Weak Rush is a tufted, grass-like perennial. Its stems are erect, round, smooth, and only 10 to 25 cm tall and 1-2mm wide. There are 1-3 dark green to purplish leaves along the stem, from 1 to 12.5 cm long and .5 to 1.5 mm wide. The flowers are borne in 3 to 35 heads on widely-spreading branches, each head with from 2-10 (20) flowers. As with all rushes, each flower has 6 tepals (reduced petal-like structures); in J. debilis these are green to straw-colored, up to 2.3 mm long, and have pointed tips. The fruit are capsules 2.8 to 3.7 mm long, exceeding the length of the perianth (tepals).

This species is best identifed when mature fruit are present.

Juncus acuminatus and J. brachycarpus each have tepals which are much longer than the fruit. J. subcaudatus has seeds .7 to 2.6 mm long, each with a tail and a white translucent veil, while those of J. debilis are only .3 to .4 mm long and lack tails or veils (FNA 2000).