Annual wetland herb, 2–15 (–20) cm high. Leaves ± terete, decumbent to erect, 1–16 cm long, 0.4–0.75 mm diam.; basal sheath with fine acute auricles 0.6–2.1 mm long. Scape at fruiting erect or ascending, 0.9–14 cm long, 0.25–1.2 mm diam. Infructescence an open raceme, 0.7–3.3 (–6) cm long, 5.3–17 mm diam.; pedicels (0.2–) 0.8–9.1 mm long. Fruits (1–) 4–23 per infructescence, semi-erect, with body narrowly ovoid to ovoid but distally more abruptly narrowed, widest below middle, expanded at base into widespreading spines and usually also with downward spines or points, 3.5–7.5 mm long (excluding spines), 1.6–2.3 mm diam. at greatest body width. Carpels 6, 3 fertile alternating with 3 sterile, united; mature fertile carpels with dorsal surface strongly rounded in T.S., sometimes with a shallow longitudinal groove and/or a low narrow ridge, readily separating; basal spines 4 per carpel; 2 spines long, fine, directed ±laterally, upcurved to inrolled, (0.25–) 1.5–3.7 mm long; 2 spines down-pointed, connected along basal portions or along whole length by a thin membrane, 0.5–1.3 mm long, alternatively these spines and membrane near-absent to partially developed; central axis of 3 narrow wings, almost obscured by much wider fertile carpels.
In sand to sandy clay damp soils or shallow water of lake and watercourse edges or ephemerally flooded depressions, in various habitats, also limestone coastal sands and damp shallow soils of rock outcrops; chiefly freshwater situations, sometimes semi-saline.
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