Plants annual, perennial, or of indefinite duration; monoecious or dioecious, inflorescences unisexual and dimorphic; stoloniferous and mat-forming. Culms 1-15(30) cm, not woody. Leaves not clustered, not strongly distichous; sheaths open, keeled; ligules membranous, not ciliate; blades flat. Inflorescences terminal, with spikes or spikelike branches on elongate rachises. Staminate inflorescences panicles of 1-6 spikelike pectinate branches on elongate rachises, exserted well above the uppermost leaves; branches 0.5-2 cm, terminating in a point; staminate spikelets glabrous, with 1 floret; glumes unequal, much shorter than the florets; lemmas 3-veined, unawned. Pistillate inflorescences 1-sided spikes, with 6-12 spikelets, often partially enclosed by the subtending sheaths, sometimes with branches to 6 mm long at the lower nodes; disarticulation below the glumes, the spikelets falling intact; pistillate spikelets laterally compressed, with 1 bisexual floret and a conspicuously 3-awned rudiment; lower glumes rudimentary or absent; upper glumes subequal to the lemmas, membranous, flat, not enclosing the florets, unawned; calluses blunt, with hairs; lemmas coriaceous, keeled, 3-veined, 3-awned, awns emanating from between 4 short, hyaline teeth; palea keels adnate to the rachilla basally, widely winged distally. x = unknown. Named for Philippe Maximilian Opiz (1787-1858), a Czech botanist and administrator.