"Tufted annual with few–several decumbent to ascending or seldom erect culms to 3 dm, sometimes rooting at the lower nodes; sheaths loose; blades soft, 2–3 mm wide; infl ovoid, 2–8 cm, with few ascending branches bearing rather crowded spikelets above the middle; spikelets green, 3–5 mm, 3–6-fld; glumes broadly lanceolate, acute, scarious-margined, indistinctly veined, the first 1.5–2.4 mm, the second 1.8–2.8 mm; lemmas thin, elliptic, 5-veined, obtuse, ± hairy on the veins only, not webbed, the lowest one 2.4–3.4 mm; anthers 0.8–1 mm; 2n=14, 28. Native of Eurasia; abundant in moist soil nearly throughout the U.S. and n. to Lab. and Alas.; often a lawn-weed."
Annuals, sometimes over wintering, infrequently stoloniferous. Culms loosely tufted, erect or oblique, often decumbent, often geniculate, soft, 6–30(–45) cm tall, smooth, nodes 1 or 2(or 3), 1(or 2) exserted. Leaf sheath slightly compressed, thin, smooth, uppermost closed for ca. 1/3 of length; blade light to dark green, flat or folded, thin, 2–12 cm × (0.8–)1–3.5 mm, margins slightly scabrid, apex acutely prow-tipped; ligules 0.6–3 mm, abaxially smooth, glabrous, apex obtuse, margin irregularly dentate, smooth. Panicle open, moderately congested, broadly ovoid to pyramidal, (1–)3–10 cm, as long as wide; branches ascending, spreading, or a few reflexed, 1 or 2(–3) per node, smooth, longest with usually 3–5 spikelets in distal 1/2. Spikelets ovate to oblong, dark to light green, (3–)4–5.5 mm, florets 3–5, distal fertile florets often female; vivipary absent; rachilla internodes 0.5–1.5 mm, smooth, glabrous, hidden or exposed; glumes unequal, smooth or rarely keeled with hooks, lower glume lanceolate and acute to subflabellate and obtuse, 1.5–2(–3) mm, 1-veined, upper glume elliptic, 2–3(–4) mm, 3-veined, the margin angled; lemmas ovate, 2.2–3.5 mm, apex and margins broadly membranous, intermediate veins prominent, keel and marginal, and usually intermediate, veins villous in the lower 1/2, rarely glabrous throughout; callus glabrous; palea keels smooth, densely pilulose to short villous. Anthers 0.6–1 mm, usually at least 2 × as long as wide, or vestigial. Fl. Apr–May, fr. Apr–Jul. 2n = 28.
Poa annua is easily distinguished from other short-anthered Poa, other than P. infirma, by the annual habit, absence of a web on the callus, and the near absence of hooks on the panicle branches and spikelet bracts, in combination with densely pubescent palea keels that lack hooked prickle hairs at the apex. Plants with glabrous florets are sporadically encountered.
Plants perennating by short stolons rooting at the nodes appear to develop repeatedly but sporadically at various elevations with prolonged, cool, mesic growing conditions, possibly in response to trampling. These are sometimes placed in var. reptans. Such plants have been recorded from Yunnan.