Parthenium hysterophorus L., Sp. Pl. 988. 1753; R.S. Rao in J. Bombay Nat. Hist. Soc. 54: 218. 1956.

Herbs up to 1 m high, annual; stems puberulous to strigillose. Leaves usually lanceolate above to pinnately dissected downwards, 5 - 15 x 0.4 - 5 cm; pinnules oblong- lanceolate, entire to deeply lobed, puberulous, acute with decurrent base; petiole 3 - 4 cm long. Inflorescence a lax panicle of numerous small heads; peduncles 4 - 10 mm long. Heads radiate, 4 - 8 mm across. Receptacle convex, small. Involucre campanulate. Involucral bracts green, 5, elliptic-ovate, acute, 3 - 4 x 1 - 2 mm, pubescent. Ray florets 5, with 2 seriate disc florets on either side; corolla white to light yellow, ovate-orbicular, bilobed, obtuse. Style 1 mm long with stout, obtuse stigma. Disc florets many; corolla light yellow, narrowly campanulate, 1.5 - 2 mm long, 5 lobed, ovate-acute. Stamens 5; anthers linear. Style 1 - 5 mm, puberulous, entire. Achenes flattened, 2 mm long. Pappus of 2 broad, strongly reflected awns, 0.3 - 0.5 mm long, puberulous along the sides.

Fl. & Fr. May - March.

Distrib. India: Jammu & Kashmir, Himachal Pradesh, Uttar Pradesh, Bihar, Assam, Arunachal Pradesh, Maharashtra, Karnataka and Tamil Nadu.

A native of America now fairly naturalised in all states of India, causing a serious threat to agriculture.