The Collaborative International Dictionary of English v.0.48:

Polygonaceous \Pol`y*go*na"ceous\, a. [See Polygonum.] (Bot.)
   Of or pertaining to a natural order of apetalous plants
   (Polygonace[ae]), of which the knotweeds (species of
   Polygonum) are the type, and which includes also the docks
   (Rumex), the buckwheat, rhubarb, sea grape (Coccoloba),
   and several other genera.
   [1913 Webster]