U.S. Frost Dates by Weather Station

NOAA station USW00024011 · North Dakota

Frost Dates for Bismarck, ND

NOAA weather station Bismarck (USW00024011), North Dakota · elevation 1658 ft (505 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 27only 1 spring in 10 still has a frost later
  • Typical last frostMay 1450% chance of a frost after it
  • Typical first fall frostSeptember 2750% chance of a frost before it
  • Frost-free season136 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Bismarck, NDSpring: a frost after May 1 in 9 years out of 10, after May 14 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 136 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 14First frost ~Sep 27Frost season at Bismarck, NDSpring: a frost after May 1 in 9 years out of 10, after May 14 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 136 days.JFMAMJJASONDLast frost ~May 14First frost ~Sep 27
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)May 27September 15
50% (typical)May 14September 27
90% (likely)May 1October 9
Frost-free season (32°F, 50% chance)
136 days
Hard frost (28°F), last in spring, 50% chance after
May 3
Hard frost (28°F), first in fall, 50% chance before
October 5

The last 15 years (2006–2020 normals)

Typical last frost May 14 (same as the 30-year normal), typical first fall frost September 29 (2 days later). Frost-free season 136 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 Bismarck: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 27, 20% May 22, 30% May 19, 40% May 16, 50% May 14, 60% May 11, 70% May 8, 80% May 5, 90% May 1. Observed last spring frost in recent years: 2021 May 28, 2022 May 3, 2023 May 3, 2024 May 4, 2025 April 29, 2026 May 10.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 27May 14May 12021: May 282022: May 32023: May 32024: May 42025: April 292026: May 10Fall at Bismarck: chance of a frost before this dateFall, chance of a frost before the date: 10% September 15, 20% September 20, 30% September 23, 40% September 25, 50% September 27, 60% September 29, 70% October 2, 80% October 5, 90% October 9. Observed first fall frost in recent years: 2021 October 15, 2022 October 6, 2023 October 6, 2024 October 1, 2025 October 19.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 15Sep 27Oct 92021: October 152022: October 62023: October 62024: October 12025: October 19
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 28October 15
2022May 3October 6
2023May 3October 6
2024May 4October 1
2025April 29October 19
2026May 10season not over yet

What these dates mean

The 136 days between the 50% dates in Bismarck frame the period when light frosts are equally likely to occur. The 50% date for a 32°F frost in spring is May 14, and the 50% date in fall is September 27; a gardener can expect a 50% chance of a light frost later than May 14 and a 50% chance of a light frost earlier than September 27.

Using the cautious approach, tender crops should be set out after the May 27 spring date, which carries only a 10% chance of a later frost, and should be harvested by the September 15 fall date, which carries only a 10% chance of an earlier frost. Planting on May 1 or keeping crops until October 9 involves a 90% chance of a light frost and is therefore riskier. A hard frost at 28°F, marked by the median dates May 3 and October 5, can cause more severe damage than 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 May 27" 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 15" 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 Bismarck, NDNumbered nearest stations: 1 — Mandan, ND — 8 mi; 2 — Hazelton, ND — 26 mi; 3 — New Salem, ND — 35 mi; 4 — Steele, ND — 39 mi; 5 — Napoleon, ND — 50 mi.25 mi · 40 km1. Mandan, ND — 8 mi12. Hazelton, ND — 26 mi23. New Salem, ND — 35 mi34. Steele, ND — 39 mi45. Napoleon, ND — 50 mi5
  1. Mandan, ND 8 mi (12 km)
  2. Hazelton, ND 26 mi (42 km)
  3. New Salem, ND 35 mi (57 km)
  4. Steele, ND 39 mi (63 km)
  5. Napoleon, ND 50 mi (81 km)

Sources

* Hardiness zone 4a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 46.7708, -100.7603.