Stems diffusely branched, trailing or later ascending; lower lvs often 5-foliolate, the upper 3-4-foliolate; lfls cuneate-obovate, 1-2 cm, with 5-9 sharp, antrorse teeth near the tip; fls yellow, 12-18 mm wide, solitary on slender axillary pedicels, many of or all them 4-merous; 2n=56. Native of Europe, intr. locally in Nf., N.S., Pa., and on the Pacific coast. May-Sept. (P. procumbens)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.