The Chicago year in pollen
Pollen seasons in Chicago: tree pollen February to May, grass May to July, and ragweed from early August until the first frost, around October 31.
First frost lands around October 31 in Chicago — about 11 days later than Illinois overall — which shifts when ragweed season ends here.
When does ragweed season end in Chicago?
Ragweed season in Chicago typically ends with the first frost, around October 31. Some years it comes as early as October 26, some as late as November 5.
A light frost around October 31 knocks the season down; the hard freeze that finishes it usually follows by November 11. Based on 30 years of NOAA frost records (1991–2020).
The rest of the Chicago pollen year
| Allergen | Typical season |
|---|---|
| Oak | April to May |
| Maple | March to April |
| Elm | February to April |
| Birch | April to May |
| Ash | April to May |
| Hickory and walnut | April to May |
| Grass | May to July |
Season-by-season detail is on the Illinois page.
Nearby calendars
Frost dates for Chicago come from the Chicago Midway Airport weather station's 30-year records. Season windows reviewed August 2026.