The Collaborative International Dictionary of English v.0.48:

Polygonum \Po*lyg"o*num\, n. [NL., fr. Gr. ? a kind of plant;
   poly`s many + go`ny the knee, a joint of a plant. So called
   in allusion to the numerous joints.] (Bot.)
   A genus of plants embracing a large number of species,
   including bistort, knotweed, smartweed, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Polygonum
    n 1: diverse genus of herbs or woody subshrubs of north
         temperate regions [syn: Polygonum, genus Polygonum]