When is pollen worst in Fort Collins?

Fort Collins runs the classic three-wave calendar: tree pollen Mar–May, grass May–Jul, then ragweed Aug–Oct. The two worst stretches are the spring tree peak and the late-summer ragweed peak. Currently, ragweed pollen is in season.

What's in the air in Fort Collins right now?

In September, ragweed pollen is in season in Fort Collins, peaking Aug–Oct. On Fort Collins's high-elevation Mountain West calendar the typical September level is High, led by ragweed pollen — a seasonal estimate, not a live count.

Is tree or grass pollen higher in Fort Collins in spring?

In spring, tree pollen leads in Fort Collins — trees pollinate Mar–May, ahead of grass (May–Jul). The handoff is the tail of the tree window: tree counts taper as grass climbs, so an early-spring flare is more likely tree pollen and a late-spring one more likely grass.

What makes Fort Collins's pollen season distinctive?

Fort Collins sits in the high-elevation Mountain West zone, which means short, sharp seasons set late by elevation, with sagebrush adding to the late-summer weed load. That shapes when symptoms hit and which allergen to watch.

How do I reduce pollen exposure in Fort Collins?

Through Fort Collins's peak windows (tree Mar–May, grass May–Jul, ragweed Aug–Oct), keep windows shut and run AC on recirculate; counts run highest on dry, warm, windy mornings, so push outdoor activity to late afternoon or just after rain, which clears pollen from the air. A HEPA purifier indoors, a saline rinse after being outside, showering before bed, and starting antihistamines a week or two before your worst local window all measurably cut symptoms.

When does the next pollen season start in Fort Collins?

Tree pollen is next in Fort Collins: it opens around week 10 (early March), peaks around week 15, and runs about 91 days.

What is the allergy forecast in Fort Collins today?

With no live reading for Fort Collins today, its allergy forecast is the seasonal estimate above: a typical September level of High, led by ragweed pollen.

Fort Collins, Colorado Pollen Count & Allergy Forecast

Fort Collins pollen count and allergy forecast — tree, grass, and ragweed seasons and what’s pollinating now

Fort Collins, CO · Pollen season

In September, ragweed pollen is in season in Fort Collins, peaking Aug–Oct.

Based on the seasonal pollen calendar for this region.

Fort Collins on the WeatherStory Allergy Index

Fort Collins isn't one of the 100 metros on the WeatherStory Allergy Index, but the nearest ranked metro — Denver, CO, about 55 miles away — sits at #92 with a 244-day combined pollen season.

#92of 100nearest ranked metro

Load index: 27
Combined season: 244 days
Heaviest load: Tree · 91 days
Nearest ranked metro: Denver, CO · ~55 mi

See the full 100 worst U.S. cities for pollen — a modeled seasonal load ranking (see methodology).

Typical now — Septemberseasonal estimate, not a live reading

Ragweed pollen typically leads in Fort Collins in September, at an estimated High level.

Pollen by type this season

TreeOut of seasonpeaks Mar–May
GrassOut of seasonpeaks May–Jul
Weed / RagweedIn seasonpeaks Aug–Oct

What pollinates near Fort Collins

The tree, grass, and weed species that drive Fort Collins’s pollen seasons, with each plant’s typical peak.

Trees

Juniper Juniperuspeaks March–April
Cottonwood Populuspeaks March–April
Aspen Populus tremuloidespeaks April–May
Ponderosa pine Pinus ponderosapeaks May–June
Box elder Acer negundopeaks March–April

Grasses

Kentucky bluegrass Poa pratensispeaks May–July
Smooth brome Bromus inermispeaks May–July
Timothy Phleum pratensepeaks June–July
Orchard grass Dactylis glomeratapeaks June–July

Weeds

Sagebrush Artemisia tridentatapeaks August–September
Common ragweed Ambrosia artemisiifoliapeaks August–September
Russian thistle Salsola traguspeaks August–September
Kochia Bassia scopariapeaks August–September

Species are the region’s common wind-pollinated plants; peak months are a seasonal estimate, not a measured count — see methodology.

Fort Collins’s allergy year

Typical month-by-month pollen intensity for Fort Collins, tuned to its latitude and climate region — darker means higher typical counts. The highlighted column is the current month.

JFMAMJJASOND

TreeMar–May

GrassMay–Jul

RagweedAug–Oct

How Fort Collins compares to the U.S. average

Each colored bar is Fort Collins’s typical season against the national-average window (grey). The offset is the per-city timing shift.

Treeabout average

Grassabout average

Ragweedabout average

“U.S. average” is the textbook tree Mar–May, grass May–Jul, ragweed Aug–Oct calendar. The shift is a phenology estimate (Hopkins’ bioclimatic law), not a measured count — see methodology.

How Fort Collins’s pollen count works

The calendar above is tuned to Fort Collins’s high-elevation Mountain West climate, not a national average: tree pollen runs from week 10 (early March) to week 22 (late May), grass from week 18 (early May) to week 30 (late July), and ragweed from week 31 (early August) to week 43 (late October) here — a 244-day combined pollen season. Those windows are why ragweed pollen is the one in season in Fort Collins right now.

No live count is wired up for Fort Collins today, so the seasonal calendar above is your guide to which allergen is in season.

For the science behind Fort Collins’s pollen seasons — how counts are measured, when each type peaks, and what the numbers mean — see the pollen count scale and the other pollen & allergy guides.

Frequently asked

When is pollen worst in Fort Collins?: Fort Collins runs the classic three-wave calendar: tree pollen Mar–May, grass May–Jul, then ragweed Aug–Oct. The two worst stretches are the spring tree peak and the late-summer ragweed peak. Currently, ragweed pollen is in season.
What's in the air in Fort Collins right now?: In September, ragweed pollen is in season in Fort Collins, peaking Aug–Oct. On Fort Collins's high-elevation Mountain West calendar the typical September level is High, led by ragweed pollen — a seasonal estimate, not a live count.
Is tree or grass pollen higher in Fort Collins in spring?: In spring, tree pollen leads in Fort Collins — trees pollinate Mar–May, ahead of grass (May–Jul). The handoff is the tail of the tree window: tree counts taper as grass climbs, so an early-spring flare is more likely tree pollen and a late-spring one more likely grass.
What makes Fort Collins's pollen season distinctive?: Fort Collins sits in the high-elevation Mountain West zone, which means short, sharp seasons set late by elevation, with sagebrush adding to the late-summer weed load. That shapes when symptoms hit and which allergen to watch.
How do I reduce pollen exposure in Fort Collins?: Through Fort Collins's peak windows (tree Mar–May, grass May–Jul, ragweed Aug–Oct), keep windows shut and run AC on recirculate; counts run highest on dry, warm, windy mornings, so push outdoor activity to late afternoon or just after rain, which clears pollen from the air. A HEPA purifier indoors, a saline rinse after being outside, showering before bed, and starting antihistamines a week or two before your worst local window all measurably cut symptoms.
When does the next pollen season start in Fort Collins?: Tree pollen is next in Fort Collins: it opens around week 10 (early March), peaks around week 15, and runs about 91 days.
What is the allergy forecast in Fort Collins today?: With no live reading for Fort Collins today, its allergy forecast is the seasonal estimate above: a typical September level of High, led by ragweed pollen.