Plants usually perennial; herbaceous, usually cespitose, occasionally rhizomatous. Culms 10-150 cm, not woody, sometimes branched above the base; internodes usually pith-filled, sometimes hollow. Leaves sometimes predominantly basal, sometimes predominantly cauline; sheaths open; auricles lacking; ligules of hairs or very shortly membranous and long-ciliate, the 2 types generally indistinguishable. Inflorescences terminal, usually panicles, sometimes racemes, occasionally spikes; primary branches without axillary pulvini and usually appressed to ascending, or with axillary pulvini and ascending to strongly divergent or divaricate. Spikelets with 1 floret; rachillas not prolonged beyond the florets; disarticulation above the glumes. Glumes often longer than the florets, thin, usually 1-3-veined, acute to acuminate; florets terete or weakly laterally compressed; calluses well-developed, hirsute; lemmas fusiform, 3-veined, convolute, usually glabrous or scabridulous, usually enclosing the palea at maturity, usually with 3 terminal awns, lateral awns reduced or obsolete in some species, lemma apices sometimes narrowed to a straight or twisted beak below the awns; awns ascending to spreading, usually straight, bases sometimes twisted together into a column or the bases of the individual awns coiled, twisted, or otherwise contorted, occasionally disarticulating at maturity; paleas shorter than the lemmas, 2-veined, occasionally absent; anthers 1 or 3. Caryopses fusiform; hila linear, x = 11, 12.

Distribution

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

Discussion

Aristida is a tropical to warm-temperate genus of 250-300 species. It grows throughout the world in dry grasslands and savannahs, sandy woodlands, arid deserts, and open, weedy habitats and on rocky slopes and mesas. All 29 species in this treatment are native to the Flora region.

The divergent awns aid in wind and animal transportation of the florets and, by holding the florets and the caryopses they contain at an angle to the ground, in establishment. The presence of Aristida frequently indicates soil disturbance or abuse. Although generally poor forage grasses and, because of the calluses, potentially harmful to grazing animals, some species of Aristida are an important source of spring forage on western rangelands. Quail and small mammals eat small amounts of the seed.

Selected References

Allred, K.W. 1984. Morphologic variation and classification of the North American Aristida purpurea complex (Graraineae). Brittonia 36:382-395

Allred, K.W. 1984, 1985, 1986. Studies in the genus Aristida (Gramineae) of the southeastern United States. Rhodora 86:73-77, 87:137-145, 145-155, 88:367-387

Henrard, J.T. 1926, 1927, 1928, 1933. A critical revision of the genus Aristida. Meded. Rijks-Herb. 54:1-701; 55G703-747

Henrard, J.T. 1929, 1933. A monograph of the genus Aristida. Meded. Rijks-Herb. 58:1-325

Kesler, T.R. 2000. A taxonomic reevaluation of Aristida striata (Poaceae) using anatomy and morphology. Master's thesis. Florida State University, Tallahassee, Florida, U.S.A. 33 pp.

Peet, R.K. 1993. A taxonomic study of Aristida striata and A. beyricbiana. Rhodora 95:25-37

Reeder, J.R. and R.S. Felger. 1989. The Aristida californica-glabrata complex (Gramineae). Madrono 36:187-197

Trent, J.S. and K.W. Allred. 1990. A taxonomic comparison of Aristida ternipes and Aristida hamulosa (Gramineae). Sida 14:251-261

Vaughn, J.M. 1981. Systematics of Aristida dichotoma, basiramea, and curtissii (Poaceae). Master's thesis. University of Oklahoma, Norman, Oklahoma, U.S.A. 38 pp.

Walters, T.E., D.S. Decker-Walters, and D.R. Gordon. 1994. Restoration considerations for wiregrass (Aristida striata): Allozymic diversity of populations. Conservation Biol. 8:581-585.

Lower Taxa

Aristida adscensionis, Aristida arizonica, Aristida basiramea, Aristida californica, Aristida condensata, Aristida desmantha, Aristida dichotoma, Aristida divaricata, Aristida floridana, Aristida gypsophila, Aristida gyrans, Aristida havardii, Aristida lanosa, Aristida longespica, Aristida mohrii, Aristida oligantha, Aristida palustris, Aristida pansa, Aristida patula, Aristida purpurascens, Aristida purpurea, Aristida ramosissima, Aristida rhizomophora, Aristida schiedeana, Aristida simpliciflora, Aristida spiciformis, Aristida stricta, Aristida ternipes, Aristida tuberculosa

Key

1	Lower glumes 3-7-veined.	> 2
2	Awns nearly equal, the lateral awns 8-66 mm long and at least 3/4 as long as the central awns.	Aristida oligantha
2	Awns markedly unequal, the lateral awns 1-4 mm long, no more than 1/2 as long as the central awns, sometimes absent.	> 3
3	Plants annual; inflorescences 5-12 cm long, 2-4 cm wide	Aristida ramosissima
3	Plants perennial; inflorescences 10-30 cm long, 4-26 cm wide	Aristida schiedeana
1	Lower glumes 1-2(3)-veined.	> 4
4	Central awns spirally coiled at the base.	> 5
5	Lateral awns 1-4 mm long, erect	Aristida dichotoma
5	Lateral awns 5-13 mm long, spreading	Aristida basiramea
4	Central awns straight to curved, sometimes loosely contorted but not spirally coiled, at the base.	> 6
6	Lateral awns markedly reduced, usually 1/3 or less as long as the central awns.	> 7
7	Panicles 1-6 cm wide, the branches erect-appressed to strongly ascending, without axillary pulvini or the pulvini only weakly developed.	> 8
8	Plants annual; culms often highly branched above the base.	> 9
9	Awns flattened at the base	Aristida adscensionis
9	Awns terete at the base.	> 10
10	Lemmas 2.5-10 mm long; central awns curving up to 100° at the base	Aristida longespica
10	Lemmas 8-22 mm long; central awns with a semicircular bend at the base	Aristida ramosissima
8	Plants perennial; culms rarely branched above the base.	> 11
11	Collars densely pilose, the hairs 1-3 mm, often densely tangled and deflexed; blades usually tightly involute, about 0.5 mm in diameter	Aristida gypsophila
11	Collars mostly glabrous or with straight hairs, often with long hairs at the sides; blades usually flat to loosely involute, sometimes tightly involute.	> 12
12	Lateral awns absent; panicle branches spikelet-bearing to the base; plants of the Florida keys	Aristida floridana
12	Lateral awns usually present, varying from much shorter than to equaling the central awns; panicle branches sometimes naked near the base; plants rarely found east of the Mississippi and not known at all from Florida.	> 13
13	Primary panicle branches 3-6 cm long; lateral awns (1)8-140 mm long	Aristida purpurea
13	Primary panicle branches 6-16 cm long; lateral awns absent or to 1(3) mm long	Aristida schiedeana
7	Panicles 6-45 cm wide, at least the lower branches spreading and having well-developed axillary pulvini.	> 14
14	Lateral awns absent or no more than 3 mm long.	> 15
15	Central awns often deflexed at a sharp angle when mature; lemma apices often twisted at maturity.	> 16
16	Blades usually flat, sometimes folded, 1-2 mm wide; plants of juniper, oak, or pine woodlands	Aristida schiedeana
16	Blades usually tightly involute, about 0.5 mm in diameter; plants of thorn-scrub deserts	Aristida gypsophila
15	Central awns usually straight or arcuate; lemma apices not twisted.	> 17
17	Panicle branches spikelet-bearing from the base; lower glumes longer than the upper glumes	Aristida floridana
17	Panicle branches usually naked at the base; lower glumes about equal to the upper glumes	Aristida ternipes
14	Lateral awns 3-23 mm long.	> 18
18	Anthers 0.8-1 mm long.	> 19
19	Spikelets usually divergent and the pedicels with axillary pulvini; secondary branches usually absent; primary branches 2-6 cm long; lemma apices with 0-2 twists when mature	Aristida havardii
19	Spikelets usually appressed and the pedicels without axillary pulvini; secondary branches usually well-developed; primary branches 5-13 cm long; lemma apices with 4 or more twists when mature	Aristida divaricata
18	Anthers 1.2-3 mm long.	> 20
20	Collars glabrous or strigillose; blades with scattered hairs 1.5-3 mm long above the ligule on the adaxial surface; lower glumes about equal to or slightly shorter than the upper glumes	Aristida ternipes
20	Collars pubescent, with hairs 0.2-0.8 mm long; blades glabrous, sometimes scabridulous, above the ligule on the adaxial surface; lower glumes slightly longer than the upper glumes	Aristida patula
6	Lateral awns well-developed, usually at least 1/2 as long as the central awns.	> 21
21	Blades tightly involute, the adaxial surfaces densely scabrous or densely short-pubescent	Aristida stricta
21	Blades flat or folded and lax, or, if involute, the adaxial surfaces neither densely scabrous nor densely short pubescent.	> 22
22	Rachis nodes and leaf sheaths usually lanose or floccose, sheaths occasionally glabrous	Aristida lanosa
22	Rachis nodes glabrous, scabrous, or with straight hairs; leaf sheaths glabrous, pilose, or floccose.	> 23
23	Junction of the lemma and awns evident; awns disarticulating at maturity.	> 24
24	Plants perennial.	> 25
25	Culms 45-100 cm tall; culms unbranched or sparingly branched; blades 12-28 cm long	Aristida spiciformis
25	Culms 10-40 cm tall; culms much branched; blades usually less than 6 cm long	Aristida californica
24	Plants annual.	> 26
26	Awns divergent but not arcuate or entwined above the column; cauline internodes pubescent or glabrous	Aristida californica
26	Awns strongly arcuate, often entwined above the column or no column present; cauline internodes glabrous.	> 27
27	Glumes 10-17 mm long; lemmas beaked, the beak 2-7 mm long; awns not forming a column; calluses 1-2.5 mm long	Aristida desmantha
27	Glumes 20-30 mm long; lemmas not beaked; awns forming a column 8-15 mm long; calluses 3-4 mm long	Aristida tuberculosa
23	Junction of the lemma and awns not evident; awns not disarticulating at maturity. 28. Lemmas terminating in a beak 7-30 mm long; upper glumes awned, the awns 10-12 mm long	Aristida spiciformis
28	Lemmas not beaked or with a beak less than 7 mm long; upper glumes unawned or with an awn to 6 mm long.	> 29
29	At least the lower primary panicle branches divergent and with axillary pulvini.	> 30
30	Lateral awns about 1/2 as thick as the central awns	Aristida patula
30	Lateral awns nearly as thick as the central awns.	> 31
31	Panicles narrow and contracted above, usually only the lower 1-2 branches spreading and with a pulvinus; lemma apices 0.2-0.3 mm wide	Aristida purpurea
31	Almost all panicle branches spreading and with axillary pulvini; lemma apices 0.1-0.2 mm wide.	> 32
32	Anthers 0.8-1 mm long.	> 33
33	Spikelets usually divergent and the pedicels with axillary pulvini; secondary branches absent or nearly so; primary branches 2-6 cm long; lemma apices straight or with 1 or 2 twists	Aristida havardii
33	Spikelets usually appressed and the pedicels without axillary pulvini; secondary branches usually well-developed; primary branches 5-13 cm long; lemma apices with 4 or more twists at maturity	Aristida divaricata
32	Anthers 1-3 mm long.	> 34
34	Base of the blades with scattered hairs 1.5-3 mm long on the adaxial surfaces	Aristida ternipes
34	Base of the blades glabrous or puberulent on the adaxial surface, the hairs, if present, less than 0.5 mm long.	> 35
35	Glumes reddish, the lower glumes often shorter than the upper glumes; awns ascending to divaricate, (8)13-140 mm long; terminal spikelets usually appressed and without axillary pulvini	Aristida purpurea
35	Glumes brownish, equal or unequal; awns spreading to horizontal, 5-15 mm long; terminal spikelets often spreading from axillary pulvini	Aristida pansa
29	Lower primary panicle branches (pedicels in racemose species) appressed, without axillary pulvini.	> 36
36	Plants with well-developed rhizomes; basal sheaths shredding into threadlike segments at maturity	Aristida rhizomophora
36	Plants tufted, without rhizomes; basal sheaths not fibrous, not shredding into threadlike segments even when old.	> 37
37	Lower inflorescence nodes with only 1 spikelet; inflorescences spicate or racemose	Aristida mohrii
37	Lower inflorescence nodes with 2 or more spikelets; inflorescences racemose or paniculate.	> 38
38	Lower glumes usually 1/3– 3/4 as long as the upper glumes.	> 39
39	Plants annual	Aristida adscensionis
39	Plants perennial.	> 40
40	Lemma awns 8-15 mm long; lemmas 5-7 mm long	Aristida gyrans
40	Lemma awns (8)15-140 mm long; lemmas 6-16 mm long	Aristida purpurea
38	Lower glumes usually more than 3/4 as long as the upper glumes.	> 41
41	Plants annual.	> 42
42	Awns flat at the base	Aristida adscensionis
42	Awns terete at the base	Aristida longespica
41	Plants perennial.	> 43
43	Lemma apices prominently twisted for 3-6 mm; blades usually curled at maturity; leaves forming a basal tuft	Aristida arizonica
43	Lemma apices straight or only slightly twisted; blades usually not curled at maturity; leaves variously distributed.	> 44
44	Lower glumes prominently 2-keeled, (7.5)9-13 mm long; central awns 15-40 mm long	Aristida palustris
44	Lower glumes usually 1-keeled, if 2-keeled, 5-9 mm long; central awns 8-25 mm long, [revert to left]	> 45
45	Central awns about twice as thick as the lateral awns, divergent to arcuate-reflexed.	> 46
46	All 3 awns divergent to reflexed and contorted at the base; lower rachis nodes usually associated with 2 spikelets (occasionally 1 or 3), 1 pedicellate and 1 sessile	Aristida simpliciflora
46	Lateral awns usually erect to ascending and not contorted at the base; lower rachis nodes usually associated with more than 2 spikelets, pedicellate to subsessile	Aristida purpurascens
45	Central awns about the same thickness as the lateral awns, erect to spreading.	> 47
47	Lower glumes 1-4 mm longer than the upper glumes	Aristida purpurascens
47	Lower glumes from shorter than to 1 mm longer than the upper glumes.	> 48
48	Culms usually 3-6 mm thick at the base; primary panicle branches 4-20 cm long; lower glumes 1-veined	Aristida condensata
48	Culms usually 1-4 mm thick at the base; primary panicle branches 1-5 cm long; lower glumes 1-2-veined.	> 49
49	Calluses 0.4-0.8 mm long	Aristida purpurascens
49	Calluses 1-2 mm long	Aristida gyrans