18. Aira L. (hairgrass)

Plants annual, forming delicate tufts. Flowering stems erect or ascending, glabrous. Leaf sheaths rounded on the back, roughened, the ligule 1.5–3.5 mm long, often somewhat torn or irregularly lobed or toothed. Leaf blades 0.5–5.0 cm long, 0.3–1.0 mm wide, usually with strongly inrolled margins, roughened. Inflorescences open (narrow elsewhere), erect panicles with numerous loosely ascending to spreading, hairlike branches. Spikelets 1.8–3.0 mm long, somewhat flattened laterally, disarticulating above the glumes, with 2 perfect florets. Glumes similar in length and shape, both about the same width, slightly longer than the rest of the spikelet, 1‑ or 3‑nerved, keeled, ovate, narrowed to a sharply pointed tip, awnless, roughened on the midnerve. Lemmas broadly lanceolate, tapered to 2 narrow teeth at the tip, rounded on the back, faintly 3‑ or 5‑nerved, somewhat roughened, one or both lemmas with a well‑developed, bent or twisted awn attached below the middle, the lemma bases glabrous or minutely hairy. Palea slightly shorter than the lemmas, membranous, 2‑nerved. Stamens 3, the anthers 0.3–0.6 mm long. Fruits 0.8–1.2 mm long, narrowly elliptic in outline, pale yellow, shiny. Eight to 12 species, Europe, Asia, Africa, introduced nearly worldwide.

Species of Aira are delicate annuals that are easily overlooked in the field. They tend to occur in disturbed or mowed sites where there is little competition from other plant species.