Plants usually perennial; usually cespitose, sometimes rhizomatous or stoloniferous. Culms (3)5–120 cm, usually erect. Sheaths open, usually smooth and glabrous, sometimes scabrous to scabridulous, rarely hairy; collars not strongly developed; auricles absent; ligules membranous, smooth or scabridulous dorsally, apices truncate, obtuse, rounded, or acute, usually erose to lacerate, the lacerations sometimes obscuring the shape, or entire; blades flat, folded, or involute, usually smooth and glabrous, sometimes scabridulous, adaxial surfaces somewhat ridged. Inflorescences terminal panicles, narrowly cylindrical and dense to open and diffuse; branches usually in whorls, usually more or less scabrous, rarely smooth, some branches longer than 1 cm; secondary panicles sometimes present in the leaf axils. Spikelets 1.2–7 mm, pedicellate, laterally compressed, lanceolate to narrowly oblong or ovate, with 1(2) florets; rachillas not prolonged beyond the base of the floret(s); disarticulation above the glumes, beneath the florets, sometimes initially at the panicle base. Glumes (1)1.3–2(4) times longer than the lemmas, 1(3)-veined, glabrous, usually mostly smooth, vein(s) often scabrous to scabridulous, backs keeled or rounded, apices acute to acuminate or awn-tipped; lower glumes usually 0.1–0.3 mm longer than the upper glumes, rarely equal; calluses poorly developed, blunt, glabrous or hairy, hairs to about 1/2 as long as the lemmas; lemmas thinly membranous to hyaline, usually smooth and glabrous, sometimes scabridulous, occasionally pubescent, rarely warty-tuberculate, 3–5-veined, veins not convergent, sometimes excurrent as 2–5 teeth, apices acute to obtuse or truncate, sometimes erose, unawned or awned, sometimes varying within an inflorescence, awns arising from near the lemma bases to near the apices, usually geniculate, sometimes straight; paleas absent, or minute to subequal to the lemmas, usually thin, veins not or only weakly developed; lodicules 2, free; anthers (1)3, 0.1–2 mm, not penicillate; styles 2, free to the base, white; ovaries glabrous. Caryopses with a hard, soft, or liquid endosperm, the latter resulting from the substitution of lipids for starch. x = 7. Name from the Greek agros, ‘pasture’ or ‘green fodder’.

Agrostis in the older, broad sense is a genus comprised of species with the spikelets reduced to single florets. As such, it is found in all inhabited continents, is presumably of ancient origins, and many of the 150–200 species may be only distantly related. The shortage of clear-cut morphological features has hindered its subdivision into more natural units. This treatment follows Edgar (1995), Edgar and Connor (2000), and Jacobs (2001) in placing A. avenacea J.F. Gmel. in the Australasian genus Lachnagrostis, as L. filiformis; Rúgolo de Agrasar (1982) in treating A. tandilensis (Kuntze) Parodi as Bromidium tandilense; and Soreng (2003) in placing A. aequivalvis(Trin.) Trin. and A. humilis —together with several Central and South American species, including A. sesquiflora E. Desv.—in the genus Podagrostis.

Agrostis usually differs from both Podagrostis and Lachnagrostis in having no, or very reduced, paleas, and in rachillas that are not prolonged beyond the base of the floret. Some of the Eurasian species of Agrostis are exceptional in having paleas at least 2/5 as long as the lemmas. Agrostis also differs from Lachnagrostis in certain features of the lemma epidermes (Jacobs 2001).

Agrostis is sometimes confused with Apera, Calamagrostis, or Polypogon. It differs from Apera in having lemmas that are less firm than the glumes, paleas that are often absent or minute, and in lacking a rachilla prolongation. There is no single character that distinguishes all species of Agrostis from those of Calamagrostis. In general, Agrostis has smaller plants with smaller, less substantial lemmas and paleas than Calamagrostis, and tends to occupy drier habitats. It differs from Polypogon in having spikelets that disarticulate above the glumes.

Some taxonomists used the presence of a trichodium net for circumscribing Trichodium Michx. This net is formed by a series of transverse thickening bars developed on the inner wall of the dorsal epidermal cells of the lemma, and is found in several different genera, usually in species with a reduced palea.

Species of Agrostis growing in the temperate regions of the Northern Hemisphere and on tropical mountains are mostly perennials, with the annual species predominantly in warmer climates, such as the Mediterranean and the Southern Hemisphere. Of the 26 species known from the Flora region, 21 are native and 5 are introductions. Two additional species, A. tolucensis and A. anadyrensis, have been reported; the reports are dubious.

Some species of Agrostis make a modest contribution to forage, a few are agricultural weeds, and some are excellent lawn grasses in cool climates. Most North American native species are narrow habitat specialists, with many being western endemics. The introduced species are all widely distributed in temperate regions of the world.

Unusual specimens of Agrostis with elongate or leafy spikelets are caused by infection with the nematode Anguillina agrostis. Other pathogens may cause stunting.

Species with awns on the lemmas frequently exhibit a developmental gradient within the inflorescence. Upper florets may possess a well-developed geniculate awn inserted at the base or on the lower half of the lemma; mid-inflorescence spikelets may have a shorter, possibly non-geniculate awn inserted high on the lemma, while basal spikelets may possess only a terminal bristle on the lemma. This phenomenon is particularly sharply shown in Agrostis castellana, where a single side branch of only a dozen or so spikelets can show the whole sequence. When using the key, it is advised to examine spikelets from the upper parts of an inflorescence. Many species key more than once, due to the potential for awns to be either present or absent.

SELECTED REFERENCES Björkman, S.O. 1960. Studies in Agrostis and related genera. Symb. Bot. Upsal. 17:1–112; Carlbom, C.G. 1967. A biosystematic study of some North American species of Agrostis L. and Podagrostis (Griesb.) Scribn. & Merr. Ph.D. dissertation, Oregon State University, Corvallis, Oregon, U.S.A. 232 pp.; Edgar, E. 1995. New Zealand species of Deyeuxia P. Beauv. and Lachnagrostis Trin. (Gramineae: Aveneae). New Zealand J. Bot. 33:1–33; Edgar, E. and H.E. Connor. 2000. Flora of New Zealand, vol. 5. Manaaki Whenua Press, Lincoln, New Zealand. 650 pp.; Hitchcock, A.S. 1951. Manual of the Grasses of the United States, ed. 2, rev. A. Chase. U.S.D.A. Miscellaneous Publication No. 200. U.S. Government Printing Office, Washington, D.C., U.S.A. 1051 pp.; Jacobs, S.W.L. 2001. The genus Lachnagrostis (Gramineae) in Australia. Telopea 9:439–448; Romero García, A.T., G. Blanca Lopez, and C. Morales Torres. 1988. Revisión del género Agrostis L. (Poaceae) en la Península Ibérica. Ruizia 7:1–160; Rúgolo de Agrasar, Z.E. 1982. Revalidación del género Bromidium Nees et Meyen emend. Pilger (Gramineae). Darwiniana 24:187–216; Rúgolo de Agrasar, Z.E. and A.M. Molina. 1997. Las especies del género Agrostis L. (Gramineae: Agrostideae) de Chile. Gayana, Bot. 54:91–156; Soreng, R.J. 2003. Podagrostis. Contr. U.S. Natl. Herb. 48:581; Tercek, M.T., D.P. Hauber, and S.P. Darwin. 2003. Genetic and historical relationships among thermally adapted Agrostis (Bentgrass) of North America and Kamchatka: Evidence for a previously unrecognized, thermally adapted taxon. Amer. J. Bot. 90:1306–1312; Tsvelev, N.N. 1976. Zlaki SSSR. Nauka, Leningrad [St. Petersburg], Russia. 788 pp.