Ragweed pollen allergy

What it is

Ragweeds are weeds in the genus Ambrosia, part of the daisy family (Asteraceae).⁴⁶ ACAAI counts 17 species in the United States.¹ Extension weed guides focus on two of them, common ragweed (Ambrosia artemisiifolia) and giant ragweed (Ambrosia trifida), both native to North America.⁴

A ragweed plant lives for only one season.² Giant ragweed is an annual that can stand up to 12 feet tall.⁵ Common ragweed is shorter, usually 1 to 2 feet tall.⁴ Its leaves are divided and hairy.⁶

Ragweed flowers are plain and easy to overlook, without the showy petals of most of its daisy relatives. They grow in spikes about 6 inches long at the ends of the branches. The pollen comes from the male flowers toward the tips of the spikes and is carried by the wind.⁶ A single plant can produce up to 1 billion pollen grains, and the grains are light enough to float easily through the air.² They measure about 20 to 50 microns, small enough to drift long distances.⁶

Goldenrod is often wrongly blamed because it blooms at the same time. Its pollen is too heavy for the wind and is moved by insects instead.⁶

Symptoms it causes

Breathed in, ragweed pollen can cause sneezing, a runny nose, nasal congestion, headaches, irritated eyes and an itchy throat.¹ It can also make asthma symptoms worse.¹ On the skin, ragweed can cause contact dermatitis and hives.²

Some people with ragweed allergy get an itchy mouth, a scratchy throat, or swelling of the lips, mouth, tongue and throat after eating certain raw foods. This is pollen food allergy syndrome, also called oral allergy syndrome.⁸ The foods linked to ragweed include banana, cucumber, melon and zucchini.⁷

Where and when it grows

Ragweed grows throughout the United States, especially in the eastern and midwestern states.³ It favors ground that has been disturbed: roadsides, riverbanks, vacant lots and fields, especially where water, farming, chemicals or road salt have broken up the soil.³ Common ragweed is most concentrated in the eastern half of the country.⁶ Giant ragweed grows in crop fields, flood plains and field edges.⁵

Common ragweed flowers from July to September in most cases, and giant ragweed from August to October.⁴ Ragweed typically blooms and releases pollen from August into November, and its pollen usually peaks in mid-September.¹ Longer nights trigger flowering, and warm, humid weather and breezes after sunrise help release the pollen. Depending on location, the season can last six to 10 weeks, and symptoms can continue until the first frost kills the plants.³

The season has been getting longer. Ragweed pollination used to start in mid-August and run through September. In many parts of the country it now begins in early August and extends through October.²