Plants perennial, coarse, rhi-zomatous; rhizomes slender, elongate. Stems decumbent or ascending, straggling, diffusely branched, smoothly 4-angled, 20-90 cm, brittle, glabrous. Leaves sessile; blade linear-lanceolate to narrowly lanceolate, widest near base, 1.5-4 cm × 1-6 mm, base round, margins smooth, apex acute, often ciliate near base, otherwise glabrous, not glaucous. Inflorescences terminal, 5-many-flowered, open, conspicuously branched cymes; bracts narrowly lanceolate, 1-5 mm, wholly scarious, margins ciliate, apex acuminate. Pedicels divaricate, 10-30 mm, glabrous. Flowers 5-12 mm diam., rarely larger; sepals 5, distinctly 3-veined, narrowly lanceolate to triangular, 3-7 mm, margins narrow, straight, scarious, apex acute, glabrous; petals 5, 3-7 mm, equaling or longer than sepals; stamens 10, all, some, or none fully developed and fertile; styles 3, ascending, ca. 3 mm. Capsules green or straw colored, narrowly ovoid, 5-7 mm, longer than sepals, apex acute, opening by 3 valves, splitting into 6; carpophore absent. Seeds reddish brown, reniform-rotund, ca. 1 mm diam., rugose in concentric rings. 2n = 39, 52. Flowering late spring-early summer. Rough grasslands, pastures, hayfields, roadsides; 0-1200 m; introduced; St. Pierre and Miquelon; B.C., N.B., Nfld. and Labr., N.S., Ont., P.E.I., Que.; Calif., Colo., Conn., D.C., Idaho, Ill., Ind., Iowa, Kans., Ky., Maine, Mass., Mich., Minn., Mo., Mont., Nev., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., S.C., Tenn., Vt., Va., Wash., W.Va., Wis.; Europe. In Europe, both diploid and tetraploid cytotypes of Stellaria graminea occur with occasional triploid hybrids. Only the tetraploid form has been found in North America, except for a triploid colony in Newfoundland. This species is often confused with S. longifolia but differs in its stems, which are very angular, glabrous, and not scabrid; the narrowly triangular leaves on the flowering stems; the smooth leaf margins; the stiff, triangular, prominently 3-veined sepals; and the larger, rugulose seeds.

The sterile overwintering shoots of Stellaria graminea have broader elliptic to elliptic-lanceolate leaf blades measuring 5-15 × 1.5-4 mm. They are broadest near the middle. This state of the plant has been named var. latifolia Petermann. Usually S. graminea has perfect flowers but occasionally plants that are entirely staminate-sterile are encountered. The flowers in these are partially fertile depending on the occurrence of cross- pollination.