Poa siphonoglossa is a perennial grass with short rhizomes forming a thickened culm base. The culms are solid, flattened, smooth, striate, and tufted, often forming extensive tufts cascading from banks in masses up to 4 meters long. The older culms appear naked and rushlike, adorned with bladeless sheaths, and their internodes can reach up to 65 cm in length. The leaves are alternate, with flat, lax blades typically shorter than 10 cm and 2-3 mm wide. The upper surfaces of the leaves are scabrous, while the lower surfaces are glabrous, and older basal sheaths shed their leaves. The sheaths are glabrous, closed to the mouth, and shorter than the internodes, with a membranous ligule 2-3 mm long that is dentate to erose at the margins, encircling the culm except where it is ruptured by sheath splitting. The flowers are ovoid panicles usually less than 5 cm long, with primary branches 2-3 cm long that are appressed to ascending. The spikelets, laterally compressed and containing 2-5 flowers, measure 4-7 mm long. The rachilla is elongated, spacing the florets 1 mm apart, and ranges from glabrous to moderately hispid. The glumes are slightly unequal, narrow, glabrous, and 3-nerved, with the second glume approximately 4 mm long. The lemmas are narrow, about 4-5 mm long, with a scabrous keel and marginal nerves, or scabrous toward the base, lacking cobwebby hairs and featuring prominent intermediate nerves. The palea (upper bract of the floret of a grass) is nearly as long as the lemma, with scabrous-ciliate keels and a bifid apex. The fruit is a reddish-brown caryopsis and ovoid approximately 0.6 mm long. It is found in wet, shady banks and around 1,000–1,300 meters along ridges.