2. Ambrosia artemisiifolia L. (common ragweed, bitter weed, Roman wormwood, hogweed)

A. artemisiifolia var. elatior (L.) Descourt.

A. artemisiifolia f. villosa Fernald & Griscom

Pl. 271 d, e; Map 1143

Plants annual, with taproots. Stems 30–120 cm long, sparsely to densely pubescent with relatively long, spreading hairs usually with minute, pustular bases and/or shorter, appressed hairs. Leaves opposite toward the stem base, alternate toward the stem tip, with short to long, narrowly winged petioles. Leaf blades 3–10 cm long, ovate to broadly ovate in outline (the uppermost leaves sometimes lanceolate to linear), 2–3 times pinnately lobed with more than 5 primary lobes (the uppermost leaves sometimes few-lobed to nearly entire), the ultimate lobes lanceolate to narrowly oblong, entire or few-toothed, the surfaces sparsely to moderately pubescent with short, somewhat broad-based hairs and sometimes appearing somewhat grayish, the undersurface also usually with longer hairs along the main veins, not or only slightly paler than the upper surface. Staminate heads in spikelike racemes, these usually not in paniculate clusters, the staminate involucre 2–4 mm wide, with 3–9 shallow lobes, glabrous or sparsely hairy. Pistillate heads in small axillary clusters (or sometimes solitary), the involucre enclosing 1 floret and with 1 beak, 3–5 mm long at fruiting, more or less ovoid, with 1 series of not or only slightly flattened, short spines in a ring toward the tip, sparsely to moderately hairy. 2n=34, 36. July–November.

Common nearly throughout the state (U.S., Canada; introduced in Hawaii, Europe). Upland prairies, savannas, glades, tops of bluffs, banks of streams, rivers, and spring branches, marshes, margins of ponds, lakes, and sinkhole ponds, and openings of bottomland to mesic or dry upland forests; also pastures, old fields, fallow fields, crop fields, levees, ditches, farmyards, railroads, roadsides, and open, disturbed areas.

This is a variable species well adapted to disturbed sites. Steyermark (1963) noted that putative hybrids with A. trifida (A. ×helenae Rouleau) occur in surrounding states and eventually may be discovered in Missouri. To the north of Missouri, hybrids with A. psilostachya (A. ×intergradiens W.H. Wagner & Beals) also have been documented (Wagner and Beals, 1958). Steyermark (1963) also noted that although the fruits provide food for wild turkey and other wildlife, grazing of the plants apparently causes a nauseous effect in cattle. The species is a problem agricultural weed, and anecdotal reports indicate that a strain resistent to glyphosate-based herbicides such as Roundup has evolved in central Missouri (Bradley, 2005).