North Dakota frost dates

Across North Dakota, the average last spring frost is around May 15 and the average first fall frost around September 20 — roughly 128 frost-free days. USDA zones 3a, 3b, 4a, 4b.

Risk you accept	Last spring frost by	First fall frost by
Cautious (~10%)	May 29	September 6
Average (~50%)	May 15	September 20
Gambler (~90%)	May 1	October 4

→ Turn these into sowing dates with the North Dakota planting calendar, or look up an exact ZIP code.

Statewide average from NOAA/NCEI freeze normals. Cautious/gambler dates are about two weeks either side of the average.