Tropical whiteweed

Ageratum conyzoides is an erect annual herb of the Asteraceae family growing 30-100 cm tall with a slender taproot and aromatic, somewhat goat-scented foliage (whence the name 'goatweed'). Stems are erect, terete, longitudinally ribbed, branched in the upper part and softly covered with spreading white hairs. Leaves are opposite (the upper sometimes alternate), ovate to broadly ovate, 2-8 cm long, with coarsely crenate to serrate margins, an acute to obtuse apex, a rounded to truncate or shallowly cordate base, and softly pubescent and glandular-dotted on both surfaces. Petioles are 1-4 cm long. The inflorescence is a dense, flat-topped corymb of numerous small cylindrical heads borne in the upper part of the plant. Each head is 5-7 mm long with an involucre of two or three rows of narrow, acute, often slightly glandular green bracts. Florets are all tubular, 30-60 per head, with a slender five-lobed corolla 2-3 mm long that is pale lavender-blue (sometimes white or pale pink) and slightly exserted from the involucre. The achene is small, narrowly oblong, black and bears a pappus of five lanceolate awned scales. Native to tropical Central and South America and now pantropical, growing as a weed on disturbed ground, roadsides and fallow fields.