Poa siphonoglossa is a perennial member of the grass family (Poaceae). It differs from P. sandvicensis principally by its longer culms (stems), lack of a prominent tooth on the ligule (a thin outgrowth at the junction of leaf and leafstalk), and shorter panicle (a branched cluster of flowers in which the branches are racemes) branches. Poa siphonoglossa has extensive tufted and flattened culms that cascade from banks in masses up to 4 m (13 ft) long. The naked, rush-like older culms have bladeless sheaths; the sheaths do not split with age. The ligule has no hard tooth. The flat, loosely packed leaf blades are usually less than 10 cm (4 in) long and 3 mm (0.1 in) wide. The primary panicle branches are about 3 cm (0.1 in) long. The lemmas (the lowermost of two chaff-like bracts enclosing the grass floret) lack cobwebby hairs. The fruits are reddish brown and oval.