Annual sedge

C. compressus is an erect glabrous, caespitose annual (sometimes biannual), 5-75 cm high. Its roots are tufted, fine and numerous. The stems are erect, tufted, slender or rigidulous, glabrous, compressed, trigonous and 0.5-2.0 mm thick. The base is covered with red-purple, usually entire and non-fibrous, loose, open leaf sheaths. The leaves are as long as or shorter than the stem. They are subcoriaceous, flat, 1.5-4.0 mm, broad, greyish-green, narrowly linear and taper gradually to a fine acuminate apex.
The inflorescence has umbellate spikes, some of the rays (usually 3-4) are well developed and up to 8 cm long. Occasionally, all the spikelets are grouped as a sessile umbel. There are usually three or four leaf-like, unequal brackets which are longer than or as long as the primary rays of the umbel. The spikelets are strongly compressed laterally and there are 4-7 in each ultimate condensed umbellate spike. These are 1.5-3.5 cm long, 3-5 mm wide and grey-green, streaked with crimson. The rachis or rachilla are very much flattened and with hyaline wings (in fresh specimens). The two or three lowest glumes are small, hyaline and empty. The flowering glumes are broadly ovate when spread out, tightly imbricating, 3.0-4.5 mm long, mucronate, pale yellowish or brownish, several-nerved, broadly scariously margined, mucro-stout and sometimes excurved. There are three stamens with anthers 0.7 mm long which have an ovate-rotundate, reddish crest. The style is long with branches 3-4 mm long, divided about halfway down. The nuts are shortly stipitate, obovioid, broadly triquetrous, 1.5-1.7 mm long with three prominent angles and three concave sides. They are dark brown or blackish-brown, apiculate and glossy.