Ragweed season in Montana ends around September 26
Ragweed doesn't stop on a schedule. It keeps flowering until a freeze kills the plants, which is why the end of the season is a weather question rather than a calendar one — and in Montana, the answer sits around September 26 in a typical year.
Is ragweed season still going in Montana?
- Ragweed In season now — about 5 weeks to the typical first frost
Status comes from the typical season window for this area, not from a pollen count — a cool spring or a late frost moves the real dates by a week or two either way. These are the windows for Montana.
Why the end of the season is a frost date
Two thresholds matter, and NOAA records both. The first 32°F frost is the one that ends ragweed season. The first 28°F hard freeze, which follows it, is what finishes off the plants for good. City tables on this site show both.
These are fifty-fifty dates, not deadlines. At each city's own weather station, NOAA's 30-year record puts half of the first freezes before the date and half after — so ragweed ends early in a cold autumn and hangs on well past the date in a warm one.
Many people plan ahead of the season; if that includes anything medical, ask your doctor.
Where in Montana it ends first
Frost doesn't switch a state off all at once. It reaches the coldest corners of Montana around September 24 and the mildest around October 7 — 13 days apart, so ragweed season can be finished at one end of the state while it's still going at the other.
| City | First frost (32°F) | Hard freeze (28°F) |
|---|---|---|
| Great Falls | September 24 | October 3 |
| Missoula | September 26 | October 6 |
| Billings | October 7 | October 16 |
Based on 30 years of NOAA frost records (1991–2020).
What comes after
Once frost has been through, the pollen year starts over: Montana then gets its one genuinely quiet stretch of the year, and tree pollen opens the next one in early March.
The full Montana pollen year — tree, grass and ragweed, with the dates each one starts — is on the Montana season page. For how ragweed works nationally, see ragweed season.
Common questions
When does ragweed season end in Montana?
Around September 26 in a typical year — the day the first freeze usually arrives in Montana. A warm autumn pushes it later; an early cold snap ends it sooner.
Does the first frost really end it?
Effectively, yes — and that is what makes the date worth looking up. Common ragweed is an annual: it grows from seed every spring and has no way to survive a freeze, so the cold kills the plants outright rather than slowing them down. A dead plant releases no more pollen, which is why the first frost is an ending and not just a quiet spell.
Does it end everywhere in Montana at once?
No. The first freeze crosses Montana over roughly 13 days — earliest around September 24, latest around October 7 — so ragweed season is finished at one end of the state while it is still running at the other. The city table above gives the local date.
What comes after ragweed in Montana?
Nothing, for a while — ragweed is the last pollen of the year. The next season to open in Montana is tree pollen in early March, usually starting with juniper.
Sources: NOAA U.S. Climate Normals, 1991–2020 (first-frost and hard-freeze dates) · Asthma and Allergy Foundation of America · Iowa State University Extension — common ragweed. Season windows reviewed August 2026.