U.S. Frost Dates by Weather Station

NOAA station USC00132110 · Iowa

Frost Dates for Decorah, IA

NOAA weather station Decorah (USC00132110), Iowa · elevation 860 ft (262 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 16only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostOctober 750% chance of a frost before it
  • Frost-free season157 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Decorah, IASpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 16 in 1 year out of 10. Fall: the first frost comes before September 25 in 1 year out of 10, before October 7 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 157 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Oct 7Frost season at Decorah, IASpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 16 in 1 year out of 10. Fall: the first frost comes before September 25 in 1 year out of 10, before October 7 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 157 days.JFMAMJJASONDLast frost ~May 3First frost ~Oct 7
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 16September 25
50% (typical)May 3October 7
90% (likely)April 19October 21
Frost-free season (32°F, 50% chance)
157 days
Hard frost (28°F), last in spring, 50% chance after
April 22
Hard frost (28°F), first in fall, 50% chance before
October 17

The last 15 years (2006–2020 normals)

Typical last frost May 1 (2 days earlier than the 30-year normal), typical first fall frost October 10 (3 days later). Frost-free season 161 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 Decorah: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 16, 20% May 12, 30% May 9, 40% May 5, 50% May 3, 60% April 30, 70% April 28, 80% April 24, 90% April 19. Observed last spring frost in recent years: 2021 May 29, 2022 May 23, 2023 May 3, 2024 April 22, 2025 April 20, 2026 May 11.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 16May 3Apr 192021: May 292022: May 232023: May 32024: April 222025: April 202026: May 11Fall at Decorah: chance of a frost before this dateFall, chance of a frost before the date: 10% September 25, 20% September 29, 30% October 1, 40% October 4, 50% October 7, 60% October 9, 70% October 12, 80% October 15, 90% October 21. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 10, 2024 October 7, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 25Oct 7Oct 212021: October 222022: September 282023: 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 29October 22
2022May 23September 28
2023May 3October 10
2024April 22October 7
2025April 20October 23
2026May 11season not over yet

What these dates mean

May 16 is the cautious spring date for light frost in Decorah, allowing a gardener to set out tender crops with only a 10% chance of a later frost. The opposite end of the season is September 25, the cautious fall date, when there is only a 10% chance that a frost has already arrived, so tender crops can be finished harvesting with minimal risk.

The 50% dates indicate the point at which a light frost is equally likely to have occurred or not. In spring the median date is May 3 and in fall it is October 7; these dates do not mark a frost but show when the odds are even. The hard-frost median dates are April 22 in spring and October 17 in fall, and a hard frost at 28 °F can damage or kill plants that would survive 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 16" 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 Decorah, IANumbered nearest stations: 1 — Cresco, IA — 16 mi; 2 — Caledonia, MN — 27 mi; 3 — Preston, MN — 29 mi; 4 — Fayette, IA — 29 mi; 5 — New Hampton, IA — 32 mi.WIMN25 mi · 40 km1. Cresco, IA — 16 mi12. Caledonia, MN — 27 mi23. Preston, MN — 29 mi34. Fayette, IA — 29 mi45. New Hampton, IA — 32 mi5
  1. Cresco, IA 16 mi (26 km)
  2. Caledonia, MN 27 mi (43 km)
  3. Preston, MN 29 mi (47 km)
  4. Fayette, IA 29 mi (47 km)
  5. New Hampton, IA 32 mi (51 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.3042, -91.7953.