Culms 23-50 cm tall, 0.6-1.2 mm thick. Lower ligules 0.7-1.5 mm; upper ligules to 2.5 mm; blades filiform, 0.3-0.6 mm in diameter. Panicles 7.5-12 cm, open or loosely contracted; branches spreading. Glumes 4.5-6 mm; florets 2.5-4 mm; calluses 0.1-0.8 mm; lemmas with 0.2-0.8 mm lobes; awns 5-25 mm, hairy, hairs on the lowest segment 1-2 mm; anthers 1.2-3 mm, glabrous. 2n = unknown.
Ptilagrostis porteri grows on hummocks of poorly drained wetlands, at 2700-3600 m, in central Colorado. It is often associated with Salix spp. and Deschampsia cespitosa. There are 29 known populations.
The proportion of plants having closed and open panicles varies among the populations; in some populations all or almost all plants have open panicles, in others all or almost all plants have closed panicles (Johnston 2006). Barkworth (1983) treated P. porteri as a subspecies of P. mongolica (Turcz. ex Trin.) Griseb. because of their morphological similarity. They differ ecologically and geographically, P. mongolica sensu stricto growing in rocky alpine habitats of central Asia.
"decumbent" is not a number."prolonged" is not a number.