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Aquatic and Wetland Vascular Plants of the Northern Great Plains

22. Brassicaceae, the Mustard Family

4. Rorippa Scop. -- Yellow cress

4. Rorippa palustris (L.) Bess. -- Bog yellow cress

Annual, biennial or shortlived perennial; stems erect, rarely prostrate or decumbent, usually single from the base, simple or branched upward, 3-12 dm long, glabrous or sparingly to densely hirsute below, sparingly hirsute to glabrous above. Leaves short-petioled to sessile, auriculate and clasping to nonauriculate and nonclasping, the blades oblong to oblanceolate, 4-20(30) cm long, 1-5(8) cm wide, glabrous or sparingly to densely hirsute, narrowly to broadly acute at the apex, the margin irregularly serrate, incised, deeply cleft, repand or variously pinnate-divided. Racemes terminal and axillary, the terminal one developing earliest, the oldest siliques on the lower portion of the terminal raceme. Sepals greenish, 1.2-2.5 mm long, caducous; petals yellow, drying whitish, 0.8-2.5 mm long, shorter than or equal to the sepals. Siliques globose to oblong or cylindrical, straight or slightly curved upward, 2-2.8 mm long, 1.2-3.4 mm wide, mostly 1-4X longer than wide, not at all to slightly tapering to the apex, truncate, obtuse or acute at the apex, the valves glabrous, the style 0.2-1.2 mm long; pedicels ascending, divergent or slightly to strongly recurved, 3-10 mm long. Jun--Sep. Marshes, wet meadows, shores, stream banks, ditches and other wet places; common; (Labr. to AK, s to n S.Amer.). R. islandica (Oeder) Bourbas.

Stuckey (op. cit.) recognizes several subspecies within this polymorphic species of which two occur in our region. R. palustris subsp. glabra (Schulz) Stuckey is characterized by leaves glabrous to sparingly hirsute on the upper surface, glabrous below, and stems glabrous to sparingly hirsute near the base, glabrous upward. This is the most common form in our area. R. palustris subsp. hispida (Desv.) Jonsell is somewhat less common, with leaves hirsute on the lower surface and stems hirsute usually up to the terminal raceme.

Stuckey has further divided these subspecies into varieties mainly on the basis of fruit characters. In our area there are two varieties of subsp. glabra. In R. palustris subsp. glabra var. fernaldiana (Butt. & Abbe) Stuckey, the style is expanded in fruit and the silique is constricted at the middle so that the replum has concave margins and becomes twisted with age and upon drying. Our other variety is R. palustris subsp. glabra var. glabrata (Lunell) Stuckey, which differs from var. fernaldiana in that the style is unexpanded in fruit and the silique is not constricted at the middle, thus the replum has straight margins and remains flat with age and upon drying. There often seems to be little if any correlation between the condition of the stigma in fruit and the silique/replum shapes.

Both varieties of R. palustris subsp. hispida occur in our range. R. palustris subsp. hispida var. hispida is the common phase, with siliques globose to subglobose, mostly 2.5-6 mm long, 1.3-3 mm wide, ca. 1-2X longer than wide; replum circular to elliptic in outline; petals 1-1.5(2) mm long; pedicels usually divergent to ascending; stem usually densely hirsute below. Of much lesser occurrence is R. palustris subsp. hispida var. elongata Stuckey. This differs from var. hispida in having siliques cylindrical, mostly 5-8 mm long, 2.1-3.1 mm wide, ca. 2X or more longer than wide; replum oblong in outline; petals 1.5-2(2.8) mm long; pedicels usually divergent to recurved; stem usually sparingly hirsute below.

GIF- Species Photo/Drawing
Rorippa palustris. GIF- Distribution Map
Map key

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Reston, VA [vaww54]