2. Artemisia annua L. (sweet wormwood, annual wormwood, sweet sagewort)

Pl. 225 a, b; Map 942

Plants annual, with taproots, strongly aromatic when bruised. Stems 30–200(–300) cm long, erect or ascending, glabrous but minutely glandular. Leaves 1–10 cm long, the basal and lower leaves relatively long-petiolate, often withered by flowering time, the median and upper leaves short-petiolate to sessile, lacking stipulelike lobes or teeth at the base. Leaf blades 1 or 2 times pinnately compound or deeply lobed, lanceolate to broadly ovate in outline, the main leaves with 7–11 primary lobes, the ultimate segments or lobes 0.5–2.0 mm wide, narrowly elliptic-oblanceolate to linear but not threadlike (except sometimes on the uppermost leaves), mostly sharply pointed at the tip, the margins toothed, both surfaces glabrous but minutely glandular. Inflorescences appearing as open, leafy panicles, the branches narrowly racemose with more or less loosely spaced, stalked heads. Heads with the central florets perfect and the marginal florets perfect or pistillate, thus all of the florets potentially producing fruits. Involucre 1.0–1.5(–2.0) mm long, the bracts in 2 or 3 overlapping rows, the main body oblong-elliptic, glabrous but minutely glandular, with broad, thin, transparent margins and tip, these glabrous. Receptacle naked, without bristly hairs. Corollas 0.7–1.0 mm long. Fruits 0.7–0.9 mm long, more or less obovoid, faintly lined, tan to grayish brown, shiny. 2n=18. August–November.

Introduced, scattered in the southern half of the state and in the Big Rivers Division (native of Asia, widely naturalized in the Americas and Europe). Bottomland forests, mesic upland forests in ravines, banks of streams and rivers, and margins of lakes; also crop fields, fallow fields, old fields, pastures, gardens, railroads, roadsides, and open, disturbed areas.

For many centuries, ingredients in A. annua L. were used in Chinese traditional medicine to treat fevers and malaria under the name qinghaosu. More recently, Western medicine has come to accept that this species is highly effective against Plasmodium species, the microbial parasites that cause malaria. The main active ingredient is a sesquiterpene lactone known as artemisinin, and synthetic derivatives also have been developed from this compound (Klayman, 1985, 1993). In addition to its medical virtues, artemisinin has shown potential as a natural herbicide (Duke et al., 1987).