Nasturtium uniseriatum H. W. Howard & Manton; Rorippa microphylla (Boenninghausen ex Reichenbach) Hylander ex Á. Löve & D. Löve

Plants glabrous throughout or sparsely pubescent. Stems (1-)2-10(-18) dm. Cauline leaves: petiole not winged, base auriculate; blade (3-)5-9(-11)-foliolate, (1.5-)2.5-10 (-15) cm; lateral leaflets sessile or petiolulate, rachis not winged, blade smaller than terminal; terminal leaflet (or simple blade) suborbicular or oblong, (0.6-) 1.5-3.5(-4.5) cm × (4-)8-20(-25) mm, base obtuse, cuneate, or subcordate, margins entire or repand, apex obtuse. Fruiting pedicels divaricate or descending, straight or, occasionally, recurved, (7-)9-21(-25) mm. Flowers: sepals 2.5-4 × 1-1.5 mm; petals white or pink, spatulate or obovate, 4.5-6 × 1.5-3 mm, (base to 1 mm), apex rounded; filaments 2.5-3.5 mm; anthers 0.7-1 mm. Fruits 1.4-2.2(-2.7) cm × 1-1.5(-1.8) mm; ovules 30-38(-40) per ovary; style 0.5-1.5(-2) mm. Seeds uniseriate, reddish brown, ovoid, (0.8-)1-1.2 × (0.6-)0.7-0.8(-0.9) mm, moderately reticulate with (75-)100-150(-175) areolae on each side. 2n = 64.

Flowering Jan-Aug. Lake margins, streams, ponds, springs, river shores, seeps, swales, wet meadows; 0-1500 m; introduced; B.C., Man., N.B., Nfld. and Labr. (Nfld.), Ont., P.E.I., Que.; Conn., Idaho, Ind., Ky., Maine, Mass., Mich., N.H., N.Mex., N.Y., Oreg., Pa., R.I., Vt., Wyo.; Europe; introduced also elsewhere in the New World, Australia.