U.S. Frost Dates by Weather Station

NOAA station USW00014995 · Wisconsin

Frost Dates for Grantsburg, WI

NOAA weather station Grantsburg (USW00014995), Wisconsin · elevation 990 ft (302 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 28only 1 spring in 10 still has a frost later
  • Typical last frostMay 1150% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season141 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Grantsburg, WISpring: a frost after April 27 in 9 years out of 10, after May 11 in half of the years, after May 28 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before October 1 in half of the years, before October 14 in 9 years out of 10. Typical frost-free season: 141 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 11First frost ~Oct 1Frost season at Grantsburg, WISpring: a frost after April 27 in 9 years out of 10, after May 11 in half of the years, after May 28 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before October 1 in half of the years, before October 14 in 9 years out of 10. Typical frost-free season: 141 days.JFMAMJJASONDLast frost ~May 11First 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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 28September 16
50% (typical)May 11October 1
90% (likely)April 27October 14
Frost-free season (32°F, 50% chance)
141 days
Hard frost (28°F), last in spring, 50% chance after
April 30
Hard frost (28°F), first in fall, 50% chance before
October 11

The last 15 years (2006–2020 normals)

Typical last frost May 9 (2 days earlier than the 30-year normal), typical first fall frost October 1. Frost-free season 144 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 Grantsburg: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 28, 20% May 23, 30% May 18, 40% May 14, 50% May 11, 60% May 8, 70% May 5, 80% May 1, 90% April 27. Observed last spring frost in recent years: 2021 May 30, 2022 May 4, 2023 April 27, 2024 April 25, 2025 April 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 28May 11Apr 272021: May 302022: May 42023: April 272024: April 252025: April 19Fall at Grantsburg: chance of a frost before this dateFall, chance of a frost before the date: 10% September 16, 20% September 21, 30% September 25, 40% September 28, 50% October 1, 60% October 3, 70% October 6, 80% October 10, 90% October 14. Observed first fall frost in recent years: 2021 October 21, 2022 September 27, 2023 October 17, 2024 October 13.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 16Oct 1Oct 142021: October 212022: September 272023: October 172024: October 13
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 30October 21
2022May 4September 27
2023April 27October 17
2024April 25October 13
2025April 19incomplete record
2026no dataseason not over yet

What these dates mean

May 28 is the cautious spring date for light frost in Grantsburg, allowing a gardener to set out tender crops with only a 10% chance of a frost later. The earlier date of April 27 carries a 90% chance and would be a gamble.

The 50% date in spring is May 11; there is a 50% chance of a frost later than this date. In fall, the 50% date is October 1, representing a 50% chance of a frost earlier than this date. The cautious fall date is September 16, when there is only a 10% chance that a frost has arrived, suitable for finishing harvest of tender crops.

Hard frost at 28 °F, marked by the spring date April 30 and the fall date October 11, can injure plants that would survive a light frost at 32 °F, making it a more serious threat for gardeners.

How to read the probabilities

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

  • Spring: "10% after May 28" 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 16" 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 Grantsburg, WINumbered nearest stations: 1 — Luck, WI — 17 mi; 2 — Wild River State Park, MN — 17 mi; 3 — St. Croix Falls, WI — 25 mi; 4 — Mora, MN — 31 mi; 5 — Forest Lake, MN — 32 mi.MNWI25 mi · 40 km1. Luck, WI — 17 mi12. Wild River State Park, MN — 17 mi23. St. Croix Falls, WI — 25 mi34. Mora, MN — 31 mi45. Forest Lake, MN — 32 mi5
  1. Luck, WI 17 mi (27 km)
  2. Wild River State Park, MN 17 mi (28 km)
  3. St. Croix Falls, WI 25 mi (40 km)
  4. Mora, MN 31 mi (50 km)
  5. Forest Lake, MN 32 mi (52 km)

Sources

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

Station coordinates: 45.7728, -92.6886.