U.S. Frost Dates by Weather Station

NOAA station USC00137844 · Iowa

Frost Dates for Spencer, IA

NOAA weather station Spencer 1 N (USC00137844), Iowa · elevation 1326 ft (404 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 14only 1 spring in 10 still has a frost later
  • Typical last frostMay 150% chance of a frost after it
  • Typical first fall frostOctober 350% chance of a frost before it
  • Frost-free season154 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Spencer, IASpring: a frost after April 18 in 9 years out of 10, after May 1 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 3 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 154 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 1First frost ~Oct 3Frost season at Spencer, IASpring: a frost after April 18 in 9 years out of 10, after May 1 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 3 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 154 days.JFMAMJJASONDLast frost ~May 1First frost ~Oct 3
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 14September 20
50% (typical)May 1October 3
90% (likely)April 18October 17
Frost-free season (32°F, 50% chance)
154 days
Hard frost (28°F), last in spring, 50% chance after
April 22
Hard frost (28°F), first in fall, 50% chance before
October 14

The last 15 years (2006–2020 normals)

Typical last frost May 2 (1 day later than the 30-year normal), typical first fall frost October 6 (3 days later). Frost-free season 158 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 Spencer: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 14, 20% May 9, 30% May 6, 40% May 4, 50% May 1, 60% April 30, 70% April 26, 80% April 23, 90% April 18. Observed last spring frost in recent years: 2021 May 12, 2022 May 4, 2023 May 3, 2024 April 24, 2025 April 19, 2026 May 11.Spring: last frostChance of a frost after this date10%50%90%May 1May 14May 1Apr 182021: May 122022: May 42023: May 32024: April 242025: April 192026: May 11Fall at Spencer: chance of a frost before this dateFall, chance of a frost before the date: 10% September 20, 20% September 24, 30% September 28, 40% September 30, 50% October 3, 60% October 5, 70% October 9, 80% October 13, 90% October 17. Observed first fall frost in recent years: 2021 October 16, 2022 September 28, 2023 October 10, 2024 October 7, 2025 October 22.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 20Oct 3Oct 172021: October 162022: September 282023: October 102024: October 72025: October 22
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 16
2022May 4September 28
2023May 3October 10
2024April 24October 7
2025April 19October 22
2026May 11season not over yet

What these dates mean

September 20 marks the cautious fall date for finishing the harvest in Spencer, Iowa, when there is only a 10% chance that a light frost has arrived earlier. For tender crops, the cautious spring date is May 14, which carries a 10% chance of a frost later, while September 20 is the cautious fall date for ending production.

The 50% dates—May 1 for spring and October 3 for fall—represent the points where a light frost is equally likely to occur later or earlier, giving gardeners a neutral reference for planning. The hard-frost median dates are April 22 in spring and October 14 in fall, and hard frost (28°F) matters more than light frost (32°F) because the lower temperature can damage plants that survive a 32°F event.

How to read the probabilities

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

  • Spring: "10% after May 14" 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 20" 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 Spencer, IANumbered nearest stations: 1 — Milford, IA — 15 mi; 2 — Sioux Rapids, IA — 19 mi; 3 — Lake Park, IA — 22 mi; 4 — Primghar, IA — 25 mi; 5 — Sanborn, IA — 26 mi.MNSD25 mi · 40 km1. Milford, IA — 15 mi12. Sioux Rapids, IA — 19 mi23. Lake Park, IA — 22 mi34. Primghar, IA — 25 mi45. Sanborn, IA — 26 mi5
  1. Milford, IA 15 mi (24 km)
  2. Sioux Rapids, IA 19 mi (31 km)
  3. Lake Park, IA 22 mi (35 km)
  4. Primghar, IA 25 mi (40 km)
  5. Sanborn, IA 26 mi (42 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: 43.1653, -95.1467.