Plants perennial, often mat-forming, glabrous. Stems ascending or, more frequently, procumbent, rooting at nodes, giving rise to secondary tufts or rosettes, few- to many-branched, slender. Leaves: axillary fascicles often present on procumbent stems; basal frequently in primary rosettes in younger plants; blade linear, 8–17 mm, herbaceous, apex apiculate to somewhat aristate, glabrous; cauline not conspicuously connate basally, never forming an inflated cup, blade linear, 4–15 mm proximally, becoming shorter toward apex, 2.5–6 mm distally, sometimes slightly fleshy, apex apiculate to aristate, rarely with minute glandular cilia. Pedicels frequently recurved during capsule development, filiform, glabrous. Flowers axillary or terminal, 4-merous, occasionally 4- and 5-merous; calyx base glabrous; sepals elliptic to orbiculate, 1.5–(–2.5) mm, hyaline margins white, never purple tinged, apex obtuse to rounded, appressed during capsular development, divergent following dehiscence; petals (1–)4(–5), orbiculate to elliptic, 0.8–1(–1.5) mm, shorter than or equaling sepals, or sometimes absent; stamens 4 (8). Capsules (1.5–)2–2.5(–3) mm, slightly exceeding sepals, dehiscing to base. Seeds brown, obliquely triangular with distinct abaxial groove, (0.3–)0.4(–0.5) mm, smooth to pebbled. 2n = 22.
Phenology: Flowering late spring–early fall.
Habitat: Weedy, wet or damp, gravelly or sandy soils along roadsides, sidewalk cracks, margins of paths or lawns, pond and lake margins, coastal rocks and sands, sea cliffs
Elevation: 0-3500 m
Introduced; Greenland, St. Pierre and Miquelon, B.C., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Alaska, Ark., Calif., Colo., Conn., Del., Idaho, Ill., Iowa, Maine, Md., Mass., Mich., Minn., Mont., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., Utah, Vt., Wash., W.Va., Wis., Europe, introduced in Mexico (Chiapas, México), Central America (Costa Rica, Guatemala), s South America (Bolivia, s Argentina), Asia (w Siberia), Antarctica (sub-Antarctic Islands).