Horseweed

Ambrosia trifida is a robust annual herb of the daisy family commonly growing 1-4 m tall, occasionally to 6 m, with a deep taproot and stout, rough-hairy, much-branched stems. The opposite leaves are large, 10-25 cm long and wide, palmately three- to five-lobed (the lower sometimes simple), with serrate lobe margins and rough-scabrous surfaces; petioles are conspicuously winged. The plant is monoecious: small greenish, nodding male capitula 4-5 mm across are arranged in long, slender terminal racemes, while the female heads are clustered in the upper leaf axils below and contain a single ovule. Female involucres mature into hard, top-shaped fruits 6-12 mm long bearing several short spines around a prominent central beak. Native to floodplains, riverbanks, and disturbed bottomlands of central and eastern North America, the species has become widely naturalised. Flowering is from July to October, producing copious airborne pollen.