Ambrosia trifida, variously called Buffalo Weed, Great Ragweed, Giant Ragweed, Bitterweed, Bloodweed, Horse Cane or, Tall Ambrosia, is an annual plant in the aster family, native throughout much of North America. Its flowers are green and are pollinated by wind rather than by insects, and the pollen is one of the main causes of late summer hay fever. Flowers are borne from midsummer through early fall.

The plant is erect, growing to 3 meters tall from clumping rootstocks.