U.S. Frost Dates by Weather Station

NOAA station USC00325479 · North Dakota

Frost Dates for Mandan, ND

NOAA weather station Mandan Experiment Station (USC00325479), North Dakota · elevation 1750 ft (533 m) · hardiness zone 4a*

  • 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 2750% chance of a frost before it
  • Frost-free season136 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Mandan, NDSpring: a frost after May 1 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 15 in 1 year out of 10, before September 27 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 136 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 13First frost ~Sep 27Frost season at Mandan, NDSpring: a frost after May 1 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 15 in 1 year out of 10, before September 27 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 136 days.JFMAMJJASONDLast frost ~May 13First 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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 26September 15
50% (typical)May 13September 27
90% (likely)May 1October 10
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 4

The last 15 years (2006–2020 normals)

Typical last frost May 13 (same as the 30-year normal), typical first fall frost September 29 (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 Mandan: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 26, 20% May 21, 30% May 18, 40% May 15, 50% May 13, 60% May 10, 70% May 7, 80% May 4, 90% May 1. Observed last spring frost in recent years: 2022 May 3, 2023 May 2, 2024 May 4, 2025 April 29, 2026 May 10.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 26May 13May 12022: May 32023: May 22024: May 42025: April 292026: May 10Fall at Mandan: 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 30, 70% October 2, 80% October 5, 90% October 10. Observed first fall frost in recent years: 2021 October 20, 2022 September 22, 2024 October 13.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 15Sep 27Oct 102021: October 202022: September 222024: October 13
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
2021incomplete recordOctober 20
2022May 3September 22
2023May 2incomplete record
2024May 4October 13
2025April 29incomplete record
2026May 10season not over yet

What these dates mean

The cautious fall date in Mandan for finishing the harvest is September 15, when there is only a 10 % chance that a light frost has arrived. In spring the cautious date for setting out tender crops is May 26, a date with a 10 % chance of a light frost occurring later.

The 50% date is May 13, marking the median point where the likelihood of a light frost later in the season is even; the 50% date is September 27, the median point where the likelihood of a light frost earlier in the season is even. A hard frost at 28 °F can damage plants that survive a 32 °F light frost, so the 50% hard-frost dates of May 3 and October 4 are especially noteworthy for garden planning.

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 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 Mandan, NDNumbered nearest stations: 1 — Bismarck, ND — 8 mi; 2 — New Salem, ND — 28 mi; 3 — Hazelton, ND — 33 mi; 4 — Heart Butte Dam, ND — 45 mi; 5 — Underwood, ND — 46 mi.ND25 mi · 40 km1. Bismarck, ND — 8 mi12. New Salem, ND — 28 mi23. Hazelton, ND — 33 mi34. Heart Butte Dam, ND — 45 mi45. Underwood, ND — 46 mi5
  1. Bismarck, ND 8 mi (12 km)
  2. New Salem, ND 28 mi (45 km)
  3. Hazelton, ND 33 mi (54 km)
  4. Heart Butte Dam, ND 45 mi (73 km)
  5. Underwood, ND 46 mi (74 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: 46.8128, -100.9097.