Botanical name: Hyptis capitata Family: Lamiaceae (Mint family)
Knobweed is an erect subshrub 1/2-2 m tall, with ovate-oblong, acute, serrate, petiolate leaves, commonly 5-15 cm long and 2-6 cm wide; petiole to about 2-3 cm; blades glabrate or puberulent and dorsally glandular; peduncles axillary, commonly 2-9 cm long; heads subglobose, 1.5-2.5 cm in diameter; flowers sessile, white, subtended by oblong-obovate bracts up to 8-12 mm long; calyx-lobes subulate, nearly equal, calyx-tube glabrous at extreme base, hirsute medially, pubescent distally, 3-4 mm long in flower, enlarging to nearly 1 cm in fruit; corolla 5-6 mm long, white with faint purplish spots on upper lip; filaments pubescent basally; nutlets brownish-black with a narrow white hilum, about 1 mm long. Knobweed is native to Central America, now widespread as a weed.
Identification credit: Pudjii Widodo
The flower labeled Knobweed is ...