Flower:

Branching clusters at the top of the stem, the branches erect to ascending with up to 30 flowers, the clusters usually compact. The lowest branch is subtended by an erect, leaf-like bract that may or may not rise above the uppermost branch. Flowers are single, not in heads of 2 or more, short-stalked to nearly stalkless, with 6 tepals (petals and similar sepals) in 2 layers, 3.5 to 4.4 mm long, the inner and outer tepals about equal in size and shape, pointed at the tip and green to light brown with white papery edging. Flowers have a 3-parted style and 6 stamens, the anthers (tips) shorter than the filament (stalk).

At the base of a flower is a pair of scale-like bracts (bracteoles) that usually have a bristle tip.

Leaves and stems:

A flowering stem has 1 or 2 alternate leaves near the base of the stem. Leaves are 2 to 6 inches long, up to 1.1mm wide, half as long as the flowering stem or less, more or less flat in cross-section though sometimes the edges are rolled in (involute). The sheath is open at the front. At the tip of the sheath is a pair of rounded lobes (auricles) .2 to .4 mm long, thin and papery, whitish to purplish and drying to brown. Stems are round in cross-section, smooth, erect, unbranched, multiple from the base and creating clumps from short rhizomes.

Fruit:

The tepals persist and become brown and appressed to erect in fruit. Fruit is an elliptic to nearly round, single-chambered capsule 3.8 to 4.7 mm long, brown when mature, as long as or shorter than the tepals. Inside the capsule are numerous tiny seeds, elliptic to crescent-shaped, .4 to .7mm long, amber colored to brown when mature with a white stub at the tip but no elongated tails.

Notes:

Nationally, Inland Rush is found in a wide range of habitats, wet or dry, from conifer forests to marshes to upland prairies. In Minnesota, it is most often in or near rock outcrops, rocky shores and in prairie swales, with the densest concentration in the Minnesota River Valley.

It is similar in most respects to Dudley's Rush (Juncus dudleyi), which has a thickened auricle and tepals that are larger (4 to 5mm long) and more spreading in fruit, where Juncus interior has a thin, papery auricle, the tepals are mostly erect and 3.3 to 4.4mm long. Also similar is Path Rush (Juncus tenuis), which has long, narrowly triangular auricles and is generally a smaller plant. Neither J. dudleyi nor J. tenuis have the bristle-tip on their floral bracts (bracteoles) that is characteristic of J. interior, though these may break off especially when dry. Of note is that many references state the primary bract at the base of the cluster is usually shorter than the inflorescence, but in our own observations it's more often been longer.

Somewhat similar are Greene's Rush (Juncus greenei) and Vasey's Rush (Juncus vaseyi) both of which have capsules distinctly longer than the tepals and leaves that are nearly round in cross-section.