U.S. Frost Dates by Weather Station

NOAA station USC00204104 · Michigan

Frost Dates for Ironwood, MI

NOAA weather station Ironwood (USC00204104), Michigan · elevation 1430 ft (436 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 3only 1 spring in 10 still has a frost later
  • Typical last frostMay 2050% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season132 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Ironwood, MISpring: a frost after May 2 in 9 years out of 10, after May 20 in half of the years, after June 3 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 132 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 20First frost ~Oct 1Frost season at Ironwood, MISpring: a frost after May 2 in 9 years out of 10, after May 20 in half of the years, after June 3 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 132 days.JFMAMJJASONDLast frost ~May 20First frost ~Oct 1
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)June 3September 17
50% (typical)May 20October 1
90% (likely)May 2October 15
Frost-free season (32°F, 50% chance)
132 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 12

The last 15 years (2006–2020 normals)

Typical last frost May 17 (3 days earlier than the 30-year normal), typical first fall frost October 3 (2 days later). Frost-free season 137 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 Ironwood: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 3, 20% May 30, 30% May 27, 40% May 23, 50% May 20, 60% May 16, 70% May 11, 80% May 7, 90% May 2. Observed last spring frost in recent years: 2021 May 29, 2022 May 5, 2023 May 4, 2024 May 10, 2025 May 25, 2026 May 12.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 3May 20May 22021: May 292022: May 52023: May 42024: May 102025: May 252026: May 12Fall at Ironwood: chance of a frost before this dateFall, chance of a frost before the date: 10% September 17, 20% September 22, 30% September 26, 40% September 28, 50% October 1, 60% October 3, 70% October 6, 80% October 10, 90% October 15. Observed first fall frost in recent years: 2021 October 18, 2022 September 28, 2023 October 12, 2024 October 4, 2025 October 8.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 17Oct 1Oct 152021: October 182022: September 282023: October 122024: October 42025: October 8
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 29October 18
2022May 5September 28
2023May 4October 12
2024May 10October 4
2025May 25October 8
2026May 12season not over yet

What these dates mean

The cautious fall date for finishing the harvest in Ironwood is September 17, when there is only a 10% chance that a light frost has arrived.

The 50% light-frost date in spring is May 20, indicating a 50% chance of a frost later than that date, while the 50% light-frost date in fall is October 1, indicating a 50% chance of a frost earlier than that date. For a cautious spring plan, June 3 is the date to set out tender crops, because it carries a 10% chance of a frost later; May 2 carries a 90% chance and is a gamble. In fall, the cautious date to finish tender crops is September 17, while October 15 carries a 90% chance of a frost earlier and is a gamble.

A hard frost of 28 °F can damage plants that survive a light frost of 32 °F.

How to read the probabilities

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

  • Spring: "10% after June 3" 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 17" 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 Ironwood, MINumbered nearest stations: 1 — Hurley, WI — 0 mi; 2 — Bergland Dam, MI — 32 mi; 3 — Madeline Island, WI — 35 mi; 4 — Winter, WI — 60 mi; 5 — Gordon, WI — 79 mi.WIMI25 mi · 40 km1. Hurley, WI — 0 mi12. Bergland Dam, MI — 32 mi23. Madeline Island, WI — 35 mi34. Winter, WI — 60 mi45. Gordon, WI — 79 mi5
  1. Hurley, WI 0 mi (1 km)
  2. Bergland Dam, MI 32 mi (51 km)
  3. Madeline Island, WI 35 mi (56 km)
  4. Winter, WI 60 mi (97 km)
  5. Gordon, WI 79 mi (126 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 46.4656, -90.1892.