What are the most common fall allergies?

Three triggers overlap in autumn: ragweed pollen, outdoor mold spores such as Alternaria and Cladosporium, and, in warm regions, grasses and weeds that pollinate late. Ragweed is the largest, with about 50 million Americans reacting each fall (AAFA).

When do fall allergies start and end?

Ragweed typically begins in mid-August, peaks in mid-September, and ends at the first hard freeze — about 6 to 10 weeks in most of the country. In the South, the season can stretch into November.

Does the first frost end fall allergies?

A hard freeze — roughly 28°F or colder for several hours (NOAA/NWS) — kills ragweed and ends its pollen. Mold is different: most spores go dormant rather than die, so a mild, damp fall can keep mold counts up after ragweed is gone.

Are fall allergies getting worse?

At the northern edge, yes. A PNAS study found the ragweed season grew by up to 27 days at higher latitudes, and EPA data show gains like 18 days in Minneapolis and 21 days in Fargo as first freezes arrive later.

What triggers ragweed to release pollen in the fall?

Day length. As nights draw out in late summer, ragweed — a short-day plant — comes into bloom, so the timing follows the calendar, not a temperature threshold. That is why it shows up in mid-August almost every year.

How do I reduce fall allergy symptoms?

Start a second-generation antihistamine about 1 to 2 weeks before your local peak, keep windows shut on warm, dry, windy mornings, run AC on recirculate, shower after being outdoors, and wear a mask when raking leaves.

Fall Allergies: What Triggers Them and When They Peak

Fall allergies rarely trace to a single culprit. Three triggers overlap each autumn: ragweed pollen, outdoor mold spores, and, in warm regions, grasses and weeds that keep pollinating late. Ragweed leads by a large margin — AAFA estimates about 50 million Americans react to it each late summer and fall — which is why autumn hay fever can feel heavier, and run longer, than a spring or summer season built on one allergen.

The season keeps a reliable rhythm. Across most of the country ragweed begins around mid-August, peaks in September, and fades at the first hard freeze — a window of roughly 6 to 10 weeks (per AAFA and EPA). Outdoor mold overlaps that window and can outlast it, so where you live decides how many of the three triggers you face and for how long.

Fall is a ragweed-and-mold season

The autumn allergy window (Aug–Nov) sits over ragweed's peak and the tail of the outdoor-mold season.

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TreeMar–May

GrassMay–Jul

RagweedAug–Oct

Fall allergy season (Aug–Nov)

Tree, grass, and ragweed shading is Weather Story’s deterministic seasonal-estimate model (regional AAFA/ACAAI season windows; see methodology), not measured pollen counts. Darker = higher typical intensity.

The three drivers of fall allergies

Ragweed is the headline trigger. It is a short-day plant, cued to bloom as the nights draw out in late summer, so its pollen release tracks day length rather than temperature — which pins it to roughly the same mid-August start every year (per AAFA). Its detailed species biology is covered on its own; in fall it is one of three overlapping problems rather than the whole story.

Outdoor mold is the second driver. Fungi such as Alternaria and Cladosporium release spores from damp soil, compost, and fallen-leaf litter, and their counts climb through late summer into October. Because mold feeds on moisture, wet autumn weather sustains it instead of clearing it (per AAFA and ACAAI).

The third driver is regional. In the warm South, grasses and other weeds keep pollinating well past summer, stacking onto ragweed and mold. Farther north, the first cold snaps shut grass down earlier, leaving ragweed and mold as the main autumn pair.

When fall allergy season peaks

Ragweed usually starts releasing pollen in mid-August, peaks in mid-September, and tapers as frost nears. In most regions the run lasts 6 to 10 weeks (per AAFA). Counts climb highest on warm, dry, breezy mornings, when there is no moisture to weigh the grains down, and ease after a soaking rain.

Timing shifts with latitude. In the South, mild autumns can push the season into November; in the North it is shorter but arrives on the same day-length cue. Symptoms often lag the pollen, so allergists suggest waiting a couple of weeks after the first freeze before assuming the season is done.

Why fall allergies flare after rain

Rain has a split effect in fall. A downpour briefly washes pollen out of the air, but it also feeds outdoor mold, which is why many people feel worse in the damp days after a storm rather than better. Alternaria and Cladosporium peak in exactly these conditions — warm, wet, and still.

The numbers back it up: one analysis found Cladosporium spores about 48% higher and Alternaria about 28% higher on thunderstorm days than on calm ones (per NIH/PMC research). Damp leaf litter raked into piles is a common backyard source, so clearing wet leaves can set off a flare.

When does fall allergy season end?

The clean end point is a hard freeze. Once temperatures drop to about 28°F or colder for several hours — the NOAA and National Weather Service threshold for a hard freeze — ragweed plants die and their pollen release stops. That is why the first killing freeze, not the first cool day, marks the end of ragweed for the year.

Two things complicate that. In the South, a hard freeze may not arrive until November or later, stretching the season by weeks. And mold does not die at frost the way ragweed does — most spores simply go dormant for winter, so a mild, damp fall can keep mold counts up after ragweed is long gone (per AAFA).

Managing fall symptoms

Timing beats reacting. Track your local ragweed and mold counts and start a second-generation antihistamine about 1 to 2 weeks before your area's usual peak, so it is already working when pollen climbs (per Mayo Clinic). Keep windows shut on warm, dry, windy mornings and run air conditioning on recirculate.

Cut what you carry indoors: shower and change clothes after time outside, and dry laundry inside rather than on a line. When you rake or bag leaves, wear a dust mask to blunt the mold exposure. If symptoms persist through the season or trigger asthma, an allergist can test for your specific triggers and discuss longer-term options (per ACAAI).

Where fall allergies last longest

The heaviest fall load sits over the Midwest, Great Plains, and the Ohio and Mississippi valleys, where open farmland and field edges let ragweed spread unchecked. Its pollen is light enough to travel long distances on the wind, so even metros with little ragweed of their own inherit the regional cloud each autumn — and add outdoor mold on top.

The season is also lengthening at its northern edge. A PNAS study (Ziska 2011) found the ragweed season grew by up to 27 days at higher latitudes between 1995 and 2009, and EPA station data show gains such as 18 days in Minneapolis and 21 days in Fargo as first freezes arrive later. The metros below show where the autumn window runs longest, each linking to its current pollen forecast.

What's still in the air in autumn

Ragweed (highlighted) and lingering outdoor mold drive fall hay fever after grass has faded.

Tree

Grass

Ragweed

Mold

Pollen windows are the seasonal-estimate model (see methodology); the mold bar is the documented outdoor-spore season (AAFA).

The fall season, by the numbers

50million

Americans react to ragweed each fall

AAFA

6–10weeks

typical ragweed run in most regions

AAFA / EPA

28°F

hard freeze that finally ends ragweed

NOAA / NWS

Check your local ragweed forecast

Pollen seasons vary sharply by region. These metros see some of the worst ragweed pressure — check the current forecast for each, or look up any US city on the pollen count hub:

Columbus, OH pollen count — Ohio Valley farmland and humidity stack ragweed and outdoor mold deep into October.
Kansas City, MO pollen count — Great Plains ragweed belt plus a humid river-valley mold season makes a long, heavy fall.
Omaha, NE pollen count — Open Plains farmland drives some of the nation's heaviest ragweed counts each autumn.
Louisville, KY pollen count — Ohio Valley humidity feeds both ragweed pollen and outdoor mold well into the fall.
Wichita, KS pollen count — Ringed by Plains grassland and field edges where ragweed thrives late into the season.
St. Louis, MO pollen count — Mississippi Valley humidity stretches the ragweed-and-mold season toward November.

Frequently asked

What are the most common fall allergies?: Three triggers overlap in autumn: ragweed pollen, outdoor mold spores such as Alternaria and Cladosporium, and, in warm regions, grasses and weeds that pollinate late. Ragweed is the largest, with about 50 million Americans reacting each fall (AAFA).
When do fall allergies start and end?: Ragweed typically begins in mid-August, peaks in mid-September, and ends at the first hard freeze — about 6 to 10 weeks in most of the country. In the South, the season can stretch into November.
Why are my fall allergies worse after it rains?: Damp weather feeds outdoor mold. One analysis found Cladosporium spores about 48% higher and Alternaria about 28% higher on thunderstorm days than on calm ones (NIH/PMC), so mold-sensitive people often feel worse in the wet days after a storm.
Does the first frost end fall allergies?: A hard freeze — roughly 28°F or colder for several hours (NOAA/NWS) — kills ragweed and ends its pollen. Mold is different: most spores go dormant rather than die, so a mild, damp fall can keep mold counts up after ragweed is gone.
Are fall allergies getting worse?: At the northern edge, yes. A PNAS study found the ragweed season grew by up to 27 days at higher latitudes, and EPA data show gains like 18 days in Minneapolis and 21 days in Fargo as first freezes arrive later.
What triggers ragweed to release pollen in the fall?: Day length. As nights draw out in late summer, ragweed — a short-day plant — comes into bloom, so the timing follows the calendar, not a temperature threshold. That is why it shows up in mid-August almost every year.
How do I reduce fall allergy symptoms?: Start a second-generation antihistamine about 1 to 2 weeks before your local peak, keep windows shut on warm, dry, windy mornings, run AC on recirculate, shower after being outdoors, and wear a mask when raking leaves.