Ambrosia bidentata Michx.

Branching annual weed 3–10 dm, the stem conspicuously spreading-hirsute, especially above; lvs numerous, sessile, opposite below, alternate above, lanceolate, acuminate, 2.5–7 cm × 4–10 mm, usually with a single pair of sharp large teeth below the middle, hirsute to scabrous, or the upper side glabrous; staminate heads sessile, the invol strongly oblique, its upper side produced into a conspicuous, retrorsely spreading, hispid-hirsute, lanceolate to triangular-ovate lobe, the bractless infl thus appearing retrorse-bracteate; fruiting invol 5–8 mm, villous-hirsute, several-ribbed, the ribs produced into short, stiff spines; 2n=34. Prairies and waste places; O. and Ky. to Nebr., La., and Tex., and occasionally intr. elsewhere. July–Oct.