6. Ambrosia trifida L. (giant ragweed, great ragweed, horse weed, buffalo weed)

A. trifida var. texana Scheele

A. trifida f. integrifolia (Muhl.) Fernald

Pl. 271 i, j; Map 1147

Plants annual, with taproots. Stems 30–500 cm long, sparsely to moderately roughened with short, ascending to loosely appressed hairs with pustular bases, sometimes also with scattered, longer, spreading hairs toward the tip. Leaves mostly opposite (the uppermost ones often alternate), with short to more commonly long petioles, those of the upper leaves often narrowly winged. Leaf blades 3–30 cm long, the main stem leaves usually much more than 1 cm wide (often 10–20 cm wide), ovate to broadly elliptic or nearly circular in outline, unlobed or more commonly with 3 or 5 deeply palmate lobes, occasionally with an additional pair of smaller lobes, the lobes oblong-elliptic to more or less elliptic or oblong-triangular, usually with many fine or coarse teeth, the upper surface sparsely to moderately roughened with short, stout, pustular-based hairs and sometimes with sparse, longer hairs along the main veins, the undersurface similarly but more sparsely roughened, not or only slightly paler than the upper surface. Staminate heads in spikelike racemes, these sometimes 3 to several in paniculate clusters, the staminate involucre 1.5–3.0 mm wide, with 5–8 shallow to moderately deep lobes, minutely hairy and usually with pronounced black lines from the attachment point to the tips of the lobes. Pistillate heads in small axillary clusters (or occasionally solitary), the involucre enclosing 1 floret and with 1 stout, conical beak, 5–10 mm long at fruiting, more or less ovoid or somewhat pear-shaped, with 4–8 longitudinal angles or ridges, each terminating in a short, stout tubercle or spine, glabrous or sparsely hairy, the spines and beak often more densely hairy. 2n=24. July–September.

Common nearly throughout the state (U.S., Canada; introduced in Europe). Bottomland and upland prairies, banks of streams and rivers, sloughs, marshes, margins of ponds and lakes, and bottomland forests; also pastures, old fields, fallow fields, crop fields, levees, ditches, railroads, roadsides, and open, disturbed areas.

Giant ragweed is an exceedingly variable species. Plants varying in characters such as degree of leaf lobing and size of the fruiting involucre have been segregated as forms or varieties by some botanists, but Payne (1970) argued against such splitting of the species based on his observations of plants from throughout the range. For a discussion of uncommon putative hybrids with A. bidentata, see the treatment of that species. Hybridization also occurs with A. artemisiifolia in other states, but it has not yet been reported from Missouri.

Ambrosia trifida forms extensive colonies in disturbed bottomland and agricultural areas and is a leading cause of hay fever in the late summer. It is a problem weed in crop fields, particularly in soybean fields, and anecdotal evidence suggests that in Illinois a strain has evolved that is resistent to glyphosate-based herbicides such as Roundup (Associated Press, 2002). Payne and Jones (1962) studied archaeological remains of A. trifida with unusually large fruiting involucres from prehistoric bluff shelters in the Ozark Plateau and concluded that the species was cultivated by aboriginal Americans for its edible fruits. Moerman (1998) noted that Native Americans used the plant ceremonially and medicinally, principally to treat stings, hives, infections, fever, and pneumonia. Steyermark (1963) stated that the species was used medicinally as an astringent. He also noted that the plants are a source of food for wildlife.