Ragweed Pollen: Allergy, Season & Data

Ragweed pollen is the defining allergen of fall in North America. Pollen Atlas tracks ragweed daily as its own category in the data behind every city count — its season is the backbone of the fall city rankings — because for millions of people, ragweed is the difference between a pleasant September and a miserable one.

What is ragweed?

What is ragweed? It is a genus of weeds, Ambrosia, that grows in nearly every state — above all across the eastern and central United States — and is invasive and spreading across Europe and parts of Asia and Australia. A single ragweed plant can release a billion grains of some of the most allergenic pollen known in a season, which is why ragweed pollen dominates fall counts even where the plants look unremarkable. The plant itself — and how to recognise it — is covered in the ragweed plant guide.

When ragweed pollinates: ragweed season

Ragweed season runs from late summer to fall — August to the first frost. The ragweed pollen season opens as the grass season fades, peaks in September across much of the country, and ends by execution: the first hard frost kills the plants and stops production outright. The full calendar, region by region, is in the ragweed season guide.

Ragweed allergy symptoms: what a ragweed allergy feels like

A ragweed allergy is among the most common pollen allergies in the United States, and ragweed allergies are a main reason fall is a second allergy season across the eastern half of the country. The ragweed symptoms commonly reported are intense sneezing, congestion and itchy eyes, and ragweed season can aggravate asthma. The fuller pattern of ragweed allergy symptoms is sneezing fits, a runny or blocked nose, itchy and watery eyes, an itchy throat, ears or roof of the mouth, sinus pressure and disturbed sleep — worst outdoors on dry, windy mornings, when ragweed pollen peaks, and easing on rainy days. Symptoms that return at the same point every late summer and fall, and track the weed index on your city page, point to ragweed rather than a cold, which does not follow the pollen calendar. Because a ragweed pollen allergy is so widespread, a fall pattern that repeats every year is worth confirming with a specialist — you can find an allergist in the Pollen Atlas directory.

How ragweed appears in pollen counts

In Pollen Atlas data, ragweed is reported as its own category — the weed index on every city page is built around it — so a city's fall reading rises and falls with the ragweed in its air. Follow it on the Houston pollen count and Tulsa pollen count pages, compare cities in the city rankings, and watch the fall pattern spread on the pollen heatmap. Ragweed's fellow summer weed, covered the same way, is dock sorrel.

Ragweed in the weed family

Weed pollen closes the year. Ragweed owns the fall in the East and centre; the amaranth weeds own it in the dry West; and the docks and sorrels quietly fill the summer between grass and ragweed. Ragweed is the headliner of a short cast — its supporting players include pigweed and Russian thistle in the Southwest and Plains, and dock sorrel ahead of it in summer — so when the weed index rises in June or July, before ragweed season opens, it is usually one of those supporting players at work. The fall itself belongs to Ambrosia.