U.S. Frost Dates by Weather Station

NOAA station USC00241044 · Montana

Frost Dates near Bozeman, MT

NOAA weather station Bozeman Montana State University (USC00241044), Montana · elevation 4913 ft (1498 m) · hardiness zone 5a*

Bozeman is the nearest town to the station (1.2 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter June 14only 1 spring in 10 still has a frost later
  • Typical last frostMay 2650% chance of a frost after it
  • Typical first fall frostSeptember 2250% chance of a frost before it
  • Frost-free season116 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season near Bozeman, MTSpring: a frost after May 9 in 9 years out of 10, after May 26 in half of the years, after June 14 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 116 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 26First frost ~Sep 22Frost season near Bozeman, MTSpring: a frost after May 9 in 9 years out of 10, after May 26 in half of the years, after June 14 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 116 days.JFMAMJJASONDLast frost ~May 26First frost ~Sep 22
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)June 14September 5
50% (typical)May 26September 22
90% (likely)May 9October 5
Frost-free season (32°F, 50% chance)
116 days
Hard frost (28°F), last in spring, 50% chance after
May 8
Hard frost (28°F), first in fall, 50% chance before
October 3

The last 15 years (2006–2020 normals)

Typical last frost May 22 (4 days earlier than the 30-year normal), typical first fall frost September 27 (5 days later). Frost-free season 125 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 near Bozeman: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 14, 20% June 8, 30% June 3, 40% May 31, 50% May 26, 60% May 23, 70% May 19, 80% May 15, 90% May 9. Observed last spring frost in recent years: 2021 May 23, 2022 May 23, 2023 May 10, 2024 May 31, 2025 June 4, 2026 June 8.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 14May 26May 92021: May 232022: May 232023: May 102024: May 312025: June 42026: June 8Fall near Bozeman: chance of a frost before this dateFall, chance of a frost before the date: 10% September 5, 20% September 11, 30% September 16, 40% September 19, 50% September 22, 60% September 25, 70% September 28, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2021 September 21, 2022 October 14, 2023 October 24, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 5Sep 22Oct 52021: September 212022: October 142023: October 242024: October 12025: October 6
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 23September 21
2022May 23October 14
2023May 10October 24
2024May 31October 1
2025June 4October 6
2026June 8season not over yet

What these dates mean

Setting out tender crops on the early spring date, May 9, carries a 90% chance of a light frost later, making it a gamble for Bozeman gardeners. A more cautious plan waits until June 14, when only a 10% chance of a later frost remains.

In the autumn, the cautious date to finish harvesting tender crops is September 5, which has only a 10% chance of a light frost earlier, while counting on crops as late as October 5 is a gamble with a 90% chance of an earlier frost. The 50% dates—May 26 in spring and September 22 in fall—represent the point at which there is a 50% chance of a light frost later than the spring date and a 50% chance of a frost earlier than the fall date.

The hard-frost 50% dates are May 8 and October 3; hard frost at 28 °F can injure plants that would survive a 32 °F light frost, so it is a more critical threshold.

How to read the probabilities

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

  • Spring: "10% after June 14" 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 5" 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 Bozeman, MTNumbered nearest stations: 1 — near Four Corners, MT — 5 mi; 2 — Bozeman (Gallatin Field), MT — 10 mi; 3 — Trident, MT — 29 mi; 4 — Norris Madison Power House, MT — 31 mi; 5 — Ennis, MT — 39 mi.WY25 mi · 40 km1. near Four Corners, MT — 5 mi12. Bozeman (Gallatin Field), MT — 10 mi23. Trident, MT — 29 mi34. Norris Madison Power House, MT — 31 mi45. Ennis, MT — 39 mi5
  1. near Four Corners, MT 5 mi (8 km)
  2. Bozeman (Gallatin Field), MT 10 mi (17 km)
  3. Trident, MT 29 mi (46 km)
  4. Norris Madison Power House, MT 31 mi (50 km)
  5. Ennis, MT 39 mi (63 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: 45.6622, -111.0464.