Sometimes species within the same genus look very similar and are distinguished only by minor differences, while other species within a genus can, on casual observation, appear so different that they do not seem to be closely related. The moth mullein (Verbascum blattaria) and the wooly mullein (Verbascum thapsus) hardly resemble each other yet belong to the same genus. In contrast, common knotweed (Polygonum aviculare) and silversheath knotweed (Polygonum argyrocoleon) have flowers and leaves that resemble each other. The most immediate difference noted between these two plants is their shape: common knotweed forms a low mat while silversheath knotweed has erect branches.
Both common and silversheath knotweeds are annuals that belong to the buckwheat family. These two introduced species have become widespread throughout the United States and can be found in disturbed and waste places such as roadsides and barren fields. A problem when they infest newly planted alfalfa fields, lawns or cultivated annual crops, silversheath and common knotweed are classified as noxious weeds.
Common knotweed has a strong taproot with lateral fibrous roots. The stems are wiry, slender, extensively branched and swollen at the joints with a zig-zag appearance. The hairless, alternate leaves are blueish green and have a short stalk. Each leaf is encircled by a papery membrane at its base. The very small flowers are borne in clusters of one to five flowers in the axis of the leaf stalks. Pink or white, the flowers have five petals and five sepals. The petals are fused together into a corolla tube.
Other colloquial names for common knotweed include knotgrass, wiregrass and prostrate knotweed. Anyone whose manicured lawn has been overrun with common knotweed probably has other more colorful names for this invader.
These common knotweeds plants were (note the tense) growing in one of our fields (near Lookout CA).
Pictures of silversheath knotweed and more information on these two species will follow in my next post.