The Colorado year in pollen
Tree pollen in Colorado runs March to June, grass follows May to July, and ragweed starts in early August and holds on until the first frost — typically around October 8.
When does ragweed season end in Colorado?
Ragweed season in Colorado typically ends with the first frost, around October 8.
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 Colorado
Tree pollen in Colorado starts in early March, and juniper is usually first out. Mild springs pull the start earlier.
| Tree | Typical season | Heaviest weeks |
|---|---|---|
| Juniper | March to May | mid-March to late April |
| Cottonwood and aspen | March to May | — |
| Maple (boxelder) | March to May | — |
| Birch and alder | April to May | — |
| Pine | May to June | — |
Grass season
Grass pollen in Colorado 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 Colorado over several weeks, and ragweed season ends with it. Based on 30 years of NOAA frost records (1991–2020).
| City | Typical first frost |
|---|---|
| Colorado Springs | October 8 |
| Denver | October 8 |
| Grand Junction | October 16 |
| Pueblo | October 9 |
Common questions
When does ragweed season end in Colorado?
Ragweed season in Colorado typically ends with the first frost, around October 8.
When does allergy season end in Colorado?
The last big wave of the Colorado pollen year is ragweed, so the season ends with the first frost, around October 8. Tree pollen is done by June, grass mostly by July.
When does tree pollen start in Colorado?
Tree pollen in Colorado starts in early March, and juniper 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.