U.S. Frost Dates by Weather Station

NOAA station USC00100803 · Idaho

Frost Dates for Bern, ID

NOAA weather station Bern (USC00100803), Idaho · elevation 5964 ft (1818 m) · hardiness zone 5a*

  • Lower-risk planting dateafter July 4only 1 spring in 10 still has a frost later
  • Typical last frostJune 1750% chance of a frost after it
  • Typical first fall frostSeptember 350% chance of a frost before it
  • Frost-free season77 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Bern, IDSpring: a frost after May 31 in 9 years out of 10, after June 17 in half of the years, after July 4 in 1 year out of 10. Fall: the first frost comes before August 18 in 1 year out of 10, before September 3 in half of the years, before September 18 in 9 years out of 10. Typical frost-free season: 77 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 17First frost ~Sep 3Frost season at Bern, IDSpring: a frost after May 31 in 9 years out of 10, after June 17 in half of the years, after July 4 in 1 year out of 10. Fall: the first frost comes before August 18 in 1 year out of 10, before September 3 in half of the years, before September 18 in 9 years out of 10. Typical frost-free season: 77 days.JFMAMJJASONDLast frost ~Jun 17First frost ~Sep 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.

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)July 4August 18
50% (typical)June 17September 3
90% (likely)May 31September 18
Frost-free season (32°F, 50% chance)
77 days
Hard frost (28°F), last in spring, 50% chance after
May 25
Hard frost (28°F), first in fall, 50% chance before
September 16

The last 15 years (2006–2020 normals)

Typical last frost June 15 (2 days earlier than the 30-year normal), typical first fall frost September 4 (1 day later). Frost-free season 80 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 Bern: chance of a frost after this dateSpring, chance of a frost after the date: 10% July 4, 20% June 30, 30% June 25, 40% June 21, 50% June 17, 60% June 14, 70% June 9, 80% June 5, 90% May 31. Observed last spring frost in recent years: 2021 June 11, 2022 June 21, 2023 June 22, 2024 June 19, 2025 June 23, 2026 June 30.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jul 4Jun 17May 312021: June 112022: June 212023: June 222024: June 192025: June 232026: June 30Fall at Bern: chance of a frost before this dateFall, chance of a frost before the date: 10% August 18, 20% August 24, 30% August 29, 40% September 1, 50% September 3, 60% September 6, 70% September 10, 80% September 13, 90% September 18. Observed first fall frost in recent years: 2021 September 4, 2022 October 1, 2023 September 23, 2024 August 29, 2025 September 16.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 18Sep 3Sep 182021: September 42022: October 12023: September 232024: August 292025: September 16
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
2021June 11September 4
2022June 21October 1
2023June 22September 23
2024June 19August 29
2025June 23September 16
2026June 30season not over yet

What these dates mean

August 18 is the cautious fall date for finishing the harvest, carrying only a 10 % chance that a light frost has arrived earlier. Gardeners can use this mark to plan the last tender crops before frost risk climbs.

June 17 is the 50 % light-frost date in spring, meaning there is an even chance of a frost later in the season, while September 3 is the 50 % light-frost date in fall, giving an even chance of a frost earlier. The 50 % hard-frost dates are May 25 in spring and September 16 in fall; these indicate when a 28 °F frost is just as likely as not. A hard frost at 28 °F can damage plants that survive a light frost at 32 °F.

For a cautious plan, July 4 is the spring date to set out tender crops, and August 18 is the fall date to finish harvesting them.

How to read the probabilities

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

  • Spring: "10% after July 4" 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 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 Bern, IDNumbered nearest stations: 1 — near Paris, ID — 15 mi; 2 — Preston, ID — 30 mi; 3 — Laketown, UT — 35 mi; 4 — Richmond, UT — 37 mi; 5 — near Logan, UT — 46 mi.IDWYUT25 mi · 40 km1. near Paris, ID — 15 mi12. Preston, ID — 30 mi23. Laketown, UT — 35 mi34. Richmond, UT — 37 mi45. near Logan, UT — 46 mi5
  1. near Paris, ID 15 mi (24 km)
  2. Preston, ID 30 mi (49 km)
  3. Laketown, UT 35 mi (57 km)
  4. Richmond, UT 37 mi (59 km)
  5. near Logan, UT 46 mi (74 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: 42.3353, -111.385.