This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Polygonum Muhlengergii, Wats. (Polygonum emersum, Britton)
Native. Perennial. Propagates by seeds and by rootstocks.
Time of bloom: July to September.
Seed-time: August to October.
Range: Ontario to British Columbia, southward to Virginia,
Louisiana, and Texas. Habitat: Muddy or dry soil; fields, low meadows, swamps.
Although this weed seems to prefer moist soil, it will grow anywhere, even on mountains, varying its form to fit its surroundings. When established in field or meadow its creeping, horizontal root-stocks make it difficult to suppress, as by ordinary cultivation they are likely to be broken and further distributed.
Stems one to three feet long, decumbent or sometimes erect, usually simple, rough with short, appressed hairs, or sometimes smooth and slightly grooved, swollen at the joints. Leaves variable, those in wet places usually smooth, rather broad, and heart-shaped at base; in dry situations narrow, lance-shaped, pointed at base, and covered on both sides with appressed hairs; loose sheathing stipules nearly as long as the petioles. Flowers dark rosecolor, their peduncles rough-hairy and often glandular, growing in dense spikes, one to four inches long; calyx five-lobed; stamens five, exserted; style two-cleft to about half its length, exserted. Achenes lens-shaped, black, and shining.
Cutting and many times cutting, close to the ground, for the purpose of depriving the rootstocks of all food assimilated by the leaves and preventing seed production. Small areas should be grubbed out.