The Missoula year in pollen
Pollen seasons in Missoula: tree pollen March to June, grass May to July, and ragweed from early August until the first frost, around September 26.
Tree pollen
Grass
Ragweed
When does ragweed season end in Missoula?
Ragweed season in Missoula typically ends with the first frost, around September 26. Some years it comes as early as September 20, some as late as October 1.
A light frost around September 26 knocks the season down; the hard freeze that finishes it usually follows by October 6. Based on 30 years of NOAA frost records (1991–2020).
The rest of the Missoula pollen year
| Allergen | Typical season |
|---|---|
| Juniper | March to May |
| Cottonwood and aspen | March to May |
| Maple (boxelder) | March to May |
| Birch and alder | April to May |
| Pine | May to June |
| Grass | May to July |
Season-by-season detail is on the Montana page.
Nearby calendars
Frost dates for Missoula come from the Missoula International Airport weather station's 30-year records. Season windows reviewed August 2026.