Ragweed, a common North American plant. Its pollen is one of the chief causes of hay fever. The pollen is produced by the tiny, greenish-yellow flowers that form in late summer or early fall. The weeds are controlled by burning or mowing.
The common ragweed (also called Roman wormwood, hogweed, or bitterweed) sometimes reaches a height of seven feet (2.1 m) and is an annual. It is a hairy, many-branched plant with thin, ragged, finely cut leaves. The western ragweed is similar, but is a perennial and has thicker leaves. The great ragweed (also called bitterweed or horsecane) is an annual often found in cities. It grows to be 3 to 17 feet (0.9 to 5.2 m) high, and has a rough, stout stem with large three-lobed leaves.
Ragweeds belong to the family Compositae. The common ragweed is Ambrosia artemisiifolia; the western, A. psilostachya; the giant A. trifida.Ragweed pollen is produced by the tiny flowers that form in late summer or early fall.