The California year in pollen
Tree pollen in California runs January to June, grass follows March to July, and ragweed starts in early August and holds on until the first frost — typically around December 15.
When does ragweed season end in California?
Ragweed season in California typically ends with the first frost, around December 15. The coldest corners of the state see frost by early December; the mildest hold out until late December.
Ragweed keeps releasing pollen until a freeze kills the plants, which is why the season has no fixed end date — it ends when the cold arrives. More on how frost ends ragweed season.
Tree pollen in California
Tree pollen in California starts in mid-January, and cypress and juniper is usually first out. Oak — the bigger problem for most people — follows March to May. Mild springs pull the start earlier.
| Tree | Typical season | Heaviest weeks |
|---|---|---|
| Oak | March to May | mid-March to late April |
| Cypress and juniper | January to March | early February to early March |
| Alder | January to March | — |
| Olive | April to June | early May to late May |
| Walnut | April to May | — |
Grass season
Grass pollen in California runs March to July, heaviest mid-April to late June. Cool-season grasses flower in the first wave; warm-season grasses like Bermuda pick up through summer, which stretches the season. More on grass seasons.
First frost — and ragweed's end — city by city
The first frost sweeps across California over several weeks, and ragweed season ends with it. Based on 30 years of NOAA frost records (1991–2020).
| City | Typical first frost |
|---|---|
| Bakersfield | December 15 |
| Fresno | December 14 |
| Long Beach | rarely freezes |
| Los Angeles | rarely freezes |
| Modesto | December 5 |
| Oxnard | rarely freezes |
| Riverside | December 30 |
| Sacramento | December 16 |
| San Diego | rarely freezes |
| San Francisco | rarely freezes |
| San Jose | December 22 |
| Stockton | December 1 |
Common questions
When does ragweed season end in California?
Ragweed season in California typically ends with the first frost, around December 15. The coldest corners of the state see frost by early December; the mildest hold out until late December.
When does allergy season end in California?
The last big wave of the California pollen year is ragweed, so the season ends with the first frost, around December 15. Tree pollen is done by June, grass mostly by July.
When does tree pollen start in California?
Tree pollen in California starts in mid-January, and cypress and juniper is usually first out. Oak — the bigger problem for most people — follows March to May. Mild springs pull the start earlier.
Sources: NOAA U.S. Climate Normals (1991–2020 frost records) · Asthma and Allergy Foundation of America — Ragweed Pollen. Season windows reviewed August 2026.