The Illinois year in pollen

When does ragweed season end in Illinois?

Ragweed season in Illinois typically ends with the first frost, around October 20. The coldest corners of the state see frost by mid-October; 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 Illinois

Tree pollen in Illinois starts in mid-February, and elm is usually first out. Oak — the bigger problem for most people — follows April to May. Mild springs pull the start earlier.

Grass season

Grass pollen in Illinois runs May to July, heaviest late May to late June. 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 Illinois 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 Illinois?

Ragweed season in Illinois typically ends with the first frost, around October 20. The coldest corners of the state see frost by mid-October; the mildest hold out until late October.

When does allergy season end in Illinois?

The last big wave of the Illinois pollen year is ragweed, so the season ends with the first frost, around October 20. Tree pollen is done by May, grass mostly by July.

When does tree pollen start in Illinois?

Tree pollen in Illinois starts in mid-February, and elm is usually first out. Oak — the bigger problem for most people — follows April to May. Mild springs pull the start earlier.