Description from Flora of China
Perennial, rarely annual. Culms usually prostrate and rooting at lower nodes. Leaf blades linear, flat; ligule narrow, membranous, margin ciliate. Inflorescence composed of single axillary racemes; racemes solitary or in groups in upper leaf axils, dorsiventral, articulation line usually oblique but racemes tough, not or very tardily disarticulating; rachis internodes thickened, oblong-angular, adnate to adjacent pedicel. Sessile spikelet appressed to hollow in rachis, dorsally compressed (terete in H. sibirica); callus obtuse to cuneate, rarely truncate; lower glume narrowly elliptic, rigidly herbaceous to leathery, smooth, marginally 2-keeled, indistinctly winged above, obtuse to caudate or rarely 2-cleft; upper glume sometimes adnate to internode, mucronate or awned; lower floret barren, without palea; upper floret bisexual, with entire awnless lemma. Pedicelled spikelet similar to sessile, but base truncate and lacking callus; pedicel similar to adnate rachis internode, junction marked by a line. x = 9, 10.
This is a genus of aquatic or semi-aquatic grasses concentrated in SE Asia. At first sight it is difficult to distinguish the sessile and pedicelled spikelets because they look very similar and, since the pedicel is fused to the rachis internode, both spikelets are in effect sessile. However, the sessile spikelet of a pair can be distinguished by its basal callus, which normally has an oblique articulation line beneath it. The strongly bilateral racemes have all the sessile spikelets on one side and all the pedicelled spikelets on the other.
Fourteen species: tropical and subtropical regions of the Old World; introduced in America; six species (one endemic) in China.
(Authors: Sun Bixing (孙必兴 Sun Bi-sin); Sylvia M. Phillips)