U.S. Frost Dates by Weather Station

NOAA station USC00132171 · Iowa

Frost Dates for Denison, IA

NOAA weather station Denison (USC00132171), Iowa · elevation 1401 ft (427 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 9only 1 spring in 10 still has a frost later
  • Typical last frostApril 2750% chance of a frost after it
  • Typical first fall frostOctober 1150% chance of a frost before it
  • Frost-free season166 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Denison, IASpring: a frost after April 13 in 9 years out of 10, after April 27 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 27 in 1 year out of 10, before October 11 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 166 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 27First frost ~Oct 11Frost season at Denison, IASpring: a frost after April 13 in 9 years out of 10, after April 27 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 27 in 1 year out of 10, before October 11 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 166 days.JFMAMJJASONDLast frost ~Apr 27First 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.

Frost dates (32°F)

Probability of frost, based on 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 9September 27
50% (typical)April 27October 11
90% (likely)April 13October 25
Frost-free season (32°F, 50% chance)
166 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 29 (2 days later than the 30-year normal), typical first fall frost October 14 (3 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 Denison: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 9, 20% May 4, 30% May 2, 40% April 30, 50% April 27, 60% April 24, 70% April 21, 80% April 18, 90% April 13. Observed last spring frost in recent years: 2021 May 12, 2022 April 27, 2023 April 24, 2024 April 20.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 9Apr 27Apr 132021: May 122022: April 272023: April 242024: April 20Fall at Denison: chance of a frost before this dateFall, chance of a frost before the date: 10% September 27, 20% October 1, 30% October 4, 40% October 7, 50% October 11, 60% October 14, 70% October 17, 80% October 20, 90% October 25. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 9, 2024 October 7.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 27Oct 11Oct 252021: October 222022: September 282023: October 92024: October 7
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 22
2022April 27September 28
2023April 24October 9
2024April 20October 7
2025no datano data
2026no dataseason not over yet

What these dates mean

The 166 days between the 50% dates in Denison frame the period when a light frost (32 °F) is equally likely to occur or not, with April 27 marking the spring median and October 11 the fall median. The hard-frost 50% dates are April 17 and October 22; these are the points where a 28 °F frost has an even chance of having arrived. Both pairs of dates are reference points rather than definitive frost events.

For a cautious planting schedule, tender crops should be set out after the May 9 spring date, which carries only a 10 % chance of a later light frost, and should be harvested before the September 27 fall date, which also carries a 10 % chance of an earlier light frost. A hard frost matters more than a light frost because 28 °F can injure plant tissue that survives the milder 32 °F temperature.

How to read the probabilities

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

  • Spring: "10% after May 9" 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 27" 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 Denison, IANumbered nearest stations: 1 — Carroll, IA — 25 mi; 2 — near Mapleton, IA — 25 mi; 3 — Castana Experiment Farm, IA — 26 mi; 4 — Audubon, IA — 31 mi; 5 — Sac City, IA — 32 mi.NESD25 mi · 40 km1. Carroll, IA — 25 mi12. near Mapleton, IA — 25 mi23. Castana Experiment Farm, IA — 26 mi34. Audubon, IA — 31 mi45. Sac City, IA — 32 mi5
  1. Carroll, IA 25 mi (40 km)
  2. near Mapleton, IA 25 mi (40 km)
  3. Castana Experiment Farm, IA 26 mi (42 km)
  4. Audubon, IA 31 mi (50 km)
  5. Sac City, IA 32 mi (52 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: 42.0364, -95.3289.