U.S. Frost Dates by Weather Station

NOAA station USC00241008 · Montana

Frost Dates for Boulder, MT

NOAA weather station Boulder (USC00241008), Montana · elevation 4904 ft (1495 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 25only 1 spring in 10 still has a frost later
  • Typical last frostJune 750% chance of a frost after it
  • Typical first fall frostSeptember 850% chance of a frost before it
  • Frost-free season90 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Boulder, MTSpring: a frost after May 24 in 9 years out of 10, after June 7 in half of the years, after June 25 in 1 year out of 10. Fall: the first frost comes before August 25 in 1 year out of 10, before September 8 in half of the years, before September 21 in 9 years out of 10. Typical frost-free season: 90 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 7First frost ~Sep 8Frost season at Boulder, MTSpring: a frost after May 24 in 9 years out of 10, after June 7 in half of the years, after June 25 in 1 year out of 10. Fall: the first frost comes before August 25 in 1 year out of 10, before September 8 in half of the years, before September 21 in 9 years out of 10. Typical frost-free season: 90 days.JFMAMJJASONDLast frost ~Jun 7First frost ~Sep 8
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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 25August 25
50% (typical)June 7September 8
90% (likely)May 24September 21
Frost-free season (32°F, 50% chance)
90 days
Hard frost (28°F), last in spring, 50% chance after
May 21
Hard frost (28°F), first in fall, 50% chance before
September 21

The last 15 years (2006–2020 normals)

Typical last frost June 4 (3 days earlier than the 30-year normal), typical first fall frost September 12 (4 days later). Frost-free season 97 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 Boulder: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 25, 20% June 19, 30% June 14, 40% June 10, 50% June 7, 60% June 4, 70% June 2, 80% May 29, 90% May 24.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 25Jun 7May 24Fall at Boulder: chance of a frost before this dateFall, chance of a frost before the date: 10% August 25, 20% August 30, 30% September 2, 40% September 5, 50% September 8, 60% September 10, 70% September 14, 80% September 17, 90% September 21.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 25Sep 8Sep 21
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.

What these dates mean

The 90-day span between the 32 °F 50% dates of June 7 and September 8 frames the window in which tender crops can be grown in Boulder. The 50% dates indicate an even odds of a seasonal light frost in the spring and in the fall, providing gardeners a central reference for timing.

Spring’s cautious date is June 25, when there is only a 10 % chance of a frost later, while planting on the 90 % date of May 24 is a gamble. In the fall the cautious date is August 25, with a 10 % chance of a frost earlier, and the 90 % gamble date is September 21.

Hard-frost dates of May 21 and September 21 matter because temperatures of 28 °F can injure plants that survive a light frost at 32 °F.

How to read the probabilities

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

  • Spring: "10% after June 25" 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 August 25" 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 Boulder, MTNumbered nearest stations: 1 — near Butte, MT — 26 mi; 2 — Helena, MT — 27 mi; 3 — Townsend, MT — 32 mi; 4 — Trident, MT — 36 mi; 5 — Twin Bridges, MT — 48 mi.MT25 mi · 40 km1. near Butte, MT — 26 mi12. Helena, MT — 27 mi23. Townsend, MT — 32 mi34. Trident, MT — 36 mi45. Twin Bridges, MT — 48 mi5
  1. near Butte, MT 26 mi (42 km)
  2. Helena, MT 27 mi (43 km)
  3. Townsend, MT 32 mi (51 km)
  4. Trident, MT 36 mi (58 km)
  5. Twin Bridges, MT 48 mi (78 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: 46.2311, -112.1139.