77. Poa L. (bluegrass)

Plants annual or perennial, sometimes with rhizomes (sometimes appearing stoloniferous in P. palustris), forming tufts, clumps, or loose colonies. Flowering stems erect to spreading, sometimes weak or nodding, sometimes rooting at the lower nodes, often slender. Leaf sheaths open to below the middle (closed, with fused margins, toward the base), sometimes closed only at the very base. Leaf blades flat or folded, with boat‑shaped tips. Inflorescences open or narrow panicles with ascending to spreading or downwardly angled branches, these sometimes arched or curved, the spikelets single or in small clusters toward the branch tips, but not regularly paired, all similar in size and appearance and with fertile florets (except in P. bulbosa, with most of the florets replaced by vegetative bulblets). Spikelets elliptic‑lanceolate to ovate in outline, somewhat flattened, with 2–8 florets. Glumes much shorter than the rest of the spikelet, pointed at the tip, awnless, glabrous or more commonly roughened on the midnerve, the lower glume slightly shorter, 1‑ or 3‑nerved, the upper glume 3‑nerved. Lemmas sharply or bluntly pointed at the tip, awnless, usually somewhat keeled on the back, faintly to strongly 3‑ or 5‑nerved with the nerves converging (arched inward) toward the tip, glabrous or more commonly hairy on the back (sometimes minutely so) and usually also with a tuft of long, cobwebby hairs at the base. Paleas slightly shorter than the lemmas, elliptic. Stamens 3, the anthers yellow. Fruits elliptic in outline, slightly flattened on 1 side. About 500 species, worldwide, especially in temperate and boreal regions.

Species within the annual and perennial groups in Poa can be difficult to distinguish, and close attention must be given to rhizome, ligule, and spikelet characters.