U.S. Frost Dates by Weather Station

NOAA station USC00131277 · Iowa

Frost Dates for Castana Experiment Farm, IA

NOAA weather station Castana Experiment Farm (USC00131277), Iowa · elevation 1400 ft (427 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 7only 1 spring in 10 still has a frost later
  • Typical last frostApril 2550% chance of a frost after it
  • Typical first fall frostOctober 1250% chance of a frost before it
  • Frost-free season170 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Castana Experiment Farm, IASpring: a frost after April 11 in 9 years out of 10, after April 25 in half of the years, after May 7 in 1 year out of 10. Fall: the first frost comes before September 28 in 1 year out of 10, before October 12 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 170 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 25First frost ~Oct 12Frost season at Castana Experiment Farm, IASpring: a frost after April 11 in 9 years out of 10, after April 25 in half of the years, after May 7 in 1 year out of 10. Fall: the first frost comes before September 28 in 1 year out of 10, before October 12 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 170 days.JFMAMJJASONDLast frost ~Apr 25First frost ~Oct 12
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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 7September 28
50% (typical)April 25October 12
90% (likely)April 11October 27
Frost-free season (32°F, 50% chance)
170 days
Hard frost (28°F), last in spring, 50% chance after
April 15
Hard frost (28°F), first in fall, 50% chance before
October 23

The last 15 years (2006–2020 normals)

Typical last frost April 25 (same as the 30-year normal), typical first fall frost October 18 (6 days later). Frost-free season 175 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 Castana Experiment Farm: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 7, 20% May 2, 30% April 30, 40% April 28, 50% April 25, 60% April 22, 70% April 19, 80% April 15, 90% April 11.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 7Apr 25Apr 11Fall at Castana Experiment Farm: chance of a frost before this dateFall, chance of a frost before the date: 10% September 28, 20% October 2, 30% October 6, 40% October 9, 50% October 12, 60% October 16, 70% October 18, 80% October 22, 90% October 27.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 28Oct 12Oct 27
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.

What these dates mean

At Castana Experiment Farm the cautious spring date is May 7, when there is only a 10 % chance of a light frost later; this lets a gardener schedule the setting out of tender crops with minimal frost risk. By contrast, the early spring date of April 11 carries a 90 % chance and would be a gamble.

The 50% spring date is April 25, meaning there is a 50 % chance of a light frost later than this date, while the 50% fall date is October 12, meaning a 50 % chance of a light frost earlier than this date. The cautious fall date is September 28, where only a 10 % chance of a light frost earlier allows the finishing of tender-crop harvests. A hard frost, defined at 28 °F, is more damaging than a light frost at 32 °F; the median hard-frost dates are April 15 in spring and October 23 in fall, indicating when a 50 % chance of a hard frost exists.

How to read the probabilities

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

  • Spring: "10% after May 7" 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 28" 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 Castana Experiment Farm, IANumbered nearest stations: 1 — near Mapleton, IA — 7 mi; 2 — Denison, IA — 26 mi; 3 — Logan, IA — 29 mi; 4 — near Sioux City, IA — 36 mi; 5 — West Point, NE — 48 mi.IA25 mi · 40 km1. near Mapleton, IA — 7 mi12. Denison, IA — 26 mi23. Logan, IA — 29 mi34. near Sioux City, IA — 36 mi45. West Point, NE — 48 mi5
  1. near Mapleton, IA 7 mi (12 km)
  2. Denison, IA 26 mi (42 km)
  3. Logan, IA 29 mi (47 km)
  4. near Sioux City, IA 36 mi (58 km)
  5. West Point, NE 48 mi (77 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: 42.0633, -95.8364.