The Boise year in pollen
Pollen seasons in Boise: tree pollen March to June, grass May to July, and ragweed from early August until the first frost, around October 21.
First frost lands around October 21 in Boise — about 31 days later than Idaho overall — which shifts when ragweed season ends here.
When does ragweed season end in Boise?
Ragweed season in Boise typically ends with the first frost, around October 21. Some years it comes as early as October 14, some as late as October 27.
A light frost around October 21 knocks the season down; the hard freeze that finishes it usually follows by October 31. Based on 30 years of NOAA frost records (1991–2020).
The rest of the Boise 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 Idaho page.
Nearby calendars
Frost dates for Boise come from the Boise Air Terminal weather station's 30-year records. Season windows reviewed August 2026.