Plants perennial; stoloniferous. Culms thick, decumbent, succulent, rooting profusely at the lower nodes, without cormous bases. Ligules membranous; blades with non-Kranz anatomy.Panicles open; branches spikelike, stiffly ascending, with secund clusters of shortly pedicellate to sessile spikelets. Spikelets 6-7 mm, lanceoloid, laterally compressed, glabrous. Glumes exceeding the upper floret, apices separated at maurity; lower glumes 4/5 as long as the spikelets, nearly as long as the lower lemmas, 3-veined; upper glumes 3-veined; lower florets sterile; lower lemmas 3-veined; lower paleas small, lanceolate; upper florets about 1/3 as long as the spikelets, obovoid, stipitate; upper lemmas indurate, smooth. x = 10.
Phanopyrum differs from other members of the Paniceae in its one-sided panicle branches, large spikelets with spreading glumes, its long lower glumes, and the short, stipitate upper floret. It includes only one species and is restricted to the southeastern United States.
Phanopyrum used to be treated as Panicum subg.gymnocarpon. Aliscioni et al. (2003)showed that it is only distantly related to Panicum sensu stricto and not closely related to other members of the subgenus, such as P. hemitomon and P. trichoides. Because the appropriate treatment of other members of the subgenus is not yet evident, they have been left in Panicum for now.
1. Phanopyrum gymnocarpon (Ell.) Nash
Plants perennial; forming extensive colonies by their long, decumbent, sprawling basal branches and stolons. Culms 60-130 cm, thick, glabrous, rooting profusely at the lower nodes; nodes glabrous, often with a dark green band. Sheaths usually shorter than the internodes, glabrous, prominently veined; ligules 0.5-1.5 mm; blades 15-40 cm long, 7-25 mm wide, tapering from midlength, flat, both surfaces glabrous, bases subcordate, margins scabrous to smooth, widest at the base, apices acute. Panicles 10-40 cm long, 7-20 cm wide, open, with straight, rigid rachises; branches whorled, stiffly ascending, with short, appressed, higher order branches; ultimate branchlets 1-sided, with solitary spikelets or small clusters of spikelets; pedicels 0.1-1.5 mm. Spikelets 5.5-7 mm long, about 1 mm wide, narrowly lanceoloid, glabrous. Glumes spreading apart at maturity, keeled, prominently veined, scabrous along the midveins; lower glumes nearly as long as the lower lemmas; upper glumes and lower lemmas 3-veined, spreading, greatly exceeding the upper florets, lower lemmas longer than the upper glumes, arcuate; lower florets sterile; lower paleas thin; upper florets 1.9-2.2 mm, less than 1/3 as long as the spikelets, obovoid, lustrous, pale to brownish, acute, often short-stipitate. 2n = 40.
Phanopyrum gymnocarpon grows in swamps, wet woodlands, and the marshy shores of lakes and streams. It is also found occasionally in shallow water, often in the shade. It is restricted to the United States.