water shamrock

from 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}]

from 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]