Plants annual; cespitose. Culms 30-300+ cm, glabrous or sparsely pubescent below the nodes, branching above the bases. Leaves mostly cauline, not aromatic; sheaths sometimes with papillose-based hairs; auricles absent; ligules membranous, ciliate; blades flat. Inflorescences terminal and axillary, solitary rames with more than 1 spikelet unit, spikelets partially embedded in the rame axes; disarticulation in the rame axes. Spikelets heterogamous, in sessile-pedicellate pairs, dorsally compressed, unawned. Sessile spikelets with 2 florets; lower glumes coriaceous, smooth or scabridulous, not pitted, 2-keeled, narrowly winged above; upper glumes coriaceous, 1-keeled, winged; lower florets staminate or sterile; upper florets bisexual; lemmas and paleas hyaline; anthers 3; ovaries glabrous. Pedicels thick, fused to the rame axes. Pedicellate spikelets sterile or staminate; glumes herbaceous. Caryopses with a hard endosperm. x = 9, 10. Named for Christen Friis Rottboell (1727-1797), a Danish botanist.
Plants perennial; cespitose or rhizomatous. Culms 60-400 cm, erect. Leaves not aromatic; basal and cauline; sheaths open, glabrous, margins scarious; auricles lacking; ligules membranous, ciliate; blades flat to conduplicate, glabrous or sparsely pubescent, margins scarious, sometimes scabrous. Inflorescences terminal and axillary, composed of a solitary, pedunculate rame; rames stout; disarticulation in the rames, below the sessile spikelets. Spikelets dorsally compressed, in heterogamous sessile-pedicellate pairs. Sessile spikelets embedded in the rame axes, ovate, with 2 florets, unawned; lower glumes indurate, smooth, rugose, or pitted, 7-11-veined, not keeled; upper glumes coriaceous, keeled, 1-veined; lower florets sterile; upper florets bisexual, unawned; anthers 3. Pedicels short, thick, appressed or partly fused to the side of the rame axes. Pedicellate spikelets 1-3 mm, usually reduced. Caryopses ellipsoid to broadly ellipsoid, yellow. x = 9. Name from the Greek koilos, hollow, and rachis, axis, in reference to the axes of the inflorescence, which are concave.
Racemes spike-like, cartilaginous, cylindric, the rachis readily disarticulating, each internode thickened distally and concave at its summit; spikelets paired, the fertile one sessile, closely appressed to the hollow of the adjacent internode, its first glume coriaceous and pitted or wrinkled; sterile spikelet smaller, its stout pedicel appressed to the rachis; smooth perennials; growing in small tufts, the slender solitary racemes resembling the pistillate portion of Tripsacum. 12, mainly trop.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.