Plants perennial; usually unisexual; loosely tufted or with solitary shoots, shortly rhizom¬atous. Basal branching all or mainly extra vaginal. Culms 20-65 cm, erect or the bases decumbent, not branching above the base, terete or weakly compressed; nodes terete, 1-2 exserted. Sheaths closed for 1/2-1/3 their length, slightly compressed, keels moderately distinct, smooth or sparsely to moderately scabrous, glabrous, bases of basal sheaths glabrous, distal sheath lengths 1.5-4.4(5.7) times blade lengths; collars smooth, glabrous; ligules of cauline leaves 2-8 mm, smooth or scabrous, acute to acuminate, innovation ligules 2-5 mm; innovation blades to 20 cm, otherwise similar to the cauline blades; cauline blades gradually reduced in length distally, 1-3.5 mm wide, usually flat or folded, soft, thin, somewhat lax, smooth or sparsely scabrous, primarily over the veins and margins, distinctly keeled, apices narrowly to broadly prow-shaped, flag leaf blades (1.4)3-6(8) cm. Panicles (2)4-10 cm, nodding, ovoid, sparse, with 20-50 spikelets, proximal internodes usually 1.8-3 cm; nodes with 1-2(4) branches; branches 1.5-4.5 cm, ascending to spreading, lax, terete to weakly angled, angles sparsely to moderately scabrous, with 2-7 spikelets. Spikelets (4)6-9(12) mm, lengths to 3.5 times widths, laterally compressed, not sexually dimorphic; florets 3-8, usually unisexual; rachilla internodes smooth or sparsely scabrous, usually glabrous, infrequently sparsely puberulent. Glumes 3/5 – 4/5 as long as the adjacent lemmas, narrowly lanceolate to lanceolate, distinctly keeled, keels scabrous; lower glumes 1-3(5)-veined; calluses webbed, hairs over 1/2 the lemma length; lemmas 4-6.5 mm, lanceolate, 5-7-veined, distinctly keeled, keels and marginal veins sparsely short- to long-villous, lateral veins moderately prominent, intercostal regions sparsely scabrous, glabrous, margins glabrous, apices acute; palea keels scabrous; anthers vestigial (0.1-0.2 mm) or 2.5-4 mm. 2n =28.
Poa rhizomata is a rare species that grows in upper elevation, mixed coniferous forests on ultramafic (gabro or peridotite) rocks of the Klamath-Siskiyou region. It is subdioecious.
Poa rhizomata resembles P. pratensis (p. 522), differing in having acute ligules, sparse inflorescences, florets that are usually unisexual florets, and generally larger spikelets. It also resembles P. chambersii (see next), but has more open sheaths, longer ligules, more pubescent lemmas, and a more well-developed web. It used to include P. piperi (p. 554), which differs in having involute, adaxially hairy leaves and glabrous lemmas.