The Flagstaff year in pollen
Pollen seasons in Flagstaff: tree pollen March to June, grass May to July, and ragweed from early August until the first frost, around September 28.
Flagstaff's pollen year runs on a different clock than most of Arizona — the seasons below follow Flagstaff's own climate, not the statewide picture.
When does ragweed season end in Flagstaff?
Ragweed season in Flagstaff typically ends with the first frost, around September 28. Some years it comes as early as September 23, some as late as October 2.
A light frost around September 28 knocks the season down; the hard freeze that finishes it usually follows by October 9. Based on 30 years of NOAA frost records (1991–2020).
The rest of the Flagstaff 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 Arizona page.
Nearby calendars
Frost dates for Flagstaff come from the Flagstaff Pulliam Airport weather station's 30-year records. Season windows reviewed August 2026.