Flower:

3 to 15 flower heads in a loose to tight cluster at the top of the stem, the short branches erect to ascending, the lowest branch subtended by an erect, leaf-like bract that usually overtops the cluster, often significantly so. Each flower head is hemispheric to round, 6 to 12 mm in diameter, with 5 to 30 flowers.

Flowers have 6 tepals (petals and similar sepals) in 2 layers, the outer tepals 2.4 to 4.1 mm long, the inner about equal in length, all sharply pointed at the tip and green to light brown to reddish or orange-brown. Flowers have a 3-parted style and 3 or 6 stamens, the anthers (tips) half to nearly as long as the filament (stalk).

Leaves and stems:

A flowering stem has 2 to 4 alternate leaves and a single basal leaf. Leaves are 2½ to 12 inches long, up to 1.2mm wide, round in cross-section, with cross partitions at regular intervals (may be easier felt than seen). The sheath is open at the front. At the tip of the sheath is a pair of lobes (auricles) .5 to 1.7 mm long, membranous with a rounded tip. Stems are smooth, erect, and unbranched. Plants form colonies from creeping rhizomes.

Fruit:

Fruit is a 3-sectioned capsule 3.2 to 5 mm long, as long as or slightly longer than the tepals, the tip with a long taper to a pointed tip, maturing to chestnut brown. Inside are numerous elliptic to egg-shaped seeds with an abrupt narrowing to a blunt tip at each end. Seeds are about .5 mm long, yellowish to brown with a textured surface.

Notes:

Knotted Rush is a very common wetland species in Minnesota. The rounded heads can resemble 2 other Minnesota rushes: Torrey's Rush (Juncus torreyi) and Canada Rush (Juncus canadensis), both of which are typically larger, more robust plants with more densely flowered heads. J. torreyi flower heads are also rather larger, up to 15mm in diameter, with up to 100 flowers per head. J. canadensis has 3 to 50 heads in the cluster and 5 to 50 flowers per head, the inner tepals are slightly longer than the outer, and its sheath auricles have a papery band around the edge.