Poa annua L., annual bluegrass. Annual (with adequate water persisting into next growing season), fibrous–rooted, several–stemmed at base (appearing many–stemmed when plant growing in clump), cespitose and densely tufted, branches decumbent to spreading often rooting at base, to 25 cm tall; gynomonoecious; shoots glabrous; adventitious roots at basal nodes.
Stems (culms) cylindric to slightly compressed, < 2 mm diameter.
Leaves alternate distichous, simple with sheath or lower leaves reduced to only sheath; prophyll = membranous closed sheath (no blade), linear in outline, membranous, to 11 mm long, 2–keeled and concave between keels, minutely short–toothed along keel; sheath of foliage leaves closed at least to 1/3 increasing to 2/3 (upper leaves), 20—66 mm long, rounded, lacking lobes (auricles) at top; collar inconspicuous; ligule membranous, obtuse to truncate, 0.5—3 mm long, milky translucent, continuous with sheath margins, smooth, glabrous; blade linear, 40—100 × (1—)2.5—5.5 mm, the widest several mm from sheath, initially folded becoming flat, entire, prow–shaped at tip, parallel–veined with midrib somewhat keeled above midblade.
Inflorescence spikelets, in terminal, open, paniclelike arrays, array lanceoloid to narrowly ovoid in outline, 70—95 × 28—65 mm, with 2(3) lateral branches at lower nodes gradually reduced to 1 branchlet or spikelet at tip, with crowded to loosely arranged spikelets, spikelet stalked to sessile, with 2—6 florets, lacking awns; peduncle 43—85 mm long; rachis initially compressed aging cylindric, glabrous; branches ascending to spreading, to 60 mm long, decreasing upward, with yellowish pulvinus at base, smooth and glabrous (sparsely scabrous); stalks 0.2—5 mm long, with conspicuous swelling beneath spikelet.
Spikelet initially lanceoloid compressed side–to–side, 3—5 mm long, with spreading florets ovate to somewhat rectangular, lower florets and generally intermediate florets bisexual, other florets pistillate to vestigial, breaking above glumes and between florets; rachilla cylindric, 0.8—1.1 mm long to 0.2—0.8 mm long for terminal rachilla extending beyond upper floret, whitish, glabrous, smooth; glumes unequal, lower glume broadly lanceolate, 1.4—2 mm long and acute at tip, upper glume ovate to obovate, 2—2.5 mm long and rounded at tip, lower glume < upper glume and < lower lemma, green + membranous on margins and at tip, keeled to weakly keeled minutely scabrous above midpoint, lower glume 1–veined or weakly 3–veined, upper glume 3–veined, the veins not converging at tip; lemma lanceolate, 2.5—3.2 mm long (lowest floret) decreasing to 2—2.5 mm long (upper floret), distinctly keeled, membranous and entire on margins, 5–veined, obtuse or notched at tip, sometimes tinged red–purple in membranous tissue, short–villous woolly on veins and typically glabrous between veins; palea membranous, 0.1—0.5 mm < lemma, 2–keeled and 2–veined, veins green, keels densely short–villous, sometimes with hairs between veins at palea base.
Flower bisexual or pistillate; perianth (lodicules) 2, teardrop–shaped and bulbous to above midpoint, 0.4—0.5 mm long, colorless; stamens 3, free, or vestigial (pistillate florets); filaments threadlike, 2—2.5 mm long, white to colorless; anthers basifixed, dithecal, narrowly oblong, 0.7—1.2 × 0.3 mm, light yellow, longitudinally dehiscent; pollen light yellow; pistil 1; ovary ± spheric, ca. 0.35 mm, light transparent green; styles 2 or fused for ca. 0.1 mm at base, whitish to colorless; stigmas feathery (plumose).
Fruit achene (caryopsis), enclosed in lemma and palea, lanceoloid–ellipsoid, ca. 1.8 × 0.6 mm, grayish brown, short–tomentose at tip; dispersal unit including rachilla segment opposite palea.
A. C. Gibson & B. A. Prigge