Perennial, robust, aquatic or marshy herb; 1-3m long with aerial proproots from the lower part of stem; leaves erect, broad, spongy, semicylindric above the sheath, linear ensiform, sometimes armed with white prickles on margins and on dorsal midrib, margin serrulate; flowers in very dense, superposed, cylindric spikes often intermixed with capillary bracteoles (hairs with dilated tips), male and female spikes separated by a considerable interval; perianth of capillary hairs or absent; male flowers upper, paler and more slender; stamens 1 or more, filaments free or connate anthers basifixed, tip of connective thickened; often mixed with clavate tipped pistillodes; female flowers pale brown, subspathullate, equaling the linear stigmas; ovary superior, 1-2 celled, each 1-ovuled, styles persistent, narrowed into clavate or filiform stigma; fruit very minute nutlets, fusiform, obovate, membranous, indehiscent, brown.