The Wyoming year in pollen
Tree pollen in Wyoming runs March to June, grass follows May to July, and ragweed starts in early August and holds on until the first frost — typically around September 23.
When does ragweed season end in Wyoming?
Ragweed season in Wyoming typically ends with the first frost, around September 23.
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 Wyoming
Tree pollen in Wyoming 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 Wyoming 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 Wyoming over several weeks, and ragweed season ends with it. Based on 30 years of NOAA frost records (1991–2020).
| City | Typical first frost |
|---|---|
| Casper | September 22 |
| Cheyenne | October 1 |
| Sheridan | September 23 |
Common questions
When does ragweed season end in Wyoming?
Ragweed season in Wyoming typically ends with the first frost, around September 23.
When does allergy season end in Wyoming?
The last big wave of the Wyoming pollen year is ragweed, so the season ends with the first frost, around September 23. Tree pollen is done by June, grass mostly by July.
When does tree pollen start in Wyoming?
Tree pollen in Wyoming 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.