14.22 AIRA L.
J.K. Wipff

Plants annual; tufted. Culms 1–55 cm, erect to decumbent. Leaves cauline; sheaths open for most of their length, glabrous, usually scabridulous, occasionally smooth; auricles absent; ligules membranous; blades of the uppermost leaves greatly reduced. Inflorescences open or contracted panicles, sometimes spikelike, with more than 1 spikelet associated with most nodes; branches longer than 5 mm, capillary, appressed to strongly divergent; pedicels capillary, appressed to divergent. Spikelets 1.5–3.8 mm, laterally compressed, with 2 bisexual florets, both usually awned, the lower floret sometimes unawned, occasionally both unawned; rachillas glabrous, lowest segments about 0.2 mm, florets appearing opposite, usually not or scarcely extended beyond the base of the distal floret; disarticulation above the glumes and beneath the florets. Glumes equal to subequal, longer than the florets, membranous, 1–3-veined, unawned; calluses puberulent; lemmas subcoriaceous, glabrous, scabridulous, 5-veined, apices bifid, awned or unawned, awns attached below midlength, usually geniculate, sometimes straight; paleas membranous, 2-veined; lodicules 2, free; anthers 3; ovaries glabrous. Caryopses shorter than the lemmas, concealed at maturity, adhering to the lemmas and/or paleas, longitudinally grooved, dorsally compressed. x = 7. Name from the Greek aira, the name for a weed.

Aira is a genus of eight species; two have been introduced into the Flora region. All members of the genus are native to temperate Europe and the Mediterranean region, North Africa, and western Asia. Frequently adventive, they are now widespread outside of their native range as weeds, although they are not considered particularly troublesome. They have little forage value because most are delicate, with extremely small leaves. All the species grow in open, disturbed places on usually dry, occasionally mesic, sandy to rocky soils.

[Click here for interactive version of dichotomous key].

1. Panicle branches ascending to divergent; panicles 1.2–13.5 cm long, 1.5–10 cm wide A. caryophyllea
1. Panicle branches appressed to the rachises; panicles 0.5–4.1 cm long, 0.3–0.7 cm wide A. praecox

Aira caryophyllea L.

Plants annual; tufted. Culms 4.5–55 cm, erect. Sheaths scab-ridulous, occasionally smooth; ligules 1.2–8 mm, abaxial surfaces scabridulous, acute to subobtuse, becoming lacerate; blades 0.3–13.5 cm long, 0.3–2.5 mm wide, antrorsely scabridulous, glabrous, apices prow-tipped. Panicles 1.2–13.5 cm long, 1.5–10 cm wide, open; primary branches to 7.3 cm, ascending to divergent, antrorsely scabridulous, occasionally smooth; pedicels 0.9–11.3 mm, apices enlarged. Spikelets 1.7–3.3(3.5) mm, silvery-green to stramineous or purplish; rachillas usually not prolonged beyond the base of the distal floret, sometimes prolonged, vestigial. Glumes subequal to equal, 1.3–3.3(3.5) mm, scab-ridulous on the upper 1/2; callus hairs 0.2–0.4 mm; lemmas 1.3–2.6 mm, apices bifid, sometimes only the upper lemma awned, awns 2.1–3.9 mm, straight or geniculate; paleas 0.9–1.7 mm; anthers 0.2–0.5 mm. Caryopses 0.9–1.5 mm long, 0.3–0.5 mm wide, abaxial surfaces grooved in the distal 1/2, adaxial surfaces grooved the entire length. 2n = 14.

Aira caryophyllea is native to Eurasia and Africa; it has become established in the Flora region, primarily on the Pacific, Gulf, and Atlantic coasts, and through much of the southeastern United States. It is usually found in disturbed areas, in vernally moist to dry, sandy to rocky, open sites, from sea level to subalpine elevations. It sometimes invades lawns or rock gardens.

1. Glumes subobtuse, usually denticulate, often mucronate; pedicels with abruptly thickened
apices var. cupaniana
1. Glumes acute; pedicels gradually thickening to the apices (2)
2. Pedicels usually 2–8 times as long as the
spikelets; spikelets 1.7–2.5 mm long var. capillaris
2. Pedicels usually 1–2 times as long as the
spikelets; spikelets 2–3.5 mm long var. caryophyllea

Aira caryophyllea var. capillaris (Mert. & W.D.J. Koch) Mutel
Delicate Hairgrass

Pedicels 2–11.3 mm, usually 2–8 times as long as the spikelets, gradually thickening to the apices. Spikelets 1.7–2.5 mm, spreading, divergent from the secondary branches, often purplish to reddish-purple-tinged; rachillas not prolonged or vestigial. Glumes subequal, 1.7–2.5 mm, 1-veined, acute; lower lemmas 1.3–1.8 mm, apices bifid, teeth to 0.1 mm, awns absent or to 2.6 mm, straight or geniculate; lower paleas 1–1.3 mm; upper lemmas 1.7–2.1 mm, apices bifid, teeth 0.2–0.4 mm, awned, awns 2.1–3 mm, geniculate; upper paleas 1–1.3 mm; anthers 0.2–0.4 mm, yellow-orange or purple. Caryopses 0.9–1.2 mm long, about 0.3 mm wide, glabrous.

Aira caryophyllea var. capillaris is native to Europe, northern Africa, and western Asia. It usually grows in dry to somewhat moist, sandy loam soils of grassy banks, woodland openings, and disturbed sites such as pastures and roadsides.

Aira caryophyllea var. capillaris is the correct name for this taxon at the varietal level. If treated at the species level, its correct name is Aira elegans Willd. ex Roem. & Schult.

Aira caryophyllea L. var. caryophyllea
Silver Hairgrass

Pedicels 0.9–7 mm, usually 1–2 times as long as the spikelets, gradually thickening to the apices. Spikelets 2–3.3(3.5) mm, usually appressed to the secondary branches, silvery-green to stramineous; rachillas not prolonged. Glumes 2–3.3(3.5) mm, subequal, 1-veined or the upper glumes with 2 lateral veins, these sometimes obscure and usually less than 1/2 the length of the glumes, acute; lower lemmas 2–2.4 mm, apices bifid, teeth 0.2–0.4 mm, awned, awns 2.4–3.5 mm, geniculate; lower paleas 1.5–1.7 mm; upper lemmas 2–2.6 mm, apices bifid, teeth 0.2–0.3 mm, awned, awns 2.5–3.9 mm; upper paleas 1.4–1.7 mm; anthers 0.2–0.5 mm, yellow to orange. Caryopses 1.4–1.5 mm long, 0.4–0.5 mm wide, glabrous.

Aira caryophyllea var. caryophyllea is native to the Mediterranean region. It usually grows in dry, sandy to rocky soil and on rock outcrops, in open and disturbed sites in woods, grassy flats, pastures, paths, and roadsides; it is occasionally found in damp ground at swamp or lagoon margins.

Aira caryophyllea var. cupaniana (Guss.) Fiori
Silvery Hairgrass

Pedicels 2.8–6.5 mm, usually 1–3 times as long as the spikelets, abruptly thickened at the apices. Spikelets 1.3–2.6 mm, spreading, divergent from the secondary branches, silvery-green. Glumes equal to subequal, 1.3–2.6 mm, 1–3-veined, subobtuse, usually denticulate, often mucronate; lower lemmas 1.3–1.9 mm, usually unawned; upper lemmas 1.3–2 mm, apices bifid, awned, awns 1.8–2.5 mm, geniculate; paleas 0.9–1.4 mm; anthers 0.2–0.4 mm, yellow or purple. Caryopses about 1 mm long, 0.3–0.4 mm wide, glabrous.

Aira caryophyllea var. cupaniana is native to southern Europe and northern Africa, growing in mesic, open habitats in disturbed areas or open woodland. It was discovered in a prescribed burn area of Mount Diablo State Park in Contra Costa County, California, in 1995, but was not relocated in 1999.

Aira praecox L.
Spike Hairgrass

Plants annual; tufted. Culms 1–36 cm, erect. Sheaths usually scab-ridulous, occasionally smooth; ligules 1.4–5.3 mm, abaxial surfaces scabridulous, acute, becoming lacerate; blades (0.1) 0.25–5 cm long, 0.3–2.0 mm wide, antrorsely scabridulous, glabrous, prow-tipped. Panicles (0.5)1–4.1 cm long, 0.3–0.7 cm wide, sometimes reduced to a single spikelet in depauperate specimens; primary branches to 1.5 cm, appressed to the rachises, antrorsely scabridulous; pedicels 0.7–3.2 mm, enlarged at the apices. Spikelets 2.8–3.8 mm, green and purple-tinged; rachillas prolonged beyond the base of the distal floret. Glumes subequal, antrorsely scabridulous, especially on the midveins; lower glumes 2.8–3.6 mm, 1–3-veined; upper glumes 2.7–3.8 mm, 3-veined; callus hairs 0.3–0.5 mm; lower lemmas 2.4–3.3 mm, apices bifid, teeth 0.3–0.5 mm, awned, awns 3–4.5 mm; paleas 1.7–2.1 mm; upper lemmas similar; anthers 0.2–0.4 mm, yellow. Caryopses 1.3–1.7 mm long, 0.4–0.5 mm wide, glabrous, abaxial surfaces grooved 0.8–1 mm distally, adaxial surfaces grooved the entire length. 2n = 14.

Aira praecox is native to Europe. In the Flora region, it grows mainly along or near the Pacific and Atlantic coasts, in dry to vernally moist sand dunes or in sandy to rocky soils, on rock faces and ledges, and in disturbed areas such as the edges of roads, railways, and airports. It is usually found in lowland areas, though it occasionally grows at montane to subalpine elevations.