7. Poa pratensis L. (Kentucky bluegrass)

Pl. 180 e, f; Map 731

Plants perennial, with well‑developed rhizomes, forming clumps or loose colonies. Flowering stems 30–100 cm long, erect, circular in cross‑section or very slightly flattened, glabrous. Leaf sheaths rounded or nearly so, glabrous or less commonly somewhat roughened, the ligule 0.7–2.0 mm long, truncate and usually somewhat uneven on the margin. Leaf blades 1–25 cm long, 2–5 mm wide, flat or more commonly folded toward the base, glabrous or roughened along the margins and toward the base. Inflorescences 3–15 cm long, usually open, the lowermost nodes with usually 4 or 5 branches, these spreading or ascending at maturity. Spikelets 3–6 mm long, with 3–5 fertile florets. Lower glume 1.7–3.0 mm long, elliptic‑lanceolate, sharply pointed at the tip, with broad, thin margins, 1‑nerved, roughened along the midnerve. Upper glume 2.2–3.5 mm long, elliptic‑ovate, sharply pointed at the tip, with broad, thin margins, 3‑nerved, roughened along the midnerve. Lemmas 2.5–4.0 mm long, elliptic, sharply pointed at the tip, 5‑nerved, short‑hairy along the keel and the outermost pair of lateral nerves, and with a tuft of long, cobwebby hairs at the base. Anthers 1.0–1.5 mm long. Fruits 1.5–2.2 mm long, reddish brown, shiny. 2n=21–147 (numerous levels). May–July.

Introduced, scattered throughout the state (native of Europe, Asia [but see discussion below], naturalized widely in the U.S. and Canada). Bottomland forests, mesic upland forests, glades, banks of streams and rivers, margins of ponds and lakes; also roadsides, railroads, pastures, fallow fields, gardens, and open, disturbed areas.

The distribution of Poa pratensis in North America is not well understood. Many authors, including Holmgren and Holmgren (1977) and Sutherland (1986) believe that at least some populations from the northern and western United States and Canada are native occurrences, whereas others treat all North American materials as introduced. In Missouri, there is no evidence to suggest that this species is native, in spite of the broad range of habitats in which it has become naturalized. A number of varieties and forms have been described, based mostly on European materials, but Missouri and other North American plants do not seem to be separable into infraspecific taxa.

Kentucky bluegrass is one of the most popular grasses for lawns and golf courses in the midwestern United States, and the species is thus the most important member of the genus economically. It also is grown as a pasture grass and makes excellent forage, particularly in areas with calcareous substrates. Steyermark (1963) noted that the pollen is a principal causal agent of hay fever during the months of May and June.