U.S. Frost Dates by Weather Station

NOAA station USC00473453 · Wisconsin

Frost Dates for Hartford, WI

NOAA weather station Hartford WWTP (USC00473453), Wisconsin · elevation 961 ft (293 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 24only 1 spring in 10 still has a frost later
  • Typical last frostMay 850% chance of a frost after it
  • Typical first fall frostOctober 350% chance of a frost before it
  • Frost-free season148 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Hartford, WISpring: a frost after April 24 in 9 years out of 10, after May 8 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 3 in half of the years, before October 16 in 9 years out of 10. Typical frost-free season: 148 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 8First frost ~Oct 3Frost season at Hartford, WISpring: a frost after April 24 in 9 years out of 10, after May 8 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 3 in half of the years, before October 16 in 9 years out of 10. Typical frost-free season: 148 days.JFMAMJJASONDLast frost ~May 8First frost ~Oct 3
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.Today

Frost dates (32°F)

Probability of frost, based on 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 24September 20
50% (typical)May 8October 3
90% (likely)April 24October 16
Frost-free season (32°F, 50% chance)
148 days
Hard frost (28°F), last in spring, 50% chance after
April 27
Hard frost (28°F), first in fall, 50% chance before
October 15

The last 15 years (2006–2020 normals)

Typical last frost May 5 (3 days earlier than the 30-year normal), typical first fall frost October 8 (5 days later). Frost-free season 155 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 Hartford: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 24, 20% May 18, 30% May 14, 40% May 11, 50% May 8, 60% May 5, 70% May 1, 80% April 29, 90% April 24. Observed last spring frost in recent years: 2021 May 13, 2022 May 5, 2023 May 3, 2024 April 26, 2025 May 9, 2026 May 14.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 24May 8Apr 242021: May 132022: May 52023: May 32024: April 262025: May 92026: May 14Fall at Hartford: chance of a frost before this dateFall, chance of a frost before the date: 10% September 20, 20% September 25, 30% September 29, 40% October 1, 50% October 3, 60% October 5, 70% October 8, 80% October 12, 90% October 16. Observed first fall frost in recent years: 2021 October 17, 2022 September 29, 2023 October 9, 2024 October 16, 2025 October 20.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 20Oct 3Oct 162021: October 172022: September 292023: October 92024: October 162025: October 20
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 13October 17
2022May 5September 29
2023May 3October 9
2024April 26October 16
2025May 9October 20
2026May 14season not over yet

What these dates mean

May 24 is the cautious spring date, with a 10 % chance of a frost later, and it lets a gardener plan to set out tender crops. The 50 % dates—May 8 for spring and October 3 for fall—mark the median point where a light frost has a 50 % chance, but they are not frost dates themselves. The median hard-frost dates are April 27 in spring and October 15 in fall, and the season spans 148 days between the two 32 °F 50 % dates.

September 20 is the cautious fall date, with a 10 % chance that a light frost has already arrived, and it guides the latest time to finish harvesting tender crops. Setting out tender crops on April 24 or counting on them until October 16 carries a 90 % risk of frost and should be treated as a gamble. A hard frost at 28 °F can damage plants that survive a 32 °F light frost, so the hard-frost dates matter more than the light-frost dates.

How to read the probabilities

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

  • Spring: "10% after May 24" 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 20" 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 Hartford, WINumbered nearest stations: 1 — Horicon, WI — 13 mi; 2 — Germantown, WI — 16 mi; 3 — Oconomowoc, WI — 17 mi; 4 — Watertown, WI — 20 mi; 5 — Beaver Dam, WI — 23 mi.WI25 mi · 40 km1. Horicon, WI — 13 mi12. Germantown, WI — 16 mi23. Oconomowoc, WI — 17 mi34. Watertown, WI — 20 mi45. Beaver Dam, WI — 23 mi5
  1. Horicon, WI 13 mi (21 km)
  2. Germantown, WI 16 mi (26 km)
  3. Oconomowoc, WI 17 mi (27 km)
  4. Watertown, WI 20 mi (32 km)
  5. Beaver Dam, WI 23 mi (37 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: 43.3317, -88.4117.