U.S. Frost Dates by Weather Station

NOAA station USC00136327 · Iowa

Frost Dates for Oskaloosa, IA

NOAA weather station Oskaloosa (USC00136327), Iowa · elevation 830 ft (253 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 4only 1 spring in 10 still has a frost later
  • Typical last frostApril 2150% chance of a frost after it
  • Typical first fall frostOctober 1650% chance of a frost before it
  • Frost-free season176 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Oskaloosa, IASpring: a frost after April 8 in 9 years out of 10, after April 21 in half of the years, after May 4 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 16 in half of the years, before October 29 in 9 years out of 10. Typical frost-free season: 176 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 21First frost ~Oct 16Frost season at Oskaloosa, IASpring: a frost after April 8 in 9 years out of 10, after April 21 in half of the years, after May 4 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 16 in half of the years, before October 29 in 9 years out of 10. Typical frost-free season: 176 days.JFMAMJJASONDLast frost ~Apr 21First frost ~Oct 16
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 4September 29
50% (typical)April 21October 16
90% (likely)April 8October 29
Frost-free season (32°F, 50% chance)
176 days
Hard frost (28°F), last in spring, 50% chance after
April 11
Hard frost (28°F), first in fall, 50% chance before
October 27

The last 15 years (2006–2020 normals)

Typical last frost April 23 (2 days later than the 30-year normal), typical first fall frost October 19 (3 days later). Frost-free season 179 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 Oskaloosa: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 4, 20% May 1, 30% April 28, 40% April 24, 50% April 21, 60% April 18, 70% April 15, 80% April 12, 90% April 8.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 4Apr 21Apr 8Fall at Oskaloosa: chance of a frost before this dateFall, chance of a frost before the date: 10% September 29, 20% October 5, 30% October 9, 40% October 13, 50% October 16, 60% October 19, 70% October 22, 80% October 25, 90% October 29.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 29Oct 16Oct 29
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

May 4 is the cautious spring light-frost date for Oskaloosa; with only a 10 % chance of a frost later, it signals when a gardener can safely set out tender crops. In the same way, September 29 is the cautious fall light-frost date, carrying a 10 % chance of a frost earlier and marking the latest time to finish harvesting tender crops.

The 50 % light-frost dates are April 21 in spring and October 16 in fall; at each of these dates the likelihood of a frost is even, so a gardener should treat them as the midpoint of risk. The 50 % hard-frost dates are April 11 and October 27, indicating when a frost of 28 °F is equally likely to have occurred or not.

Hard frost matters more than light frost because a 28 °F event can damage plant tissue that survives a 32 °F light 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 4" 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 29" 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 Oskaloosa, IANumbered nearest stations: 1 — near Ottumwa, IA — 18 mi; 2 — Albia, IA — 19 mi; 3 — Sigourney, IA — 22 mi; 4 — Grinnell, IA — 28 mi; 5 — Newton, IA — 33 mi.ILMO25 mi · 40 km1. near Ottumwa, IA — 18 mi12. Albia, IA — 19 mi23. Sigourney, IA — 22 mi34. Grinnell, IA — 28 mi45. Newton, IA — 33 mi5
  1. near Ottumwa, IA 18 mi (29 km)
  2. Albia, IA 19 mi (31 km)
  3. Sigourney, IA 22 mi (35 km)
  4. Grinnell, IA 28 mi (45 km)
  5. Newton, IA 33 mi (54 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: 41.3214, -92.6467.