4. Poa compressa L. (Canada bluegrass)

Pl. 181 g, h; Map 728

Plants perennial, with well‑developed rhizomes, forming clumps or loose colonies. Flowering stems 20–70 cm long, erect but often with prostrate bases, strongly flattened and 2‑edged, at least in the apical 1/2, glabrous. Leaf sheaths keeled, glabrous or slightly roughened, the ligule 0.7–2.0 mm long, truncate on the margin. Leaf blades 2–15 cm long, 1.5–4.0 mm wide, flat or more commonly folded toward the base, glabrous or roughened along the margins. Inflorescences 2–10 cm long, usually narrow and relatively dense, the lowermost nodes with 2(–4) branches, these strongly ascending at maturity. Spikelets 3–8 mm long, with 3–6(–9) fertile florets. Lower glume 1.5–3.0 mm long, lanceolate, sharply pointed at the tip, with broad, thin margins, 1‑ or 3‑nerved, roughened along the midnerve. Upper glume 1.8–3.5 mm long, elliptic‑ovate, sharply pointed at the tip, with broad, thin margins, 3‑nerved, roughened along the midnerve. Lemmas 2–3 mm long, elliptic, sharply pointed at the tip, 3‑ or 5‑nerved, short‑hairy along the keel and the outermost pair of lateral nerves, and sometimes with a tuft of long, cobwebby hairs at the base. Anthers 1.0–1.7 mm long. Fruits 1.2–1.5 mm long, yellow to yellowish brown. 2n=14, 35, 39, 42, 45, 49, 50, 56. May–August.

Introduced, scattered nearly throughout the state (native of Europe, naturalized widely in the U.S. and Canada). Openings of mesic to dry upland forests (particularly grazed sites), upland prairies, and glades; also pastures, fallow fields, roadsides, railroads, and open, disturbed areas.

This species sometimes is cultivated as a lawn grass, mostly on athletic fields and other high‑use areas. It also has been cultivated as a pasture grass for forage in areas of poor soil where P. pratensis does not grow well. Steyermark (1963) noted that the pollen is an important causal agent of hay fever during the summer months.