U.S. Frost Dates by Weather Station

NOAA station USC00134228 · Iowa

Frost Dates for Jefferson, IA

NOAA weather station Jefferson (USC00134228), Iowa · elevation 1055 ft (322 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 8only 1 spring in 10 still has a frost later
  • Typical last frostApril 2650% chance of a frost after it
  • Typical first fall frostOctober 950% chance of a frost before it
  • Frost-free season164 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Jefferson, IASpring: a frost after April 12 in 9 years out of 10, after April 26 in half of the years, after May 8 in 1 year out of 10. Fall: the first frost comes before September 23 in 1 year out of 10, before October 9 in half of the years, before October 24 in 9 years out of 10. Typical frost-free season: 164 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 26First frost ~Oct 9Frost season at Jefferson, IASpring: a frost after April 12 in 9 years out of 10, after April 26 in half of the years, after May 8 in 1 year out of 10. Fall: the first frost comes before September 23 in 1 year out of 10, before October 9 in half of the years, before October 24 in 9 years out of 10. Typical frost-free season: 164 days.JFMAMJJASONDLast frost ~Apr 26First frost ~Oct 9
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 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 8September 23
50% (typical)April 26October 9
90% (likely)April 12October 24
Frost-free season (32°F, 50% chance)
164 days
Hard frost (28°F), last in spring, 50% chance after
April 15
Hard frost (28°F), first in fall, 50% chance before
October 21

The last 15 years (2006–2020 normals)

Typical last frost April 26 (same as the 30-year normal), typical first fall frost October 15 (6 days later). Frost-free season 171 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 Jefferson: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 8, 20% May 4, 30% May 1, 40% April 29, 50% April 26, 60% April 24, 70% April 21, 80% April 17, 90% April 12. Observed last spring frost in recent years: 2021 April 23, 2022 April 27, 2023 April 25, 2024 April 21, 2025 April 9.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 8Apr 26Apr 122021: April 232022: April 272023: April 252024: April 212025: April 9Fall at Jefferson: chance of a frost before this dateFall, chance of a frost before the date: 10% September 23, 20% September 28, 30% October 1, 40% October 5, 50% October 9, 60% October 12, 70% October 16, 80% October 19, 90% October 24. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 10, 2025 October 24.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 23Oct 9Oct 242021: October 222022: September 282023: October 102025: October 24
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
2021April 23October 22
2022April 27September 28
2023April 25October 10
2024April 21incomplete record
2025April 9October 24
2026incomplete recordseason not over yet

What these dates mean

164 days separate the two 32 °F 50 % dates in Jefferson – April 26 in spring and October 9 in fall – and that interval defines the period when a light frost is less likely than not. Within those dates a gardener can plan most above-ground activities with a roughly even chance of a frost occurring.

For a cautious spring schedule the 10 % chance date, May 8, is used to set out tender crops, while the 90 % chance date, April 12, would be a gamble. In the autumn the cautious date is September 23, the 10 % chance that a frost has arrived earlier, and the gamble date is October 24, when a frost is 90 % likely. The median hard-frost dates are April 15 in spring and October 21 in fall.

A hard frost at 28 °F matters more than a light frost at 32 °F because it can damage plant tissue that would survive the milder 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 8" 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 23" 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 Jefferson, IANumbered nearest stations: 1 — Perry, IA — 19 mi; 2 — Carroll, IA — 24 mi; 3 — Guthrie Center, IA — 24 mi; 4 — Boone, IA — 26 mi; 5 — Rockwell City, IA — 29 mi.25 mi · 40 km1. Perry, IA — 19 mi12. Carroll, IA — 24 mi23. Guthrie Center, IA — 24 mi34. Boone, IA — 26 mi45. Rockwell City, IA — 29 mi5
  1. Perry, IA 19 mi (30 km)
  2. Carroll, IA 24 mi (38 km)
  3. Guthrie Center, IA 24 mi (39 km)
  4. Boone, IA 26 mi (42 km)
  5. Rockwell City, IA 29 mi (47 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: 42.0103, -94.3953.