3. Ambrosia bidentata Michx. (lanceleaf ragweed, southern ragweed)

Pl. 271 a–c; Map 1144

Plants annual, with taproots. Stems 30–100 cm long, moderately roughened-pubescent with a mixture of short-ascending to spreading and longer, stiffly spreading hairs with pustular bases, sometimes appearing nearly glabrous toward the base and often somewhat shiny. Leaves mostly alternate (the lowermost ones occasionally opposite), sessile. Leaf blades 1–7 cm long, 0.4–1.0 cm wide, lanceolate to narrowly oblong-lanceolate, unlobed or with a pair of small lobes toward the base, the lobes triangular, the margins otherwise entire, the surfaces sparsely to moderately roughened-pubescent with short, loosely ascending, pustular-based hairs and stiff, longer hairs (mostly along the midvein), the upper surface sometimes only sparsely hairy, the undersurface often with pronounced, small, sessile glands (slightly sticky to the touch), not or only slightly paler than the upper surface. Staminate heads sessile in relatively dense, solitary spikes, the staminate involucre 2.5–4.0 mm wide, with 3(5) lobes, the lateral ones small, the terminal lobe elongate, the outer surface with minute, sessile glands, the terminal lobe with a patch of relatively long, stiff hairs. Pistillate heads in small axillary clusters (or occasionally solitary), the involucre enclosing 1 floret and with 1 stout, conical beak, 5–8 mm long at fruiting, more or less ovoid, with usually 4 longitudinal angles or ridges, each terminating in a short spine, moderately hairy. 2n=34. July–October.

Scattered nearly throughout the state, but uncommon or absent from portions of the northwestern quarter (Ohio to Louisiana west to Nebraska and Texas; introduced farther northward and eastward). Upland prairies, savannas, glades, tops of bluffs, banks of streams and rivers, and openings of mesic or dry upland forests; also old fields, fallow fields, railroads, roadsides, and open, disturbed areas.

Sterile hybrids between A. bidentata and A. trifida have been collected sporadically in Missouri, mostly in the western half of the state. These plants resemble A. bidentata in stature but have the leaves irregularly and mostly deeply 3- or 5-lobed with relatively narrow, pinnate lobes.