The Montana year in pollen

When does ragweed season end in Montana?

Ragweed season in Montana typically ends with the first frost, around September 26. The coldest corners of the state see frost by late September; the mildest hold out until early 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 Montana

Tree pollen in Montana starts in early March, and juniper is usually first out. Cottonwood and aspen — the bigger problem for most people — follows March to May. Mild springs pull the start earlier.

Grass season

Grass pollen in Montana 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 Montana over several weeks, and ragweed season ends with it. Based on 30 years of NOAA frost records (1991–2020).

Common questions

When does ragweed season end in Montana?

Ragweed season in Montana typically ends with the first frost, around September 26. The coldest corners of the state see frost by late September; the mildest hold out until early October.

When does allergy season end in Montana?

The last big wave of the Montana pollen year is ragweed, so the season ends with the first frost, around September 26. Tree pollen is done by June, grass mostly by July.

When does tree pollen start in Montana?

Tree pollen in Montana starts in early March, and juniper is usually first out. Cottonwood and aspen — the bigger problem for most people — follows March to May. Mild springs pull the start earlier.