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 involure, 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.

Distribution

Conn., N.J., N.Y., N.Mex., Wash., Va., W.Va., Del., D.C, Wis., Ariz., Mont., Oreg., Wyo., Pacific Islands (Hawaii), Md., Mass., Maine, N.H., R.I., Vt., Fla., Tex., La., Utah, Puerto Rico, N.Dak., Nebr., Tenn., N.C., S.C., Pa., Okla., Calif., Nev., Colo., Miss., Alaska, Ala., Kans., S.Dak., Ind., Ark., Ill., Ga., Iowa, Ohio, Idaho, Alta., B.C., Greenland, Man., N.B., Nfld. And Labr., N.S., N.W.T., Nunavut, Ont., P.E.I., Que., Sask., Yukon, Mo., Minn., Mich., Ky.

Discussion

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; Rugolo 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 (p. 788), Calamagrostis (p. 706), or Polypogon (p. 662). 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.

References

Key

1 Paleas at least 2/5 as long as the lemmas. > 2
2 Lemmas 0.5-0.8 mm long, transparent; paleas similar to the lemmas and almost as long; panicles usually over 1/2 the length of the culms, extremely diffuse Agrostis nebulosa
2 Lemmas 1.2-2.5 mm long, opaque to translucent; paleas shorter than the lemmas; panicles less than 1/2 the length of the culms, diffuse or not. > 3
3 Panicles narrowly contracted, sometimes open at anthesis, 0.5-4(6) cm wide; branches ascending to appressed. > 4
4 Stolons present, plants mat-forming, rhizomes absent; paleas 0.7-1.4 mm long; anthers 0.9-1.4 mm long Agrostis stolonifera
4 Stolons absent, plants usually cespitose, rhizomes sometimes present; paleas to 0.5 mm long; anthers 0.3-0.6 mm long Agrostis exarata
3 Panicles open at maturity, sometimes somewhat contracted after anthesis, (1)2-15 cm wide; branches spreading to ascending. > 4
5 Ligules of the upper leaves longer than wide, 2-7.5 mm long; usually at least some lower panicle branches with spikelets to the base. > 6
6 Stolons absent, rhizomes present; culms 20-120 cm tall; panicles 8-30 cm long; longest lower panicle branches 4-9 cm long Agrostis gigantea
6 Stolons present, rhizomes absent; culms 8-60 cm tall; panicles 3-20 cm long; longest lower panicle branches 2-6 cm long Agrostis stolonifera
5 Ligules of the upper leaves usually shorter than wide, 0.3-3 mm long; lower panicle branches with spikelets confined to the distal 1/3-1/2. > 6
7 Calluses glabrous or with a few hairs to 0.1 mm long; adjacent pedicels divergent, giving well-separated spikelets; panicles stiffly erect, 3-20 cm long; awns rarely present, to 2 mm; lemmas glabrous Agrostis capillaris
7 Calluses abundantly hairy, hairs to 0.6 mm long; adjacent pedicels not divergent, spikelets appearing clustered; panicles somewhat lax, 10-30 cm long; awns, if present, to 5 mm long on the terminal spikelet of a cluster; lemmas occasionally with hairs on the lower 1/2 Agrostis castellana
1 Paleas absent or less than 1/2 as long as the lemmas. > 2
8 Lemmas awned [for opposite lead, see p. 637]. > 9
9 Panicles dense, often spikelike, 0.2-4 cm wide; lower branches usually shorter than 2 cm, appressed to ascending, usually hidden by the spikelets. > 10
10 Lemma awns 3.5-10 mm long; calluses with hairs to 1 mm long; plants annual. > 11
11 Lemmas 2.5-4 mm long, teeth to 1.5 mm long; awns (5)8-10 mm long; blades 1-4.5 cm long Agrostis hendersonii
11 Lemmas 1.5-2.3 mm long, teeth to 0.5 mm long; awns 3.5-8 mm long; blades 3-15 cm long Agrostis microphylla
10 Lemma awns to 3.5 mm long; calluses glabrous or with hairs to 0.3 mm long; plants perennial. > 11
12 Blades less than 2 mm wide, usually involute or folded. > 13
13 Calluses glabrous; panicles often partly enclosed by the upper sheaths at maturity; lemma awns to 0.7 mm long Agrostis blasdalei
13 Calluses hairy; panicles exserted from the upper sheaths at maturity; lemma awns to 3.5 mm long. > 14
14 Lemma apices acute, entire; lemma awns to 2.8 mm long, usually not exserted from the spikelets. Agrostis variabilis
14 Lemma apices truncate, denticulate; lemma awns 2-3.5 mm long, exserted from the spikelets Agrostis tolucensis
12 Blades 2-10 mm wide, usually flat, sometimes folded. > 13
15 Lemma apices truncate to acute; blades to 4 mm wide, flat or involute; ligules 2-6.2 mm long; panicles 0.5-1.5 cm wide Agrostis tolucensis
15 Lemma apices acute to obtuse; blades to 10 mm wide, flat; ligules 1-11.2 mm long; panicles 0.5-4 cm wide. > 16
16 Anthers 0.3-0.6 mm long; paleas usually absent, rarely to 0.5 mm long and about 1/5 the length of the lemmas; lemmas entire or with teeth to 0.12 mm long Agrostis exarata
16 Anthers 0.5-2 mm long; paleas 0.3-0.7 mm long, to about 1/3 the length of the lemmas; lemmas usually with teeth to 0.3 mm long Agrostis densiflora
9 Panicles open or diffuse, or somewhat contracted but not spikelike, 0.4-20 cm wide; lower branches 1.5-12 cm long, erect to spreading, readily visible. > 10
17 Leaves usually involute or becoming so, sometimes only the basal leaves involute, less than 1 mm in diameter when involute, 0.5-2 mm wide when flat; plants without rhizomes or stolons. > 18
18 Anthers 1, 0.1-0.2 mm long, usually persistent at the apices of the caryopses; awns attached just below the apices of the lemmas, flexuous but not geniculate, deciduous Agrostis elliottiana
18 Anthers 3, 0.4-1.5 mm long, usually shed at anthesis; awns attached below midlength on the lemmas, usually geniculate, persistent. 19. Basal leaves usually withered at anthesis; lower sheaths finely tomentose; callus hairs abundant; plants endemic to coastal California Agrostis hooveri
19 Basal leaves persistent; lower sheaths smooth or scabrous; callus hairs sparse; plants widespread, especially in northern and montane parts of the Flora region, including California. > 20
20 Panicles (2)3-10 cm long; branches not capillary, fairly stiff, smooth or sparsely scabridulous; callus hairs to 0.4 mm long; caryopses 1.4-2 mm long, endosperm solid Agrostis mertensii
20 Panicles (4)8-25(50) cm long; branches capillary, flexible, scabrous; callus hairs to 0.2 mm long; caryopses 0.9-1.4 mm long; endosperm liquid Agrostis scabra
17 Leaves usually remaining flat, 0.5-6 mm wide; plants with or without rhizomes or stolons. > 18
21 Lemmas with 4 teeth up to 0.5 mm long, lemmas 2.5-3 mm long; awns 4-6 mm long Agrostis howellii
21 Lemmas usually entire, sometimes minutely toothed or erose, teeth to 0.4 mm long, lemmas 1-3 mm long; awns to 5 mm long. > 22
22 Rhizomes and stolons absent; blades to 30 cm long; anthers 0.4-1.2 mm long. > 23
23 Panicle branches widely divergent, the whole panicle often detaching at the base at maturity, forming a tumbleweed; cauline nodes usually 1-3; blades 1-2 mm wide; glume apices acuminate Agrostis scabra
23 Panicle branches usually erect to ascending, if widely divergent then the panicle not forming a tumbleweed; cauline nodes 2-10; blades 0.5-5 mm wide; glume apices acute to acuminate. > 24
24 Lemma awns 1-4.4 mm long, geniculate, exserted; blades to 13 cm long. > 25
25 Leaf blades 0.5-3 mm wide, flat to involute; panicles 2-10 cm long, usually open; awns 2-4.4 mm long, inserted just below midlength on the lemmas Agrostis mertensii
25 Leaf blades 3-4 mm wide, flat; panicles 6-20 cm long, somewhat contracted; awns 1-1.5 mm long, inserted just above midlength on the lemmas Agrostis anadyrensis
24 Lemma awns minute or to 2 mm long, straight, usually not exserted; blades 6-30 cm long. > 25
26 Basal leaves usually withered by anthesis; culm leaves 3-10, as broad and substantial as the lower leaves; callus hairs dense; plants primarily from east of the 100th Meridian Agrostis perennans
26 Basal leaves persisting; culm leaves 5 or fewer, usually less substantial than the lower leaves; callus hairs sparse; plants primarily western Agrostis oregonensis
22 Rhizomes or stolons present; blades 1-10 cm long; anthers 0.7-1.8 mm long. > 23
27 Rhizomes absent; stolons present, to about 25 cm long, producing tufts of shoots at the nodes; glumes 1.7-3 mm long; panicles open, branches erect to spreading Agrostis canina
27 Rhizomes present, to about 10 cm long; stolons absent; glumes 2-4 mm long; panicles open to constricted, branches more or less erect to ascending. > 28
28 Lemma apices blunt, entire; lemmas usually awned from near the base, rarely unawned, awns 2-4.5 mm long, geniculate Agrostis vinealis
28 Lemma apices acute, entire or toothed; lemmas usually unawned, rarely awned from below the apices, awns to 0.5(2.7) mm long, straight Agrostis pallens
8 Lemmas unawned [for opposite lead, see p. 635]. > 9
29 Mature panicles dense; lower panicle branches to 3(4) cm long, often hidden by the spikelets; spikelets crowded. > 30
30 Blades 0.5-2 mm wide, in dense basal tufts; panicles 0.2-2 cm wide; culms 5-30 cm tall. > 31
31 Lemma veins not excurrent; anthers 0.4-1 mm long; plants of western alpine and subalpine zones Agrostis variabilis
31 Lemma veins excurrent to 0.2 mm; anthers 0.7-2 mm long; plants of western coastal cliffs, dunes, and shrublands Agrostis blasdalei
30 Blades 2-10 mm wide, not basally concentrated; panicles 0.5-4 cm wide; culms 8-100 cm tall. > 31
32 Anthers 0.3-0.6 mm long; paleas usually absent, rarely to 0.5 mm long and about 1/5 the length of the lemmas; lemmas entire or with teeth to 0.12 mm long Agrostis exarata
32 Anthers 0.5-2 mm long; paleas 0.3-0.7 mm long, to about 1/3 the length of the lemmas; lemmas usually toothed, teeth to 0.3 mm long Agrostis densiflora
29 Mature panicles open to diffuse; lower panicle branches often longer than 3 cm, usually not hidden by the spikelets; spikelets crowded or not. > 30
33 Blades 0.5-14 cm long, 0.5-2 mm wide, usually involute or becoming so; anthers 0.1-0.9 mm long. > 34
34 Anthers 1, 0.1-0.2 mm long; callus hairs dense, to 0.6 mm long; plants annual Agrostis elliottiana
34 Anthers 3, 0.2-0.9 mm long; callus hairs sparse, to 0.3 mm long; plants perennial or annual. > 35
35 Panicles (4)8-50 cm long; lower panicle branches 4-15 cm long. > 36
36 Lemmas 1.4-2 mm long, exceeding the ripe caryopses by 0.3+ mm; anthers 0.4-0.8 mm long; pedicels to 9.6 mm long, spikelets not appearing clustered Agrostis scabra
36 Lemmas 0.8-1.2 mm long, exceeding the ripe caryopses by no more than 0.2 mm; anthers 0.2-0.5 mm long; pedicels to 3.5 mm long, spikelets appearing clustered Agrostis hyemalis
35 Panicles 1.5-13 cm long; lower panicle branches 1-4 cm long. > 36
37 Anthers 0.3-0.6 mm long; upper culm sheaths not inflated; plants perennial, of western seepage areas and bogs Agrostis idahoensis
37 Anthers 0.5-0.9 mm long; upper culm sheaths inflated; plants annual, near hot springs Agrostis rossiae
33 Blades 1-30 cm long, 1-7.5 mm wide, usually flat; anthers 0.3-2.3 mm long. > 34
38 Rhizomes present, to 50 cm long, stolons absent; panicle branches branching from midlength or to near the base; lower panicle branches 1-5 cm long; anthers 0.7-2.3 mm long. > 39
39 Lemma apices blunt, entire; callus hairs sparse, to 0.1 mm long; panicles 2-15 cm long; pedicels 0.5-2 mm long; blades 1-3 mm wide Agrostis vinealis
39 Lemma apices usually acute, entire or toothed, teeth to about 0.2 mm long; callus hairs sparse or abundant, to 2 mm long; panicles 5-22 cm long; pedicels 0.5-7 mm long; blades 1-6 mm wide. > 40
40 Anthers 0.7-1.8 mm long; callus hairs to 0.3(1) mm long, sparse; leaf blades 1.5-11.5 cm long; caryopses 1-1.5 mm long Agrostis pallens
40 Anthers 1.5-2.3 mm long; callus hairs 0.8-2 mm long, abundant; leaf blades 6-20 cm long; caryopses 1.5-2 mm long Agrostis hallii
38 Rhizomes absent, stolons sometimes present, to 25 cm long; panicle branches mostly branching at or beyond midlength; lower panicle branches 1-12 cm long; anthers 0.3-1.5 mm long. > 39
41 Stolons present, to about 25 cm long, producing tufts of shoots at the nodes; anthers 1-1.5 mm long; blades 1-3 mm wide; glume apices acute Agrostis canina
41 Stolons absent; anthers 0.3-1.2 mm long; blades 0.5-7 mm wide; glume apices acute to acuminate. > 42
42 Blades to 2 mm wide, 1-14 cm long, flat or involute; leaves mostly basal. > 43
43 Lower panicle branches 4-12 cm long; whole panicle often detaching at the base at maturity, forming a tumbleweed; blades 4-14 cm long Agrostis scabra
43 Lower panicle branches 1-4 cm long; panicle not detaching at maturity and forming a tumbleweed; blades 1-7 cm long Agrostis idahoensis
42 Blades to 7 mm wide, usually at least some wider than 2 mm, 5-30 cm long, flat; leaves mostly cauline to mostly basal. > 43
44 Plants annual or short-lived perennials; glumes 1.5-2.8 mm long, subequal; lemmas smooth and glabrous; anthers 0.3-0.6 mm long; caryopses 0.9-1.3 long Agrostis clavata
44 Plants perennial; glumes 1.8-3.6 mm long, unequal; lemmas smooth or scabridulous, sometimes pubescent; anthers 0.4-1.2 mm long; caryopses 1-1.9 mm long. > 45
45 Basal leaves usually withered by anthesis; cauline nodes 3-10; blades of the upper leaves as broad and substantial as those of the lower leaves; callus hairs abundant; plants primarily from east of the 100th Meridian Agrostis perennans
45 Basal leaves persisting to anthesis; cauline nodes 5 or fewer; blades of the upper leaves usually less substantial than those of the lower leaves; callus hairs sparse; plants primarily western Agrostis oregonensis