The Riverside year in pollen
Pollen seasons in Riverside: tree pollen January to June, grass March to July, and ragweed from early August until the first frost, around December 30.
First frost lands around December 30 in Riverside — about 15 days later than California overall — which shifts when ragweed season ends here.
Tree pollen
Grass
Ragweed
When does ragweed season end in Riverside?
Ragweed season in Riverside typically ends with the first frost, around December 30. Some years it comes as early as December 19, some as late as January 8.
The rest of the Riverside pollen year
| Allergen | Typical season |
|---|---|
| Oak | March to May |
| Cypress and juniper | January to March |
| Alder | January to March |
| Olive | April to June |
| Walnut | April to May |
| Grass | March to July |
Season-by-season detail is on the California page.
Nearby calendars
Frost dates for Riverside come from the Riverside Municipal Airport weather station's 30-year records. Season windows reviewed August 2026.