Much branched, hairy annual 2-8 dm; lvs elliptic or ovate to lance-ovate, 2-6 cm, glabrous or nearly so except for the scabrous margins and the midrib on the lower surface; fls mostly 6-merous, in dense terminal glomerules surrounded by 2 or 4 involucral lvs with broad, sessile base; cor-tube 2-8 mm, lobes 0.5-2.5 mm; fr 2-3.5 mm, muricate-tuberculate, its 3(-6) cocci nearly cylindric, the adaxial scar nearly closed; 2n=28, 56. Native of tropical Amer., established as a weed in se. U.S., and occasionally adventive in our range. Summer.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.