The Collaborative International Dictionary of English v.0.48:

bogbean \bogbean\ n.
   a perennial plant of Europe and America (Menyanthes
   trifoliata) having racemes of white or purplish flowers and
   intensely bitter trifoliate leaves; often rooting at the
   water margin and spreading across the surface; -- called also
   bog myrtle, water shamrock and marsh trefoil.

   Syn: water shamrock, buckbean, bog myrtle, marsh trefoil.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

water shamrock
    n 1: perennial plant of Europe and America having racemes of
         white or purplish flowers and intensely bitter trifoliate
         leaves; often rooting at water margin and spreading across
         the surface [syn: water shamrock, buckbean, bogbean,
         bog myrtle, marsh trefoil, Menyanthes trifoliata]