Juncus effusus is a perennial herb with stout rhizomes covered in overlapping scales. The stems are crowded along the rhizome, reddish-brown to purple at the base, striate, terete (cylindrical), and 40–80 cm long, with a solid pith. The leaves are simple and basal but scale-like and bladeless, with entire margins. The sheaths at the base are dark reddish-brown to purple, becoming paler toward the apex. Stipules are absent. The flowers are numerous, arranged in usually contracted or sometimes more open cymose inflorescences, up to 8 cm long, appearing lateral. The lowest bract is terete and appears to continue the stem. There are three bracteoles, which are colorless and about ¼ as long as the perianth. The flowers are bisexual (perfect). The perianth parts are pale, rigid, and narrow, not obscuring the capsule. The outer perianth parts are about 2.5 mm long, and the inner parts are about 2 mm long, roughly as long as the capsule, with a long acuminate apex. There are three stamens, with the anthers and filaments about equal length. Pollen grains remain in tetrads, surrounded by a common exine. The ovary is superior, with one carpel. The fruit is a pale to medium brown, shiny capsule, contrasting with the pale perianth. It is 3-celled, ovoid, and has a truncate apex. The seeds are pale brown, about 0.5 mm long, and approximately three times longer than they are wide. It is cultivated and naturalized along the margins of ponds and streams, in open boggy sites, and in wet areas along trails between 1,000–2,000 meters.