This species and Vicia americana are often confused but are easily separated by the fact that Vicia americana usually has 8-14 leaflets while this species has 4-8. Besides the generic distinction between the styles, the basal section of the stipules of Vicia americana is generally sharply toothed while, if the stipules of this species are not entire or nearly so, they are never sharply toothed. [Deam delineates three varieties of L. palustris.] The varieties intergrade so completely that it is not possible to separate them satisfactorily. [The typical variety has winged stems, flowers 1.5-2 cm long, and wide leaflets (7-23 mm). This form is] infrequent in the lake area in swamps and marshes, on the low borders of lakes, in springy places along streams, and in a drier habitat in prairies. It has been reported from several places south of the lake area and, without doubt, it was formerly found in prairie habitats and springy places along streams and on the borders of ponds and swamps. [Variety linearifolius has stem and flower of the typical variety but narrower leaflets, mostly 3-9 mm wide. It] has the habitat of the [full] species but is much less frequent. [Variety myrtifolius is wingless and has smaller flowers, 1-1.5 cm long. Its leaflets are 6-20 mm wide. Deam reports this variety as] infrequent to very rare, mostly in the lake area in habitats similar to those of the species. There are several reports from the dune area of Lake Michigan. My Posey County specimen is not typical and may be a southern representative of the species.
Slender rhizomatous climbing perennial, to 1 m, the stem often winged; stipules semisagittate, often serrate; lfls 4-8(10), linear to elliptic, 2-8 cm נ3-20 mm; peduncles about equaling the subtending lf, with a raceme of 2-6(-9) red-purple (whitish) fls 12-20 mm; cal distinctly oblique, the lower lobes much longer than the upper; 2n=42. Wet meadows, swamps, shores, and wet woods; circumboreal, in Amer. s. to N.J., Pa., O., Ind., Mo., Colo., and Calif. June, July. Var. palustris, with winged stems and usually 6-8 lfls, occurs throughout the range of the sp. Typical var. palustris is glabrous or nearly so; a pubescent form, called var. pilosus (Cham.) Ledeb., has the same distribution but is more common along the seacoast; it is only doubtfully significant. Forms with narrow and broad lfls occur in both typical var. palustris and the hairy phase. The well marked but still not wholly distinct var. myrtifolius (Muhl.) A. Gray, consistently glabrous, with wingless (but sometimes acutely ridged) stems and usually 4(6) lfls that tend to be shorter (2-4 cm), broader, and more rounded than in var. palustris, occurs chiefly from N.Y. to Wis. and Ill., especially about the Great Lakes.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.