U.S. Frost Dates by Weather Station

NOAA station USC00477997 · Wisconsin

Frost Dates for Sparta, WI

NOAA weather station Sparta (USC00477997), Wisconsin · elevation 782 ft (238 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 22only 1 spring in 10 still has a frost later
  • Typical last frostMay 950% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season144 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Sparta, WISpring: a frost after April 26 in 9 years out of 10, after May 9 in half of the years, after May 22 in 1 year out of 10. Fall: the first frost comes before September 18 in 1 year out of 10, before October 1 in half of the years, before October 13 in 9 years out of 10. Typical frost-free season: 144 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 9First frost ~Oct 1Frost season at Sparta, WISpring: a frost after April 26 in 9 years out of 10, after May 9 in half of the years, after May 22 in 1 year out of 10. Fall: the first frost comes before September 18 in 1 year out of 10, before October 1 in half of the years, before October 13 in 9 years out of 10. Typical frost-free season: 144 days.JFMAMJJASONDLast frost ~May 9First 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)May 22September 18
50% (typical)May 9October 1
90% (likely)April 26October 13
Frost-free season (32°F, 50% chance)
144 days
Hard frost (28°F), last in spring, 50% chance after
April 29
Hard frost (28°F), first in fall, 50% chance before
October 11

The last 15 years (2006–2020 normals)

Typical last frost May 8 (1 day earlier than the 30-year normal), typical first fall frost October 3 (2 days later). Frost-free season 147 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 Sparta: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 22, 20% May 18, 30% May 15, 40% May 12, 50% May 9, 60% May 6, 70% May 4, 80% May 1, 90% April 26. Observed last spring frost in recent years: 2021 May 29, 2022 May 23, 2023 May 26.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 22May 9Apr 262021: May 292022: May 232023: May 26Fall at Sparta: chance of a frost before this dateFall, chance of a frost before the date: 10% September 18, 20% September 23, 30% September 26, 40% September 29, 50% October 1, 60% October 3, 70% October 6, 80% October 9, 90% October 13. Observed first fall frost in recent years: 2021 October 17, 2022 September 28, 2023 October 11.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 18Oct 1Oct 132021: October 172022: September 282023: October 11
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 17
2022May 23September 28
2023May 26October 11
2024no datano data
2025no datano data
2026no dataseason not over yet

What these dates mean

The 144 days between the 50% dates in Sparta—May 9 and October 1—frame the middle of the growing season. By definition each of these dates marks a point where there is a fifty-percent chance of a light frost (32 °F) occurring later in spring or earlier in fall; they are not themselves frost dates but simply the median of the historic record.

A cautious plan for tender crops would wait until the May 22 light-frost date, which carries only a ten-percent chance of a frost later, and would aim to finish harvesting by September 18, when there is only a ten-percent chance that a frost has arrived earlier.

A hard frost at 28 °F, recorded median on April 29 and October 11, can damage plants more severely than a light frost, so its timing is especially critical.

How to read the probabilities

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

  • Spring: "10% after May 22" 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 18" 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 Sparta, WINumbered nearest stations: 1 — La Crosse, WI — 22 mi; 2 — Viroqua, WI — 26 mi; 3 — Mather, WI — 28 mi; 4 — near Trempealeau, WI — 31 mi; 5 — Genoa Dam 8, WI — 33 mi.WIMN25 mi · 40 km1. La Crosse, WI — 22 mi12. Viroqua, WI — 26 mi23. Mather, WI — 28 mi34. near Trempealeau, WI — 31 mi45. Genoa Dam 8, WI — 33 mi5
  1. La Crosse, WI 22 mi (36 km)
  2. Viroqua, WI 26 mi (42 km)
  3. Mather, WI 28 mi (46 km)
  4. near Trempealeau, WI 31 mi (50 km)
  5. Genoa Dam 8, WI 33 mi (52 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: 43.9364, -90.8164.