Polygonum aviculare subsp. buxiforme L.
Family: Polygonaceae
American knotweed, box knotweed, boxwood knotweed
[Polygonum buxiforme Small]
Polygonum aviculare subsp. buxiforme image
from USDA Plants website  
Polygonum aviculare subsp. buxiforme image
University of Wisconsin-Milwaukee (UWM)  
Etymology: Polygonum: derived from the Greek words polys, "many," and gonu, "knee or joint," hence "many joints" because of the thickened joints on the stem
Plants: sprawling to matting annual forb; stems with many branches
Leaves: alternate, flat, oblong, jointed at the base
Flowers: white to pink edges, 5-parted, petals and petal-like sepals joined at the base and hooded, 3 outer longer than other 2; inflorescence small clusters from the leaf axils
Fruits: dark brown, dull, dry seed
Habitat: marsh edges, dunes; in sandy soil
Conservation Status: Native