U.S. Frost Dates by Weather Station

NOAA station USC00243915 · Montana

Frost Dates for Hardin, MT

NOAA weather station Hardin (USC00243915), Montana · elevation 2880 ft (878 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 26only 1 spring in 10 still has a frost later
  • Typical last frostMay 1350% chance of a frost after it
  • Typical first fall frostSeptember 2950% chance of a frost before it
  • Frost-free season135 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Hardin, MTSpring: a frost after April 30 in 9 years out of 10, after May 13 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before September 29 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 135 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 13First frost ~Sep 29Frost season at Hardin, MTSpring: a frost after April 30 in 9 years out of 10, after May 13 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before September 29 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 135 days.JFMAMJJASONDLast frost ~May 13First frost ~Sep 29
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 26September 14
50% (typical)May 13September 29
90% (likely)April 30October 10
Frost-free season (32°F, 50% chance)
135 days
Hard frost (28°F), last in spring, 50% chance after
May 1
Hard frost (28°F), first in fall, 50% chance before
October 7

The last 15 years (2006–2020 normals)

Typical last frost May 13 (same as the 30-year normal), typical first fall frost October 1 (2 days later). Frost-free season 138 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 Hardin: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 26, 20% May 22, 30% May 19, 40% May 16, 50% May 13, 60% May 10, 70% May 7, 80% May 4, 90% April 30. Observed last spring frost in recent years: 2021 May 14, 2022 May 25, 2023 April 29, 2024 June 16, 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 26May 13Apr 302021: May 142022: May 252023: April 292024: June 162025: May 22026: May 19Fall at Hardin: chance of a frost before this dateFall, chance of a frost before the date: 10% September 14, 20% September 19, 30% September 23, 40% September 25, 50% September 29, 60% October 1, 70% October 3, 80% October 5, 90% October 10. Observed first fall frost in recent years: 2021 September 17, 2022 October 13, 2023 October 6, 2024 October 1, 2025 October 7.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 14Sep 29Oct 102021: September 172022: October 132023: October 62024: October 12025: October 7
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 14September 17
2022May 25October 13
2023April 29October 6
2024June 16October 1
2025May 2October 7
2026May 19season not over yet

What these dates mean

September 14 is the cautious fall date for finishing the harvest in Hardin, representing a 10% chance that a light frost has arrived earlier. In spring the cautious date for setting out tender crops is May 26, which carries a 10% chance of a light frost later.

The 50% dates are May 13 and September 29; each marks the point where a gardener has an even chance of a light frost occurring on either side. The hard-frost 50% dates are May 1 and October 7, indicating the median days for a 28 °F frost. The span between the two 32 °F 50% dates—May 13 and September 29—covers 135 days of the growing season.

A hard frost matters more than a light frost because the colder 28 °F temperature can cause more severe damage to tender plant tissue.

How to read the probabilities

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

  • Spring: "10% after May 26" 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 14" 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 Hardin, MTNumbered nearest stations: 1 — Hysham 25 SSE, MT — 27 mi; 2 — Yellowtail Dam, MT — 33 mi; 3 — Busby, MT — 34 mi; 4 — Billings, MT — 42 mi; 5 — Hysham, MT — 43 mi.25 mi · 40 km1. Hysham 25 SSE, MT — 27 mi12. Yellowtail Dam, MT — 33 mi23. Busby, MT — 34 mi34. Billings, MT — 42 mi45. Hysham, MT — 43 mi5
  1. Hysham 25 SSE, MT 27 mi (43 km)
  2. Yellowtail Dam, MT 33 mi (53 km)
  3. Busby, MT 34 mi (55 km)
  4. Billings, MT 42 mi (68 km)
  5. Hysham, MT 43 mi (69 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: 45.7317, -107.6092.