Sisymbrium amphibium Linnaeus var. palustre Linnaeus, Sp. Pl. 2: 657. 1753; Cardamine palustre (Linnaeus) Kuntze; Nasturtium densiflorum Turczaninow; N. palustre (Linnaeus) de Candolle; N. palustre f. longipes Franchet; N. palustre f. stoloniferum Franchet.
Herbs annual or rarely short-lived perennial, (5-)10-100(-140) cm tall, glabrous, rarely hirsute. Stems erect, simple or often branched above, ribbed. Basal leaves rosulate, withered early; leaf blade lyrate-pinnatisect, (4-)6-20(-30) × 1-5(-8) cm. Cauline leaves petiolate or subsessile, auriculate or amplexicaul, lyrate-pinnatisect, (1.5-)2.5-8(-12) × (0.5-)0.8-2.5(-3) cm; lateral lobes oblong or ovate, smaller than terminal lobe, (1 or)2-6(or 7) on each side of midvein, sometimes absent, margin subentire or irregularly dentate, sinuate, serrate, or crenate. Racemes ebracteate. Fruiting pedicels divaricate or slightly to strongly reflexed, (2.5-)3-8(-12) mm, slender, straight or curved. Sepals oblong, 1.5-2.4(-2.6) × 0.5-0.8 mm. Petals yellow or pale yellow, spatulate, (1.5-)1.8-2.6(-3) × 0.5-1.3(-1.5) mm. Filaments 1-2.5 mm; anthers ovate, 0.3-0.5 mm. Ovules 20-90 per ovary. Fruit oblong, ellipsoid, or oblong-ovoid, often slightly curved, (2.5-)4-10(-14) × (1.5-)1.7-3(-3.5) mm; valves not veined; style 0.2-1(-1.2) mm. Seeds brown to yellowish brown, ovoid or subglobose, colliculate, biseriate, 0.5-0.9 × 0.4-0.6 mm. Fl. and fr. Mar-Oct. 2n = 32.
Marshlands, pastures, meadows, roadsides, shores of lakes and ponds, streamsides, thickets, grasslands; near sea level to 4000 m. Anhui, Gansu, Guangxi, Guizhou, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jilin, Liaoning, Nei Mongol, Ningxia, Qinghai, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xinjiang, Xizang, Yunnan [Afghanistan, Bhutan, India, Japan, Kazakhstan, Korea, Mongolia, Ne-pal, Pakistan, Russia, Sikkim, Tajikistan, Turkmenistan, Uzbekistan; Europe, North America; introduced in Australia, South America, and elsewhere].
All records of Rorippa islandica (Oeder ex Murray) Borbás from China are based on misidentified plants of R. palustris. Plants of the former are prostrate diploids (2n = 16) restricted to Europe and W Russia; they have nonauriculate cauline leaves, sepals and petals 1-1.5 mm, and fruit often 2-3(-5) × longer than the pedicels. By contrast, plants of R. palustris are widespread, erect tetraploids (2n = 32) with auriculate cauline leaves, sepals and petals 1.5-2.6 mm, and fruit shorter than to 2 × longer than the pedicels.
Rorippa palustris is highly variable and has been divided into as many as four subspecies and seven varieties, all of which are indigenous to North America; only subsp. palustris is also indigenous in Eurasia.