Poa annua is an annual or winter annual C3 grass (Bassinger 2002), sometimes a short-lived perennial (2-3 years), tufted, to 30 cm tall, often much shorter, sometimes rooting at nodes and forming mats.
Leaves soft, 0.1-0.5 cm wide, hairless, tip boat-shaped, usually bright green, basal leaves to 15 cm long, often wrinkled when young, sheaths loose, open half their length, margins overlapping, keeled, ligule membranous, to 0.5 cm long.
Flower spikelets crowded, above middle of branch, 0.3-0.5 cm long, 3-8 flowers, first glume about 0.2 cm, second slightly longer, papery margins, lemmas 5-veined (base not webbed); inflorescence to 11 cm long, oval, pale to bright green (occasionally reddish), branches 1-2 per node, branching beyond midpoint, hairy on veins; self compatible but can outcross as well; blooms and fruits March-Dec., with a maximum in May-June. Often the smallest, earliest grass blooming. Seed dispersal principally by lawn mowers, shoes and other human activities. May also be dispersed by birds. Seed germination and seedling establishment greatest in autumn and early spring. Seeds retain viability for four or more years (Warwick 1979).
Wetland status: FACU.
Frequency in NYC: Occasional in natural areas. Ubiquitous in lawns, mowed roadsides, vacant lots.
Habitat: Common in lawns, open ground, gardens, disturbed roadsides, tree pits, soil pH 4.8-8, moderately tolerant of shade, intolerant of drought, fire, salt (USDA, NRCS 2010). Susceptible to high temperatures and smog. Tolerates compacted or waterlogged soil and shade (Hightshoe 1988).
Notes: Plants and seeds eaten by birds and mammals (Martin et al. 1951). Cannot compete with dense vegetation. Pollen is allergenic. Attacked by various insects including: Acyrthosiphon chandrani, Anoecia haupti, Schizaphis agrostis (Aphidae); Calendra parvulus (Coleoptera, blue grass billbug grubs); Crambus teterrellus (blue grass webworm) and Pseudaletia unipuncta (Lepidoptera). Infected by numerous fungi including: Pellicularia filamentosa (Basidiomycotina) Puccinia poae-nemoralis, P. graminis (a stem rust, Basidiomycotina), Rhizoctonia solani (rots and damping off of seedlings, Basidiomycotina), Gaeumannomyces graminis (take-all disease, Ascomycetes), Polymyxa graminis (a root disease, Plasmodiophoromycetes), Septoria macropoda and several species of Fusarium (wilts, Deuteromycotina). Similar to P. pratensis, which has lemmas webbed at base and is colonial from rhizomes (Agrios 1988; Warwick 1979).