Ambrosia artemisiifolia L.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Annual weed 3–10 dm, branching at least above, variously hairy or in part subglabrous; lvs opposite below, alternate above, once or more often twice pinnatifid, the blade narrowly to broadly ovate or elliptic in outline, 4–10 cm, the middle and lower ones, at least, evidently petiolate; staminate invols short-pedunculate, symmetrical or slightly oblique, inconspicuously nerved; fruiting invols 3–5 mm, 1-fld and 1 beaked, with a single series of short, sharp, erect spines near or above the middle; 2n=34, 36. Waste places; throughout our range, w. to the Pacific. (July) Aug.–Oct. (A. elatior; A. media)