U.S. Frost Dates by Weather Station

NOAA station USC00476678 · Wisconsin

Frost Dates for Plymouth, WI

NOAA weather station Plymouth WWTP (USC00476678), Wisconsin · elevation 828 ft (252 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 20only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostOctober 1050% chance of a frost before it
  • Frost-free season159 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Plymouth, WISpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 28 in 1 year out of 10, before October 10 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 159 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Oct 10Frost season at Plymouth, WISpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 28 in 1 year out of 10, before October 10 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 159 days.JFMAMJJASONDLast frost ~May 3First frost ~Oct 10
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 20September 28
50% (typical)May 3October 10
90% (likely)April 19October 25
Frost-free season (32°F, 50% chance)
159 days
Hard frost (28°F), last in spring, 50% chance after
April 20
Hard frost (28°F), first in fall, 50% chance before
October 23

The last 15 years (2006–2020 normals)

Typical last frost May 3 (same as the 30-year normal), typical first fall frost October 12 (2 days later). Frost-free season 162 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 Plymouth: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 20, 20% May 15, 30% May 10, 40% May 7, 50% May 3, 60% May 1, 70% April 28, 80% April 24, 90% April 19. Observed last spring frost in recent years: 2021 May 12, 2022 May 4, 2023 May 18, 2024 April 26, 2025 May 9, 2026 May 14.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 20May 3Apr 192021: May 122022: May 42023: May 182024: April 262025: May 92026: May 14Fall at Plymouth: chance of a frost before this dateFall, chance of a frost before the date: 10% September 28, 20% October 1, 30% October 4, 40% October 7, 50% October 10, 60% October 14, 70% October 17, 80% October 20, 90% October 25. Observed first fall frost in recent years: 2021 October 22, 2022 September 29, 2023 October 17, 2024 October 17, 2025 October 9.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 28Oct 10Oct 252021: October 222022: September 292023: October 172024: October 172025: October 9
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
2022May 4September 29
2023May 18October 17
2024April 26October 17
2025May 9October 9
2026May 14season not over yet

What these dates mean

The 159 days between the 32°F 50% dates of May 3 and October 10 mark the span in which a light frost is less likely than not for Plymouth gardeners. The 50% dates themselves indicate the point in the season where the probability of a frost later (spring) or earlier (fall) is exactly even, giving a reference for planning around the median risk.

In spring the cautious date for setting out tender crops is May 20, when only a 10% chance of a later light frost remains; planting on April 19 would be a gamble with a 90% chance of frost later. In fall the cautious date to finish harvesting tender crops is September 28, carrying only a 10% chance that a frost has already arrived, while October 25 would be a gamble with a 90% chance of frost already present. Hard-frost dates—April 20 in spring and October 23 in fall—are noted because a 28°F frost can damage plants that survive 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 20" 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 28" 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 Plymouth, WINumbered nearest stations: 1 — Chilton, WI — 23 mi; 2 — Fond du Lac, WI — 24 mi; 3 — Manitowoc, WI — 29 mi; 4 — Germantown, WI — 35 mi; 5 — Two Rivers, WI — 35 mi.WIMI25 mi · 40 km1. Chilton, WI — 23 mi12. Fond du Lac, WI — 24 mi23. Manitowoc, WI — 29 mi34. Germantown, WI — 35 mi45. Two Rivers, WI — 35 mi5
  1. Chilton, WI 23 mi (36 km)
  2. Fond du Lac, WI 24 mi (39 km)
  3. Manitowoc, WI 29 mi (47 km)
  4. Germantown, WI 35 mi (56 km)
  5. Two Rivers, WI 35 mi (56 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: 43.73, -87.9714.