Clustered sedge is a graminoid, 40-80cm tall, with 5-9 sessile spikes, each 7-10mm long, aggregated into such a dense cluster that spikelets are nearly at right angles to the rachis. The staminate portion is 2mm or less. The perigynia are flattened, appressed, greenish to dark brown, conspicuously 3-7-veined abaxially, inconspicuously veined adaxially, obovate, broadest at about the middle, and concavo-convex, achenes are lenticular and elliptic. It has 2-4 blades per fertile culm, 3-5mm wide with sheaths adaxially, green-veined nearly to collar, with a white-hyaline band or sharp Y-shaped hyaline region at collar. Basal sheaths are somewhat fibrous, adaxially firm, summits truncate, prolonged 0.3mm beyond collar, and abaxially finely papillose, distal ligules are 2-7mm. Pistillate scales are white-hyaline or brown, with a green or white midstripe, extend to tip, are broadly lanceolate, and shorter and narrower than perigynia, the apex is firm, obtuse, or acute.
It can be distinguished from C merritt-fernaldii, by its adaxially green-veined sheaths, obovate perigynia, and usually broader (3-5mm) leaves.