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.