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 3" 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 Princeton, ILNumbered nearest stations: 1 — Walnut, IL — 16 mi; 2 — Peru, IL — 18 mi; 3 — Kewanee, IL — 25 mi; 4 — Ottawa, IL — 28 mi; 5 — Paw Paw, IL — 32 mi.IA25 mi · 40 km1. Walnut, IL — 16 mi12. Peru, IL — 18 mi23. Kewanee, IL — 25 mi34. Ottawa, IL — 28 mi45. Paw Paw, IL — 32 mi5
  1. Walnut, IL 16 mi (25 km)
  2. Peru, IL 18 mi (29 km)
  3. Kewanee, IL 25 mi (40 km)
  4. Ottawa, IL 28 mi (45 km)
  5. Paw Paw, IL 32 mi (52 km)

Sources

* Hardiness zone 5b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 41.3661, -89.4519.