Ambrosia trifida L.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Coarse annual weed to 2(–5) m; stem spreading-hairy above, often glabrous below; lvs opposite, petiolate, ± scabrous, broadly elliptic to more often ovate or suborbicular, often 2 dm, serrate, palmately 3–5-lobed, or, especially in depauperate individuals, lobeless; staminate invols ca 1.5 mm, slightly oblique, unilaterally evidently 3-nerved; fruiting invols 5–10 mm, several- ribbed, each rib ending in a tubercle or spine; 2n=24. Moist soil and waste places throughout our range, s. to Fla., w. to B.C. and n. Mex. (July) Aug.–Oct. The common form in most of our range is var. trifida, with the petioles of at least the upper lvs ± wing-margined, and the ribs of the fr ending in short spines. (A. striata) From s. Ill. and Mo. to Miss., Ariz., and n. Mex. is found var. texana Scheele, with wingless petioles, and with the ribs of the mostly smaller fr ending in blunt or almost obsolete tubercles. (A. aptera)