Ragweed is an annual or perennial plant of the genus Ambrosia, family Compositae or Asteraceae. Fifteen species are native to North America; 3 occur across Canada: common ragweed (A. artemisiifolia), perennial ragweed (A. coronopifolia) and giant ragweed (A. trifida). Ragweed pollen is the most prevalent cause of hay fever in Canada, and common ragweed can also cause dermatitis. Common ragweed, the most abundant, is an erect, hairy-stemmed, coarse, annual herbaceous plant, 5-200 cm high, usually with deeply divided, dark green leaves. Greenish, male flowers with obvious yellow stamens are borne in clusters along an erect stalk. One-seeded fruits (achenes) occur singly or in clusters, and are found at bases of upper leaves. Ragweed is an abundant weed in cultivated fields, around open disturbed areas and along roadsides, especially in southern Ontario and Québec.