Green smartweed
Polygonum scabrum

Introduced from Europe but almost identical to the smaller plants of Pale smartweed with ocrea and undersurface of upper leaves as in; distinguished from Pale smartweed by having its flowers in shorter and plumper pale green to greenish-white spikes (1-5cm long by 0.8-1.5cm wide; 2/5-2in. by 1/3-2/3in.), its lateral spikes unstalked (no bare stem) in the axils of leaves or on stalks mostly less than 1 cm (2/5in.) long, its sepals only as long as or slightly shorter than the "seed," and its flattened "seed" usually 2mm (1/12in.) in diameter or larger.