The North Dakota year in pollen
Tree pollen in North Dakota runs February to May, grass follows May to August, and ragweed starts in early August and holds on until the first frost — typically around September 27.
When does ragweed season end in North Dakota?
Ragweed season in North Dakota typically ends with the first frost, around September 27.
Ragweed keeps releasing pollen until a freeze kills the plants, which is why the season has no fixed end date — it ends when the cold arrives. More on how frost ends ragweed season.
Tree pollen in North Dakota
Tree pollen in North Dakota starts in mid-February, and eastern red cedar is usually first out. Elm — the bigger problem for most people — follows February to April. Mild springs pull the start earlier.
| Tree | Typical season | Heaviest weeks |
|---|---|---|
| Eastern red cedar | February to April | early March to early April |
| Elm | February to April | — |
| Maple | March to April | — |
| Oak | April to May | mid-April to early May |
| Cottonwood | April to May | — |
Grass season
Grass pollen in North Dakota runs May to August, heaviest mid-May to early July. Cool-season grasses flower in the first wave; warm-season grasses like Bermuda pick up through summer, which stretches the season. More on grass seasons.
First frost — and ragweed's end — city by city
The first frost sweeps across North Dakota over several weeks, and ragweed season ends with it. Based on 30 years of NOAA frost records (1991–2020).
| City | Typical first frost |
|---|---|
| Bismarck | September 27 |
| Fargo | September 30 |
| Grand Forks | September 27 |
Common questions
When does ragweed season end in North Dakota?
Ragweed season in North Dakota typically ends with the first frost, around September 27.
When does allergy season end in North Dakota?
The last big wave of the North Dakota pollen year is ragweed, so the season ends with the first frost, around September 27. Tree pollen is done by May, grass mostly by August.
When does tree pollen start in North Dakota?
Tree pollen in North Dakota starts in mid-February, and eastern red cedar is usually first out. Elm — the bigger problem for most people — follows February to April. Mild springs pull the start earlier.
Sources: NOAA U.S. Climate Normals (1991–2020 frost records) · Asthma and Allergy Foundation of America — Ragweed Pollen. Season windows reviewed August 2026.