Sisymbrium nasturtium-aquaticum Linnaeus, Sp. Pl. 2: 657. 1753; Rorippa nasturtium-aquaticum (Linnaeus) Hayek.

Herbs perennial, rhizomatous, aquatic, 10-70(-200) cm tall, glabrous throughout or sparsely pubescent with simple trichomes. Stems decumbent or prostrate, erect in emergent plants, branched above, rooting at proximal nodes. Leaves all cauline, pinnately compound, 3-9(-13)-foliolate; petiole auriculate at base; terminal leaflet suborbicular or oblong, 1-4 cm, base obtuse, cuneate, or subcordate, margin entire or repand, apex obtuse; lateral leaflets smaller, usually sessile. Fruiting pedicels slender, divaricate or recurved, 5-12(-20) mm. Sepals oblong, 2-3.5 mm, lateral pair slightly saccate. Petals white or pink, spatulate or obovate, 2.8-4.5(-6) × 1.5-25 mm, apex rounded; claw ca. 1 mm. Filaments white, 2-3.5 mm; anthers oblong, ca. 0.6 mm. Fruit cylindric, 1-1.5(-2) cm × (1.8-)2-2.5(-3) mm; valves with an obscure midvein; style 0.5-1(-1.5) mm. Seeds biseriate, ovoid, 1-1.3 × 0.7-1 mm, reddish brown, coarsely reticulate, with 25-50(-60) areolae on each side. Fl. Apr-Sep, fr. May-Sep. 2n = 32*.

Cultivated as a vegetable and used medicinally.

Streams, ditches, lakes, swamps, marshes; near sea level to 3700 m. Anhui, Guangdong, Guangxi, Guizhou, Hebei, Heilongjiang, Henan, Hubei, Jiangsu, Jiangxi, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xinjiang, Xizang, Yunnan [native to SW Asia and Europe; widely naturalized elsewhere].