U.S. Frost Dates by Weather Station

NOAA station USC00135198 · Iowa

Frost Dates for Marshalltown, IA

NOAA weather station Marshalltown (USC00135198), Iowa · elevation 870 ft (265 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 11only 1 spring in 10 still has a frost later
  • Typical last frostApril 2850% chance of a frost after it
  • Typical first fall frostOctober 750% chance of a frost before it
  • Frost-free season162 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Marshalltown, IASpring: a frost after April 13 in 9 years out of 10, after April 28 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 22 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: 162 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 28First frost ~Oct 7Frost season at Marshalltown, IASpring: a frost after April 13 in 9 years out of 10, after April 28 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 22 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: 162 days.JFMAMJJASONDLast frost ~Apr 28First 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 11September 22
50% (typical)April 28October 7
90% (likely)April 13October 21
Frost-free season (32°F, 50% chance)
162 days
Hard frost (28°F), last in spring, 50% chance after
April 17
Hard frost (28°F), first in fall, 50% chance before
October 18

The last 15 years (2006–2020 normals)

Typical last frost April 27 (1 day earlier than the 30-year normal), typical first fall frost October 12 (5 days later). Frost-free season 167 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 Marshalltown: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 11, 20% May 6, 30% May 3, 40% April 30, 50% April 28, 60% April 25, 70% April 22, 80% April 18, 90% April 13. Observed last spring frost in recent years: 2021 May 12, 2022 April 27, 2024 April 22, 2025 April 17, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 11Apr 28Apr 132021: May 122022: April 272024: April 222025: April 172026: May 7Fall at Marshalltown: chance of a frost before this dateFall, chance of a frost before the date: 10% September 22, 20% September 27, 30% October 1, 40% October 4, 50% October 7, 60% October 10, 70% October 13, 80% October 17, 90% October 21. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 10, 2024 October 15, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 22Oct 7Oct 212021: October 222022: September 282023: October 102024: October 152025: 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
2022April 27September 28
2023incomplete recordOctober 10
2024April 22October 15
2025April 17October 23
2026May 7season not over yet

What these dates mean

Setting out tender crops on the early spring date of April 13 in Marshalltown is a gamble, because there is a 90 % chance of a light frost later that season.

The 50% light-frost dates are April 28 in spring and October 7 in fall; each marks an even-odds point for a frost to occur. Gardeners who prefer a low-risk schedule use the cautious dates, May 11 for planting tender crops in spring and September 22 for finishing harvest in fall. The 50% hard-frost dates are April 17 and October 18, the median points for a 28 °F frost.

A hard frost at 28 °F can injure plants that would survive a light frost at 32 °F, making it a more critical threshold. The interval between the two 32°F 50% dates spans 162 days, showing the typical frost-free window. The probabilities are based on a 30-year NOAA record for this station.

How to read the probabilities

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

  • Spring: "10% after May 11" 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 Marshalltown, IANumbered nearest stations: 1 — Toledo, IA — 18 mi; 2 — Grundy Center, IA — 22 mi; 3 — Eldora, IA — 23 mi; 4 — Newton, IA — 25 mi; 5 — Grinnell, IA — 25 mi.25 mi · 40 km1. Toledo, IA — 18 mi12. Grundy Center, IA — 22 mi23. Eldora, IA — 23 mi34. Newton, IA — 25 mi45. Grinnell, IA — 25 mi5
  1. Toledo, IA 18 mi (29 km)
  2. Grundy Center, IA 22 mi (36 km)
  3. Eldora, IA 23 mi (36 km)
  4. Newton, IA 25 mi (40 km)
  5. Grinnell, IA 25 mi (41 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: 42.0647, -92.9244.