6. Rorippa sylvestris (L.) Besser (creeping yellow cress)

Pl. 326 e, f; Map 1386

Plants perennial herbs, with rhizomes, glabrous or the stem bases sparsely hairy. Stems (5–)20–80(–100) cm long, erect or ascending, rarely rooting at the lowermost nodes. Leaves basal and alternate, (2.0–)3.5–15.0(–20.0) cm long, petiolate, the bases not clasping the stem, pinnately lobed or divided, with 5–13 lobes or divisions, these linear to oblong or irregularly obovate to ovate, the margins with several sharp teeth. Sepals 2–4 mm long. Petals (3–)4–6(–8) mm long, yellow. Styles 0.5–1.0 mm long, the stigma capitate and wider than the style. Fruits 10–20(–25) mm long, 1–2 mm wide, linear, often slightly arched upward. Seeds produced uncommonly, mostly 25–80 per fruit, in 2 rows in each locule, 0.6–0.7 mm long, oblong in outline, the surface faintly and finely bumpy, reddish brown. 2n=32, 40, 48. May–September, rarely also January.

Introduced, scattered in southwestern Missouri and along the floodplains of the Mississippi and Missouri Rivers (native of Europe, Asia; introduced widely in eastern and northwestern U.S., Canada). Bottomland forests and sloughs; also edges of crop fields, pastures, ditches, railroads, roadsides, and open, disturbed areas.

The lack of seed set in many populations of R. sylvestris may be attributed to the self-incompatibility of the plants and the relatively clonal nature of many populations. The plants are easily spread by rhizome fragments. Crosses between different clones frequently produce copious seeds (Mulligan and Munro, 1984). Mulligan and Munro also discovered a naturally occurring hybrid between this species and R. palustris in Quebec.