Plants perennial, without rhizomes, forming tufts. Flowering
stems 25–80 cm long, erect, sometimes spreading at the very base, circular in
cross‑section, glabrous. Leaf sheaths rounded, glabrous, the ligule
0.6–2.0 mm long, rounded to truncate on the margin. Leaf blades 2–20 cm long,
1–2 mm wide, flat or rarely with the margins loosely inrolled, glabrous.
Inflorescences 5–15 cm long, open, the lowermost nodes with (2)3–5 branches,
these loosely ascending. Spikelets 2.2–4.5 mm long, with 2–4 fertile florets.
Lower glume 1.5–3.0 mm long, lanceolate to narrowly ovate, sharply pointed at
the tip, with broad, thin margins, 1‑ or 3‑nerved, roughened along
the midnerve. Upper glume 1.8–3.2 mm long, narrowly ovate, sharply pointed at
the tip, with broad, thin margins, 3‑nerved, roughened along the
midnerve. Lemmas 2.0–3.2 mm long, elliptic, sharply pointed at the tip, 3‑nerved
(an additional, very faint pair of lateral nerves sometimes present), short‑hairy
along the nerves and with a tuft of long, cobwebby hairs at the base. Anthers
0.9–1.6 mm long. Fruits 1.3–2.5 mm long, reddish brown. 2n=28, 34, 42,
43, 56. May–June.
Known from a single specimen from Cass County (western U.S. across the northern Great Plains to Michigan and Vermont; Canada). Upland prairies.
In his initial report of this species for Missouri, Tim
Smith (1988) noted that there is some doubt whether P. interior is
native at the site that he discovered, because a railroad crosses the property
and a number of exotic species occur in the prairie. Also, the main range of
the species is not particularly close to Cass County, with the closest stations
in western Nebraska. However, P. interior is not generally considered a
weedy species in the floristic literature, and the plants were not growing
particularly close to the railroad at the Missouri site. Thus, pending further
inventory of surrounding sites and more study of the existing population,
Smith’s (1988) report is treated here as a native occurrence.