Bushy Knotweed - Polygonum ramosissimum
[including] Polygonum leptocarpum
Stems erect, branched, 8–70 cm high. Leaves short-petiolate, often moldy beneath, linear to narrowly lanceolate, 1–3 cm long,; stipules 5–14 mm long, lacerate, brownish. Flowers 2–4 mm long, 1 to 3 in leaf axils; pedicels 1–6 mm long, erect; tepals green with yellow or white to pink margins. Achene dark brown, 3-sided (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX)
Our plants are subspecies ramosissimum
. The Great Plains Flora includes P. leptocarpum in its treatment.