September Allergies: The Month That Rivals Spring
Ask a room of allergy sufferers to name the worst month and spring usually wins — but september allergies have a strong claim to the title. September is when the fall season stops building and starts peaking: ragweed, the dominant fall pollen, reaches its maximum release in the middle of the month across most of the United States, while outdoor mold climbs on accumulating leaf litter, and late grass pollen lingers in warmer regions. The Asthma and Allergy Foundation of America consistently describes mid-September as the peak of the fall season, and count data backs the description.
Key takeaways
- AAFA resources place the ragweed peak in mid-September for most of the United States.
- A single ragweed plant can produce up to a billion pollen grains in a season, a figure cited by the AAAAI.
- Pollen Atlas data shows the Weeds Index reaching its annual maximum most often in the second and third weeks of September.
- Outdoor mold counts climb through September as leaf litter accumulates, per National Allergy Bureau reporting patterns.
- September's burden falls unevenly: Pollen Atlas data shows central U.S. cities peaking days to weeks before northern-tier cities cool down.
September allergies, week one: the build becomes the season
Early September rarely feels like an ending to summer, and the counts agree. Ragweed plants that began releasing in August are now fully mature, and daily release becomes consistent rather than patchy. In Tulsa pollen counts and Kansas City pollen counts, Pollen Atlas data shows the Weeds Index typically stepping up in the first week and staying elevated — the difference between an occasional high day and a high plateau.
Why the plateau matters
A single high day is weather. A plateau is biology: thousands of plants across a region releasing on the same schedule, so that even a calm or briefly rainy day only dents the numbers before they rebound. The pollen heatmap shows the plateau as a broad regional band rather than isolated city spikes.
September allergies, weeks two and three: the peak
The center of the month is the center of the season. The AAAAI notes the scale behind it: one ragweed plant can produce up to a billion grains in a season, and those grains are built to travel, with ragweed pollen documented traveling hundreds of miles from its source in the literature summarized at aaaai.org.
Weather that makes peak weeks worse
Warm, dry, windy days are the peak's amplifiers — wind keeps light grains airborne and spreads them from fields and roadsides into cities. Chicago pollen counts show the pattern in a northern key: the city's highest fall readings typically cluster in these same mid-September weeks, just on a slightly later and cooler schedule than the Plains. Rain interrupts counts for a day, but during peak weeks the rebound is fast.
The mold layer underneath
September adds a second component. Fallen leaves begin accumulating, soil stays warm, and fungi colonize the litter. National Allergy Bureau stations report mold counts climbing through the month, so the total airborne load is ragweed plus a rising mold baseline — the combination the guide to fall allergies traces across the whole season.
| September window | Ragweed pattern | Mold pattern | Typical index behavior |
|---|---|---|---|
| Week 1 | Consistent daily release | Moderate, building | Weeds Index steps up to a plateau |
| Weeks 2–3 | Peak release, mid-month maximum | Climbing on leaf litter | Annual maximum most common here |
| Week 4 | Declining in the North, strong in the South | High in warm, wet areas | North–South split widens |
| Month overall | Dominant fall pollen | Second component all month | Rival to spring peak weeks |
Week four: the split begins
By late September, geography reasserts itself. Northern cities cool, ragweed plants past their prime release less, and the first genuinely lower readings appear — though frost, the true ending, is usually still weeks away, as the full arc in the ragweed season guide shows. Southern cities barely move: warm nights keep plants productive, and September can shade into an equally strong early October. The Asthma and Allergy Foundation of America describes this regional staggering in its fall resources.
For readers who consult a clinician about their own situation, the Find an Allergist directory lists providers by city and specialty as a general reference.
September also behaves differently at its edges depending on the summer that preceded it. A wet summer grows taller ragweed and lusher mold habitat, and the September that follows tends to sit at the high end of the range; a drought summer stunts plants but keeps what pollen exists airborne on dry wind, which can produce sharp individual days inside a lower monthly average. Station-level records published at pollen.aaaai.org let readers see this texture — the month as a sequence of weather-driven days rather than one smooth curve. School calendars add a social rhythm to the month: September is when routines move outdoors again for sports and recess across much of the country, precisely as the counts crest, which is one reason the month registers so strongly in public awareness relative even to spring. There is one more September signature worth naming: its nights. Cool evenings begin suppressing the next day's earliest release in northern states while afternoons remain warm, widening the gap between morning and midday readings. Pollen Atlas data shows this diurnal stretch in northern cities weeks before it appears in southern ones, where warm nights keep the daily cycle compressed and the counts high around the clock. By the month's final days, September has usually delivered its verdict — peak delivered, decline begun in the North, and the South settling in for a longer finish.
There is also a counting footnote: because ragweed pollen is so distinctive under the microscope, September station reports are among the most directly comparable across cities of any month in the year, which is part of why the mid-September peak shows up so consistently in records from different networks and different decades.
Frequently Asked Questions
Why are september allergies so strong?
September combines the ragweed peak — AAFA places it mid-month for most of the country — with rising outdoor mold counts and, in warmer regions, lingering grass pollen. The overlap produces some of the highest total airborne loads of the year.
When exactly does ragweed peak in September?
Pollen Atlas data shows the Weeds Index reaching its annual maximum most often in the second and third weeks of September, with exact days varying by city and weather. Northern cities tend to peak slightly later within that window than southern ones.
How much pollen does one ragweed plant make?
The AAAAI cites up to a billion grains from a single ragweed plant in one season. Multiplied across the plants in fields, roadsides, and vacant lots, this explains the scale of mid-September counts.
Do September counts fall after rain?
Rain temporarily washes pollen from the air, but during peak weeks counts typically rebound within a day or two as plants resume release in dry weather, per National Allergy Bureau reporting patterns. Mold can rise after rainy stretches.
Conclusion
September allergies earn their reputation honestly: the ragweed maximum, a climbing mold baseline, and in the South no cooling relief at all. Read week by week, the month has a predictable shape — build, peak, split — that the daily numbers follow closely. Check the pollen before it checks you — free counts and forecasts, worldwide: PollenAtlas.com
Sources
- Asthma and Allergy Foundation of America (AAFA), fall allergy and ragweed resources (2024–2025).
- American Academy of Allergy, Asthma & Immunology (AAAAI) / National Allergy Bureau, ragweed production and count reporting (2025).
- Pollen Atlas Weeds Index seasonal data (2026).