The Portland year in pollen
Pollen seasons in Portland: tree pollen February to May, grass May to July, and ragweed from mid-August until the first frost, around November 19.
First frost lands around November 19 in Portland — about 27 days later than Oregon overall — which shifts when ragweed season ends here.
When does ragweed season end in Portland?
Ragweed season in Portland typically ends with the first frost, around November 19. Some years it comes as early as November 11, some as late as November 27.
A light frost around November 19 knocks the season down; the hard freeze that finishes it usually follows by December 5. Based on 30 years of NOAA frost records (1991–2020).
The rest of the Portland pollen year
| Allergen | Typical season |
|---|---|
| Alder | February to April |
| Birch | March to May |
| Cottonwood | March to May |
| Oak | April to May |
| Grass | May to July |
Season-by-season detail is on the Oregon page.
Nearby calendars
Frost dates for Portland come from the Portland International Airport weather station's 30-year records. Season windows reviewed August 2026.