3. Rorippa palustris (L.) Besser var. fernaldiana (Butters & Abbe) Stuckey (marsh yellow cress, bog yellow cress, yellow water cress)

R. islandica (Oeder ex Murray) Borbás var. fernaldiana Butters & Abbe

Pl. 325 d–f; Map 1383

Plants annual or biennial, with taproots. Stems (10–)35–120(–140) cm long, erect or ascending, not rooting at the nodes or rooting sporadically only at the lowermost few nodes, much-branched in the upper half, unbranched or few-branched from the base, glabrous or the lower portion sparsely hairy. Leaves basal and alternate, 2–22(–30) cm long, the basal and lowermost stem leaves sessile to short-petiolate, the base usually somewhat clasping the stem, with small, rounded auricles, the blades simple to pinnately divided or compound with 3–17 divisions or leaflets, the lobes linear to oblong or irregularly ovate, the margins sharply and irregularly toothed, glabrous or the lower leaves sparsely hairy on the petioles and larger veins. Sepals 1.5–3.0 mm long. Petals 1.0–3.5 mm long, yellow. Styles absent or 0.5–1.0 mm long. Fruits 2.5–9.0(–14.0) mm long, 1.0–2.5 mm wide, ovoid or oblong, straight or slightly arched upward, usually slightly constricted at about the midpoint, the surface smooth, at least some of the stalks 3–7 mm long. Seeds mostly 20–80 per fruit, in 2 rows in each locule, 0.5–1.0 mm long, circular in outline, usually with a small notch at the base, the surface slightly uneven or finely bumpy, brown or reddish brown. 2n=32. May–October.

Scattered to common throughout Missouri (eastern U.S. and Canada west to Texas, Colorado, and Montana; possibly introduced in Mexico, Central America, the West Indies, and the Old World). Openings of bottomland forests, banks of streams and rivers, sloughs, and margins of ponds and lakes; also levees, wet pastures, fallow fields, railroads, roadsides, and moist, open, disturbed areas.

This species was known as R. islandica (Oeder ex Murray) Borbás in much of the North American floristic literature until Stuckey (1972), following Jonsell (1968), indicated that the epithet islandica should be restricted to a different species that is confined to Europe and Greenland. He further noted that plants identified as var. hispida (Desv.) Jonsell from Missouri were misdetermined, and that the only variety of R. palustris present in the state is var. fernaldiana.

This is the commonest species of Rorippa in Missouri. The erect habit and relatively long fruit stalks are good characters to distinguish it from other annual species in the state. Overall, this morphologically variable species is circumboreal in its native distribution, but it has become naturalized nearly worldwide. Stuckey’s complex infraspecific classification involved recognition of four subspecies with eleven total varieties. The var. fernaldiana was classified as part of ssp. glabra (O.E. Schulz) Stuckey. Stuckey’s subspecies and varieties are distinguished by a number of minor, doubtfully useful features ranging from pubescence types to plant size and color, and fruit length and shape. They are probably oversplit, based on the large variation found in many of the critical characters (Al-Shehbaz, 1988).

For a discussion of putative hybrids between this species and R. sinuata, see the treatment of that species.