U.S. Frost Dates by Weather Station

NOAA station USC00136800 · Iowa

Frost Dates for Primghar, IA

NOAA weather station Primghar (USC00136800), Iowa · elevation 1520 ft (463 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 11only 1 spring in 10 still has a frost later
  • Typical last frostApril 2950% chance of a frost after it
  • Typical first fall frostOctober 1050% chance of a frost before it
  • Frost-free season163 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Primghar, IASpring: a frost after April 13 in 9 years out of 10, after April 29 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 10 in half of the years, before October 23 in 9 years out of 10. Typical frost-free season: 163 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 29First frost ~Oct 10Frost season at Primghar, IASpring: a frost after April 13 in 9 years out of 10, after April 29 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 10 in half of the years, before October 23 in 9 years out of 10. Typical frost-free season: 163 days.JFMAMJJASONDLast frost ~Apr 29First frost ~Oct 10
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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 11September 26
50% (typical)April 29October 10
90% (likely)April 13October 23
Frost-free season (32°F, 50% chance)
163 days
Hard frost (28°F), last in spring, 50% chance after
April 18
Hard frost (28°F), first in fall, 50% chance before
October 21

The last 15 years (2006–2020 normals)

Typical last frost April 29 (same as the 30-year normal), typical first fall frost October 14 (4 days later). Frost-free season 168 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 Primghar: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 11, 20% May 6, 30% May 3, 40% May 1, 50% April 29, 60% April 26, 70% April 22, 80% April 18, 90% April 13. Observed last spring frost in recent years: 2021 May 6, 2022 April 26, 2023 May 2, 2024 April 21, 2025 April 19, 2026 May 6.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 11Apr 29Apr 132021: May 62022: April 262023: May 22024: April 212025: April 192026: May 6Fall at Primghar: chance of a frost before this dateFall, chance of a frost before the date: 10% September 26, 20% October 1, 30% October 4, 40% October 7, 50% October 10, 60% October 13, 70% October 15, 80% October 19, 90% October 23. Observed first fall frost in recent years: 2021 October 16, 2022 September 28, 2023 October 7, 2024 October 15, 2025 October 22.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 26Oct 10Oct 232021: October 162022: September 282023: October 72024: October 152025: 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 6October 16
2022April 26September 28
2023May 2October 7
2024April 21October 15
2025April 19October 22
2026May 6season not over yet

What these dates mean

The cautious fall date in Primghar for finishing the harvest is September 26, when there is only a 10% chance that a light frost has arrived. After that date the risk of a frost rises quickly, and by October 23 a 90% chance signals a gamble for tender crops.

The 50% light-frost dates—April 29 in spring and October 10 in fall—mark the point at which a light frost is equally likely to occur before or after the date, giving gardeners a clear reference for planning. A cautious spring plan uses the late date of May 11, which carries a 10% chance of a later frost, while a cautious fall plan relies on the early September 26 date. Hard frost at 28°F, shown by the median dates April 18 and October 21, matters more than light frost because the colder temperature can freeze plant tissue that survives a 32°F frost.

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 26" 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 Primghar, IANumbered nearest stations: 1 — Sanborn, IA — 7 mi; 2 — Sheldon, IA — 13 mi; 3 — Sibley, IA — 23 mi; 4 — Cherokee, IA — 23 mi; 5 — Spencer, IA — 25 mi.IAMNSDNE25 mi · 40 km1. Sanborn, IA — 7 mi12. Sheldon, IA — 13 mi23. Sibley, IA — 23 mi34. Cherokee, IA — 23 mi45. Spencer, IA — 25 mi5
  1. Sanborn, IA 7 mi (11 km)
  2. Sheldon, IA 13 mi (21 km)
  3. Sibley, IA 23 mi (37 km)
  4. Cherokee, IA 23 mi (37 km)
  5. Spencer, IA 25 mi (40 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: 43.0864, -95.6292.