U.S. Frost Dates by Weather Station

NOAA station USC00138688 · Iowa

Frost Dates for Washington, IA

NOAA weather station Washington (USC00138688), Iowa · elevation 687 ft (209 m) · hardiness zone 5b*

  • 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 1150% chance of a frost before it
  • Frost-free season165 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Washington, 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 25 in 1 year out of 10, before October 11 in half of the years, before October 24 in 9 years out of 10. Typical frost-free season: 165 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 28First frost ~Oct 11Frost season at Washington, 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 25 in 1 year out of 10, before October 11 in half of the years, before October 24 in 9 years out of 10. Typical frost-free season: 165 days.JFMAMJJASONDLast frost ~Apr 28First frost ~Oct 11
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.Today

Frost dates (32°F)

Probability of frost, based on 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 11September 25
50% (typical)April 28October 11
90% (likely)April 13October 24
Frost-free season (32°F, 50% chance)
165 days
Hard frost (28°F), last in spring, 50% chance after
April 17
Hard frost (28°F), first in fall, 50% chance before
October 22

The last 15 years (2006–2020 normals)

Typical last frost April 28 (same as the 30-year normal), typical first fall frost October 15 (4 days later). Frost-free season 169 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 Washington: 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 24, 70% April 21, 80% April 18, 90% April 13. Observed last spring frost in recent years: 2021 April 25, 2022 April 27, 2024 April 22, 2025 April 17.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 11Apr 28Apr 132021: April 252022: April 272024: April 222025: April 17Fall at Washington: chance of a frost before this dateFall, chance of a frost before the date: 10% September 25, 20% October 1, 30% October 4, 40% October 8, 50% October 11, 60% October 13, 70% October 16, 80% October 19, 90% October 24. Observed first fall frost in recent years: 2021 October 23, 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 25Oct 11Oct 242021: October 232022: 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
2021April 25October 23
2022April 27September 28
2023incomplete recordOctober 10
2024April 22October 15
2025April 17October 23
2026incomplete recordseason not over yet

What these dates mean

Setting out tender crops on April 13 in Washington, Iowa, carries a 90 % chance that a light frost will still occur later in the season. If a gardener prefers a lower risk, the cautious spring date is May 11, when only a 10 % chance of frost remains.

The 50 % spring date, April 28, marks the point where a light frost is equally likely to occur later or not; similarly, the 50 % fall date, October 11, splits the odds for an earlier frost. For finishing tender crops, the cautious fall date is September 25, with only a 10 % chance that frost has arrived earlier; counting on crops through October 24 would be a gamble with a 90 % chance of frost already.

Hard frosts (28 °F) are more damaging than light frosts, so the median hard-frost dates of April 17 in spring and October 22 in fall signal when the temperature is equally likely to reach that more severe 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 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 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 Washington, IANumbered nearest stations: 1 — Fairfield, IA — 22 mi; 2 — Mount Pleasant, IA — 24 mi; 3 — Iowa City, IA — 25 mi; 4 — Sigourney, IA — 27 mi; 5 — Williamsburg, IA — 30 mi.IL25 mi · 40 km1. Fairfield, IA — 22 mi12. Mount Pleasant, IA — 24 mi23. Iowa City, IA — 25 mi34. Sigourney, IA — 27 mi45. Williamsburg, IA — 30 mi5
  1. Fairfield, IA 22 mi (36 km)
  2. Mount Pleasant, IA 24 mi (39 km)
  3. Iowa City, IA 25 mi (40 km)
  4. Sigourney, IA 27 mi (44 km)
  5. Williamsburg, IA 30 mi (49 km)

Sources

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

Station coordinates: 41.2825, -91.7078.