1. Poa annua L. (annual bluegrass)

Pl. 181 d, e; Map 725

P. annua var. aquatica Asch.

P. annua var. reptans Hausskn.

Plants annual, without rhizomes, forming tufts (rarely rooting at the nodes under aquatic conditions). Flowering stems 4–30 cm long, spreading to ascending (rarely erect), somewhat flattened, glabrous. Leaf sheaths somewhat keeled, glabrous, the ligule 0.8–2.5 mm long, rounded to truncate on the margin. Leaf blades 1–10(–13) cm long, 1–3 mm wide, usually flat, glabrous. Inflorescences 1–8(–10) cm long, open, the lowermost branches single or paired, spreading or loosely ascending. Spikelets 2.5–5.5 mm long, with 2–6 fertile florets. Lower glume 1.5–2.4 mm long, lanceolate, sharply pointed at the tip, with broad, thin margins, faintly 3‑nerved, roughened along the midnerve. Upper glume 1.7–2.8 mm long, narrowly ovate, sharply pointed at the tip, with broad, thin margins, 3‑nerved, roughened along the midnerve. Lemmas 2.4–3.5 mm long, elliptic, bluntly pointed at the tip, 5‑nerved, the lateral nerves all well developed, short‑hairy along the nerves but lacking long, cobwebby hairs at the base. Anthers 0.7–1.1 mm long, often exserted from the spikelet at maturity. Fruits 1.0–1.2 mm long, brownish yellow. 2n=14, 24–26, 28. January–December.

Introduced, scattered nearly throughout the state, but less commonly collected north of the Missouri River (native of Europe, Asia; escaped from cultivation and naturalized nearly worldwide). Disturbed portions of bottomland and mesic upland forests, banks of streams, spring branches, and rivers, margins of ponds and lakes, pastures, margins of crop fields, fallow fields, gardens, lawns, roadsides, railroads, and disturbed, open areas, rarely emergent aquatics.

Poa annua is a common lawn weed and is often spread as a contaminant in seed mixes of perennial bluegrasses. However, the species sometimes is planted intentionally as a lawn grass. Steyermark (1963) discussed the occurrence of occasional plants in spring branches of the Ozarks with lax, elongated stems rooting at the nodes. These are referable to var. reptans Hausskn., which often flowers very sporadically. He indicated that because plants intermediate between this aquatic phase and the more common, terrestrial phase occur along a moisture gradient at most of these sites, the two types should not be given formal taxonomic recognition. Steyermark (1963) also noted that more than 40 varieties of P. annua have been described in the literature on European plants, none of which can be distinguished, based on Missouri materials.