The Idaho year in pollen
Tree pollen in Idaho 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 20.
When does ragweed season end in Idaho?
Ragweed season in Idaho typically ends with the first frost, around September 20. The coldest corners of the state see frost by mid-September; the mildest hold out until late October.
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 Idaho
Tree pollen in Idaho 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 Idaho 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 Idaho over several weeks, and ragweed season ends with it. Based on 30 years of NOAA frost records (1991–2020).
| City | Typical first frost |
|---|---|
| Boise | October 21 |
| Idaho Falls | September 20 |
| Pocatello | September 18 |
Common questions
When does ragweed season end in Idaho?
Ragweed season in Idaho typically ends with the first frost, around September 20. The coldest corners of the state see frost by mid-September; the mildest hold out until late October.
When does allergy season end in Idaho?
The last big wave of the Idaho pollen year is ragweed, so the season ends with the first frost, around September 20. Tree pollen is done by June, grass mostly by July.
When does tree pollen start in Idaho?
Tree pollen in Idaho 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.