U.S. Frost Dates by Weather Station

NOAA station USC00133473 · Iowa

Frost Dates for Grinnell, IA

NOAA weather station Grinnell 3 SW (USC00133473), Iowa · elevation 905 ft (276 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 17only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostOctober 650% chance of a frost before it
  • Frost-free season155 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Grinnell, IASpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 17 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 6 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 155 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Oct 6Frost season at Grinnell, IASpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 17 in 1 year out of 10. Fall: the first frost comes before September 22 in 1 year out of 10, before October 6 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 155 days.JFMAMJJASONDLast frost ~May 3First frost ~Oct 6
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 17September 22
50% (typical)May 3October 6
90% (likely)April 19October 20
Frost-free season (32°F, 50% chance)
155 days
Hard frost (28°F), last in spring, 50% chance after
April 23
Hard frost (28°F), first in fall, 50% chance before
October 17

The last 15 years (2006–2020 normals)

Typical last frost May 2 (1 day earlier than the 30-year normal), typical first fall frost October 11 (5 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 Grinnell: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 17, 20% May 13, 30% May 8, 40% May 6, 50% May 3, 60% May 1, 70% April 27, 80% April 24, 90% April 19. Observed last spring frost in recent years: 2022 May 4, 2023 May 3, 2024 April 22, 2025 April 16, 2026 May 11.Spring: last frostChance of a frost after this date10%50%90%May 1May 17May 3Apr 192022: May 42023: May 32024: April 222025: April 162026: May 11Fall at Grinnell: chance of a frost before this dateFall, chance of a frost before the date: 10% September 22, 20% September 27, 30% September 30, 40% October 3, 50% October 6, 60% October 9, 70% October 12, 80% October 15, 90% October 20. Observed first fall frost in recent years: 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 22Oct 6Oct 202022: 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
2021no dataincomplete record
2022May 4September 28
2023May 3October 10
2024April 22October 7
2025April 16October 23
2026May 11season not over yet

What these dates mean

September 22 is the cautious fall date in Grinnell for finishing the harvest, when there is only a 10 % chance that a light frost has already arrived. Counting on tender crops after this date is risky, while October 20 carries a 90 % chance of a frost and is a gamble.

May 17 is the cautious spring date for setting out tender crops, with a 10 % chance of a light frost later than that day; planting on April 19 carries a 90 % chance of a frost and is a gamble. The 50% date for light frost in spring is May 3, indicating an even chance of a frost later than that day, and the 50% date for light frost in fall is October 6, indicating an even chance of a frost earlier than that day.

Hard-frost median dates are April 23 in spring and October 17 in fall; a hard frost at 28 °F can damage plants more severely than a light frost at 32 °F, so it is a more critical threshold.

How to read the probabilities

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

  • Spring: "10% after May 17" 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 22" 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 Grinnell, IANumbered nearest stations: 1 — Newton, IA — 14 mi; 2 — Toledo, IA — 23 mi; 3 — Marshalltown, IA — 25 mi; 4 — Belle Plaine, IA — 27 mi; 5 — Oskaloosa, IA — 28 mi.25 mi · 40 km1. Newton, IA — 14 mi12. Toledo, IA — 23 mi23. Marshalltown, IA — 25 mi34. Belle Plaine, IA — 27 mi45. Oskaloosa, IA — 28 mi5
  1. Newton, IA 14 mi (23 km)
  2. Toledo, IA 23 mi (38 km)
  3. Marshalltown, IA 25 mi (41 km)
  4. Belle Plaine, IA 27 mi (43 km)
  5. Oskaloosa, IA 28 mi (45 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: 41.72, -92.7489.