Plants cespitose, matted. Stems ascending, simple proximally, branched distally, (4.5-)10-40 cm, puberulent; sterile shoots matted, 3-6 cm. Leaves: sheath 0.5-1.5 mm, ca. equaling stem diam.; blade linear to linear-lanceolate (flowering stems) or oblanceolate (sterile stems), 0.6-2.6 cm, green, margins ciliate. Inflorescences open, 2-4-flowered cymes or often flowers solitary; bracts absent; bracteoles 2 (or 4), green, ovate, ca. 1/ 1/ 2 times as long as calyx, herbaceous with ± scarious margins, apex aristate. Pedicels 4-12(-30) mm. Flowers: calyx 25-30-veined, 10-17 mm, glabrous or minutely pubescent, lobes narrowly lanceolate to linear, 3-5 mm; petals light or dark pink to purple (rarely white), often with darker band at base of blade, bearded, 4-9 mm, apex dentate. Capsules 12-13 mm, somewhat shorter than calyx. Seeds 1.1 mm. 2n = 30 (Europe).

Flowering early summer. Roadsides, meadows, shores, open woods, waste places; 40-2700 m; introduced; Alta., B.C., Man., N.B., N.S., Ont., P.E.I., Que., Sask.; Ark., Calif., Colo., Conn., Ill., Iowa, Maine, Mass., Mich., Minn., Mo., Mont., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., Utah, Vt., Va., Wash., Wis., Wyo.; Europe; introduced in e Asia (China, Japan, Russian Far East).

Subspecies deltoides is often cultivated and occasionally escapes, perhaps not persisting in the northern part of its range.