5. Lepidium didymum L. (wart cress, swine cress)

Coronopus didymus (L.) Sm.

Carara didyma (L.) Britton

Pl. 318 h, i; Map 1364

Plants annual or rarely biennial, terrestrial, sparsely to moderately pubescent with short, straight hairs, less commonly glabrous, usually with a somewhat fetid odor when bruised or crushed. Stems 10–50(–70) cm long, spreading to ascending or erect, branched, sometimes forming mats. Leaves alternate and basal, (1.5–)3.0–10.0 cm long, short-petiolate, not clasping, the blades oblong to ovate in outline, deeply 1 time pinnately divided, the divisions often toothed or pinnately lobed. Inflorescences short, dense racemes, terminal and lateral in the leaf axils (or sometimes opposite the leaves). Sepals 0.5–1.0 mm long, spreading, elliptic to oval. Petals 0.4–0.5 mm long, inconspicuous, not lobed, white. Stamens 2. Styles absent. Fruits spreading, 1.5–2.0 mm long, about as long as wide or wider, with 2 lateral, kidney-shaped or oblong to circular and slightly flattened lobes, notched at the tip and base, the surfaces wrinkled or with a network of ridges and pits, the 2 lobes breaking apart at maturity and shed intact, but not releasing the seeds. Seeds 1 per locule (lobe), 1.0–1.2 mm long, kidney-shaped, appearing coiled, the surface with an indistinct, fine, netlike or honeycomb-like pattern of ridges and pits, light yellow or light brown. 2n=32. April–October.

Introduced, uncommon, known thus far only from the city of St. Louis (probably native to South America, but introduced nearly worldwide). Sidewalk cracks, lawns, railroads, roadsides, and open, disturbed areas.

The native range of this species is not well understood. Al-Shehbaz (1986) noted that although some botanists consider it a Eurasian native, its closest relatives grow in South America. This worldwide weed is sometimes eaten as a salad green or used as a wound disinfectant. When eaten by cattle, the fetid-smelling plants impart an unpleasant flavor to the milk.