U.S. Frost Dates by Weather Station

NOAA station USC00133487 · Iowa

Frost Dates for Grundy Center, IA

NOAA weather station Grundy Center (USC00133487), Iowa · elevation 985 ft (300 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 12only 1 spring in 10 still has a frost later
  • Typical last frostApril 2950% chance of a frost after it
  • Typical first fall frostOctober 950% chance of a frost before it
  • Frost-free season160 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Grundy Center, IASpring: a frost after April 15 in 9 years out of 10, after April 29 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 20 in 9 years out of 10. Typical frost-free season: 160 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 29First frost ~Oct 9Frost season at Grundy Center, IASpring: a frost after April 15 in 9 years out of 10, after April 29 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 20 in 9 years out of 10. Typical frost-free season: 160 days.JFMAMJJASONDLast frost ~Apr 29First 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 29October 9
90% (likely)April 15October 20
Frost-free season (32°F, 50% chance)
160 days
Hard frost (28°F), last in spring, 50% chance after
April 19
Hard frost (28°F), first in fall, 50% chance before
October 18

The last 15 years (2006–2020 normals)

Typical last frost April 29 (same as the 30-year normal), typical first fall frost October 13 (4 days later). Frost-free season 166 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 Grundy Center: 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 1, 50% April 29, 60% April 26, 70% April 23, 80% April 19, 90% April 15. Observed last spring frost in recent years: 2021 May 12, 2022 April 27, 2023 May 3, 2024 April 21, 2025 April 17, 2026 May 11.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 12Apr 29Apr 152021: May 122022: April 272023: May 32024: April 212025: April 172026: May 11Fall at Grundy Center: chance of a frost before this dateFall, chance of a frost before the date: 10% September 25, 20% September 30, 30% October 3, 40% October 6, 50% October 9, 60% October 11, 70% October 14, 80% October 16, 90% October 20. Observed first fall frost in recent years: 2021 October 23, 2022 October 8, 2023 October 11, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 25Oct 9Oct 202021: October 232022: October 82023: October 112025: 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 23
2022April 27October 8
2023May 3October 11
2024April 21incomplete record
2025April 17October 23
2026May 11season not over yet

What these dates mean

September 25 is the cautious fall date in Grundy Center for finishing the harvest; there is only a 10 % chance that a light frost has arrived earlier. In spring the cautious date for setting out tender crops is May 12, when a light frost later than that day occurs with just a 10 % chance.

The 50 % light-frost dates—April 29 in spring and October 9 in fall—mark the median point where a frost is as likely as not, giving gardeners a clear reference for risk. The 50 % hard-frost dates are April 19 and October 18; these dates are listed without implying “before” or “after” and indicate when a 28 °F frost has even odds of occurring.

A hard frost matters more than a light frost because the lower 28 °F temperature can cause more severe damage to plant tissue than the 32 °F 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 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 Grundy Center, IANumbered nearest stations: 1 — Eldora, IA — 17 mi; 2 — Marshalltown, IA — 22 mi; 3 — Waterloo, IA — 22 mi; 4 — Toledo, IA — 25 mi; 5 — Allison, IA — 27 mi.25 mi · 40 km1. Eldora, IA — 17 mi12. Marshalltown, IA — 22 mi23. Waterloo, IA — 22 mi34. Toledo, IA — 25 mi45. Allison, IA — 27 mi5
  1. Eldora, IA 17 mi (28 km)
  2. Marshalltown, IA 22 mi (36 km)
  3. Waterloo, IA 22 mi (36 km)
  4. Toledo, IA 25 mi (39 km)
  5. Allison, IA 27 mi (43 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: 42.3647, -92.7594.