Asteraceae (or Compositae) is a very large family with three subfamilies and 17 tribes, the majority of which are insect-pollinated and of little concern to allergists. The subfamily Asteroideae, however, contains three tribes of particular interest: Heliantheae (sunflower tribe), Astereae (aster tribe), and Anthemideae (mayweed tribe). Ragweeds are in a Heliantheae subtribe, Ambrosiinae. Giant ragweed, Ambrosia trifida, is one of the four ragweeds of major importance; the others are short, A. artemisiifolia, western. A. psilotachya, and false, A. acanthicarpa. There are numerous other Ambrosia species of regional importance. Giant ragweed is prevalent from New Brunswick west to the Continental Divide and south to the Gulf Coast, with the exception of the southeast Atlantic seaboard and Florida. Except for Ambrosia maritima, which is native to the Mediterranean region, all European Ambrosia species are introduced. Giant ragweed, A. trifida, was first found in southern Moravia in 1960. Climatic conditions are very favorable to ragweed expansion, and the ragweed pollen load in Europe has been steadily increasing over the past 15 years.

Giant ragweed is an annual weed, reproducing by seed. It will grow from a fibrous root system to a height of > 10 feet with proper moisture and fertility such as found near streambeds. In areas where giant ragweed outnumbers short ragweed, it will produce significantly greater amounts of pollen compared with short ragweed because of its larger size. Giant ragweed will generally begin pollination 2 to 3 weeks before short ragweed.

Ragweeds are prolific pollen producers and are the predominant cause of late summer hay fever in the central and eastern states. Durham had documented many years ago that the highest ragweed counts in the United States were found in the vicinity of Omaha. In portions of the northeast and south-central states, ragweed accounts for between 75 to 90% of all pollen captured between August and October.

Adapted from: Annals of Allergy, Richard Weber, MD, September 2001.