U.S. Frost Dates by Weather Station

NOAA station USC00136305 · Iowa

Frost Dates for Osage, IA

NOAA weather station Osage (USC00136305), Iowa · elevation 1170 ft (357 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 12only 1 spring in 10 still has a frost later
  • Typical last frostApril 3050% chance of a frost after it
  • Typical first fall frostOctober 950% chance of a frost before it
  • Frost-free season163 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Osage, IASpring: a frost after April 14 in 9 years out of 10, after April 30 in half of the years, after May 12 in 1 year out of 10. Fall: the first frost comes before September 25 in 1 year out of 10, before October 9 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 163 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 30First frost ~Oct 9Frost season at Osage, IASpring: a frost after April 14 in 9 years out of 10, after April 30 in half of the years, after May 12 in 1 year out of 10. Fall: the first frost comes before September 25 in 1 year out of 10, before October 9 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 163 days.JFMAMJJASONDLast frost ~Apr 30First frost ~Oct 9
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 12September 25
50% (typical)April 30October 9
90% (likely)April 14October 21
Frost-free season (32°F, 50% chance)
163 days
Hard frost (28°F), last in spring, 50% chance after
April 17
Hard frost (28°F), first in fall, 50% chance before
October 20

The last 15 years (2006–2020 normals)

Typical last frost April 30 (same as the 30-year normal), typical first fall frost October 14 (5 days later). Frost-free season 168 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 Osage: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 12, 20% May 7, 30% May 4, 40% May 2, 50% April 30, 60% April 26, 70% April 22, 80% April 19, 90% April 14. Observed last spring frost in recent years: 2021 May 12, 2023 May 4, 2024 April 25, 2025 April 27.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 12Apr 30Apr 142021: May 122023: May 42024: April 252025: April 27Fall at Osage: chance of a frost before this dateFall, chance of a frost before the date: 10% September 25, 20% September 29, 30% October 3, 40% October 6, 50% October 9, 60% October 11, 70% October 14, 80% October 17, 90% October 21. Observed first fall frost in recent years: 2021 October 22, 2023 October 10, 2024 October 7, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 25Oct 9Oct 212021: October 222023: October 102024: October 72025: October 23
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
2021May 12October 22
2022incomplete recordincomplete record
2023May 4October 10
2024April 25October 7
2025April 27October 23
2026incomplete recordseason not over yet

What these dates mean

The cautious fall date in Osage for finishing the harvest is September 25, when there is only a 10 % chance that a light frost has arrived.

For a gardener, the 50 % light-frost dates—April 30 in spring and October 9 in fall—mark the point where a light frost has an even 50 % chance of having occurred. The 50 % hard-frost dates—April 17 in spring and October 20 in fall—similarly indicate an even 50 % chance of a 28 °F temperature having been reached.

A cautious spring plan uses the May 12 date, which carries a 10 % chance of a later light frost, while the September 25 date guides the generally safe end of the tender-crop season. A hard frost at 28 °F can damage plant tissue that survives a light frost at 32 °F.

How to read the probabilities

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

  • Spring: "10% after May 12" 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 September 25" 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 Osage, IANumbered nearest stations: 1 — Charles City, IA — 17 mi; 2 — Mason City, IA — 21 mi; 3 — Northwood, IA — 24 mi; 4 — Austin, MN — 27 mi; 5 — Mason City (Airport), IA — 27 mi.IAMNWI25 mi · 40 km1. Charles City, IA — 17 mi12. Mason City, IA — 21 mi23. Northwood, IA — 24 mi34. Austin, MN — 27 mi45. Mason City (Airport), IA — 27 mi5
  1. Charles City, IA 17 mi (27 km)
  2. Mason City, IA 21 mi (34 km)
  3. Northwood, IA 24 mi (38 km)
  4. Austin, MN 27 mi (44 km)
  5. Mason City (Airport), IA 27 mi (44 km)

Sources

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

Station coordinates: 43.2794, -92.8106.