The Oregon year in pollen
Tree pollen in Oregon runs February to May, grass follows May to July, and ragweed starts in mid-August and holds on until the first frost — typically around October 23.
When does ragweed season end in Oregon?
Ragweed season in Oregon typically ends with the first frost, around October 23. The coldest corners of the state see frost by mid-September; the mildest hold out until mid-November.
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 Oregon
Tree pollen in Oregon starts in early February, and alder is usually first out. Mild springs pull the start earlier.
| Tree | Typical season | Heaviest weeks |
|---|---|---|
| Alder | February to April | early March to early April |
| Birch | March to May | early April to late April |
| Cottonwood | March to May | — |
| Oak | April to May | — |
Grass season
Grass pollen in Oregon runs May to July, heaviest early June to early July. The culprits are cool-season lawn and meadow grasses — timothy, bluegrass, orchard grass — which flower in one early-summer wave. More on grass seasons.
First frost — and ragweed's end — city by city
The first frost sweeps across Oregon over several weeks, and ragweed season ends with it. Based on 30 years of NOAA frost records (1991–2020).
| City | Typical first frost |
|---|---|
| Bend | September 14 |
| Eugene | October 23 |
| Medford | October 29 |
| Portland | November 19 |
Common questions
When does ragweed season end in Oregon?
Ragweed season in Oregon typically ends with the first frost, around October 23. The coldest corners of the state see frost by mid-September; the mildest hold out until mid-November.
When does allergy season end in Oregon?
The last big wave of the Oregon pollen year is ragweed, so the season ends with the first frost, around October 23. Tree pollen is done by May, grass mostly by July.
When does tree pollen start in Oregon?
Tree pollen in Oregon starts in early February, and alder is usually first out. 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.