10.12 NASSELLA (Trin.) E. Desv.
Mary E. Barkworth

Plants usually perennial, rarely annual; cespitose, occasionally rhizomatous. Culms 10–175(210) cm, sometimes branched at the upper nodes, branches flexible; prophylls not evident, shorter than the sheaths. Leaves mostly basal, not overwintering; sheaths open; cleistogenes sometimes present; auricles absent; ligules membranous, sometimes pubescent or ciliate; blades of basal leaves 3–60 cm long, 0.2–8 mm wide, apices narrowly acute to acute, not sharp, flag leaf blades 1–80 mm, bases about as wide as the top of the sheaths. Inflorescences terminal panicles, sometimes partially included at maturity. Spikelets 3–22 mm, with 1 floret; rachillas not prolonged beyond the base of the floret; disarticulation above the glumes, beneath the floret. Glumes longer than the floret, narrowly lanceolate or ovate, basal portion usually purplish at anthesis, color fading with age, (1)3–5-veined, sometimes awned; florets usually terete, sometimes slightly laterally compressed; calluses blunt or sharp, glabrous or antrorsely strigose; lemmas usually papillose or tuberculate, at least distally, sometimes smooth throughout, glabrous or variously hairy, strongly convolute, wrapping 1.2–1.5 times around the caryopses, apices not lobed, fused distally into crowns, these often evident by their pale color and constricted bases; crowns mostly glabrous, rims often bearing hairs with bulbous bases; awns terminal, centric or eccentric, deciduous or persistent, usually twice-geniculate, second geniculation often obscure; paleas up to 1/2 as long as the lemmas, glabrous, without veins, flat; lodicules 2 or 3, if 3, the third somewhat shorter than the other 2; anthers 1 or 3, if 3, often of 2 lengths, penicillate; ovaries glabrous; styles 2, bases free. Caryopses glabrous, not ribbed; hila elongate; embryos to 2/5 as long as the caryopses. x = 7, 8. Name not explained by Desvaux (1854), but possibly from the Latin nassa, a narrow-necked basket for catching fish.

Nassella used to be interpreted as a South American genus of approximately 14 species. It is now interpreted as including at least 116 species (Barkworth and Torres 2001), the majority of which are South American. The additional species were previously included in Stipa. There are eight species in the Flora region, one of which is introduced; two additional species treated here were found in the region at one time, but have not become established. The strongly convolute lemmas distinguish Nassella from all other genera of Stipeae in the Americas and, in combination with the reduced, ecostate, glabrous paleas, from all other genera in the tribe worldwide. Molecular data (Jacobs et al. 2006) support the expanded interpretation of Nassella. Relationships among the species have not been explored.

Many species of Nassella develop both cleistogamous and chasmogamous florets in the terminal panicle. The cleistogamous florets have 1–3 anthers that are less than 1 mm long; the chasmogamous florets have 3 anthers that are significantly longer. In addition, some species develop panicles in the axils of their basal sheaths. Spikelets of cleistogenes have reduced or no glumes, and florets with no or very short awns.

SELECTED REFERENCES Barkworth, M.E. 1990. Nassella (Gramineae: Stipeae): Revised interpretation and nomenclatural changes. Taxon 39:597–614; Barkworth, M.E. 1993. Nassella. Pp. 1274–1276 in J.C. Hickman (ed.). The Jepson Manual: Higher Plants of California. University of California Press, Berkeley and Los Angeles, California, U.S.A. 1400 pp.; Barkworth, M.E. and M.A. Torres. 2001. Distribution and diagnostic characters of Nassella (Poaceae: Stipeae). Taxon 50:439–468; Brown, W.V. 1952. The relation of soil moisture to cleistogamy in Stipa leucotricha. Bot. Gaz. 113:438–444; Desvaux, E. 1854. Gramineas. Pp. 233–469 in C. Gay. Flora Chilena [Historia Fisica y Politica de Chile], vol. 6. Museo Historia Natural, Santiago, Chile. 551 pp. [1853 on title page; printed March 1854]; Dyksterhuis, E.J. 1949. Axillary cleistogenes in Stipa leucotricha and their role in nature. Ecology 26:195–199; Hamilton, J.G. 1997. Changing perceptions of pre-European grasslands in California. Madroño 44:311–333; Jacobs, S.W.L., R. Bayer, J. Everett, M.O. Arriaga, M.E. Barkworth, A. Sabin-Badereau, M.A. Torres, F. Vázquez, and N. Bagnall. 2006. Systematics of the tribe Stipeae using molecular data. Aliso 23:349–361; Jacobs, S.W.L., J. Everett, and M.E. Barkworth. 1995. Clarification of morphological terms used in the Stipeae (Gramineae), and a reassessment of Nassella in Australia. Taxon 44:33–41; Love, R.M. 1946. Interspecific hybridization in Stipa: I. Natural hybrids. Amer. Naturalist 80:189–192; Love, R.M. 1954. Interspecific hybridization in Stipa: II. Hybrids of S. cernua, S. lepida, and S. pulchra. Amer. J. Bot. 41:107–110.

For an interactive dichotomous key, click here; the interactive, multientry key is not yet available.

1. Florets 1.5–3 mm long; blades 0.2–1.5 mm wide, usually tightly convolute ... 2
1. Florets 3.4–13 mm long; blades 0.4–8 mm wide, flat to convolute ... 4
2. Florets widest about midlength; awns 45–100 mm long, almost centric ... N. tenuissima
2. Florets widest near the top; awns 7–35 mm long, eccentric ... 3
3. Awns 15–35 mm long; lemmas strongly tuberculate, particularly distally ... N. trichotoma
3. Awns 7–10 mm long; lemmas smooth ... N. chilensis
4. Terminal segment of the awns cernuous ... 5
4. Terminal segment of the awns straight ... 6
5. Awns 12–55 mm long, 0.1–0.2 mm thick at the base ... N. lepida
5. Awns 50–110 mm long, 0.2–0.3 mm thick at the base ... N. cernua
6. Florets 3.4–5.5 mm long; lemmas not constricted below the crown; awns 19–32 mm
long ... N. viridula
6. Florets 6–13 mm long; lemmas constricted below the crown; awns 30–120 mm long ... 7
7. Lemmas hairy between the veins at maturity ... N. pulchra
7. Lemmas glabrous between the veins at maturity ... 8
8. Crowns usually wider than long, the rims with hairs to 0.5 mm long; florets widest just below the crowns ... N. neesiana
8. Crowns usually longer than wide, the rims with hairs 1–2 mm long; florets widest near or slightly above midlength ... 9
9. Florets 6.5–13 mm long; crowns often flaring distally; plants native to Texas and adjacent states ... N. leucotricha
9. Florets 6–8 mm long; crowns more or less straight-sided; plants introduced, established in California ... N. manicata

1. Nassella neesiana (Trin. & Rupr.) Barkworth
Uruguayan Tussockgrass

Plants perennial; cespitose, not rhizomatous. Culms 30–140 cm tall, 1–1.8 mm thick, erect or geniculate, internodes glabrous; nodes usually 2–4, exposed, sericeous, hairs to 1.2 mm. Sheaths glabrous or slightly hispid, basal leaf sheaths often with cleistogenes; collars glab-rous, often brown or purple-tinged, with tufts of hair at the sides, hairs 0.5–1.2 mm; ligules 1–4 mm, glabrous or pubescent, truncate; blades to 30 cm long, 2–8 mm wide, flat to convolute, sometimes scabrous, sometimes with hairs. Terminal panicles 5–40 cm, exserted, erect to nodding; branches 2.5–8.5 cm, with 2–5 spikelets; pedicels 1–8 mm, angled, scabrous, pubescent. Glumes subequal, 10–22 mm long, 1.8–2.3 mm wide, narrowly lanceolate, glabrous, 3–5-veined; florets 6–13 mm long, 1–1.5 mm wide, terete, widest just below the crown; calluses 2–4.5 mm, sharp, strigose; lemmas often purple, finely rugose-papillose, particularly near the crown, constricted below the crown, midveins pilose proximally, glabrous between the veins at maturity; crowns 0.4–1.6 mm, usually wider than long, sides usually flaring somewhat distally, rims with hairs to 0.5 mm; awns 50–120 mm, clearly twice-geniculate, terminal segment straight; anthers 3–3.5 mm, penicillate. Caryopses 3–5 mm. 2n = 28.

Nassella neesiana is native to South America, growing from Ecuador to Argentina, primarily in steppe habitats. It was found on ballast dumps in Mobile, Alabama but has not persisted in the Flora region. It has become established in Australia, where it is considered a noxious weed.

2. Nassella leucotricha (Trin. & Rupr.) R.W. Pohl
Texan Nassella, Texan Needlegrass

Plants perennial; cespitose, not rhizomatous. Culms 33–120 cm tall, 1–1.2 mm thick, erect, internodes glabrous; nodes 3–5, usually pubescent. Sheaths often conspicuously hairy, sometimes glabrous, basal leaf sheaths often with cleistogenes; collars glabrous, often brown or purple-tinged, with tufts of hair at the sides, hairs 0.5–1 mm; ligules 0.2–1.2 mm, glabrous, truncate, sometimes longest at the sides; blades 10–35 cm long, 1–3.6 mm wide, flat to convolute, abaxial surfaces sparsely coarsely hairy, adaxial surfaces glabrous. Terminal panicles 5–55 cm, open; branches 3–7 cm, ascending or spreading, angled, scabrous, glabrous or hairy, with 1–4 spikelets; pedicels 4–9 mm. Glumes subequal, 10–21 mm long, 0.7–3.2 mm wide, narrowly lanceolate, glabrous, 3–5-veined; florets 6.5–13 mm long, 1.1–1.4 mm wide, terete, widest near or slightly above midlength; calluses 1–5.5 mm, sharp, strigose; lemmas straw-colored to light brown, papillose distally, constricted below the crown, veins pubescent proximally, particularly the midveins, glabrous between the veins; crowns 0.75–2 mm, usually longer than wide, often flaring distally, rims irregular, with 1–2 mm hairs; awns 40–90 mm, clearly twice-geniculate, terminal segment straight; anthers 3 and 3.5–5 mm in chasmogamous florets, 1–3 and shorter than 0.7 mm in cleistogamous florets. Caryopses about 4 mm. 2n = 28.

The range of Nassella leucotricha extends from the southern United States, where it was once one of the dominant species, into northern Mexico. It is now established in Australia (Jacobs et al. 1995). In North America, N. leucotricha grows mostly in open grasslands, but it is also found in woodlands. It provides good spring forage and increases in abundance with moderate grazing, primarily because of its cleistogenes. It resembles N. manicata, but has longer florets and less strongly developed crowns.

The sharp callus easily sticks to skin and clothing, and can cause wounds, especially in the mouths of grazing animals. The wounds often retain the fruit, which may require surgical removal for proper healing.

Individual plants produce both chasmogamous and cleistogamous florets in their terminal panicles, with the terminal florets usually being cleistogamous. The proportion of cleistogamous florets is influenced by soil moisture, a higher proportion being produced if the soil moisture content is low (Brown 1952). Spikelets of the axillary panicles usually mature before those of the terminal panicles (Dyksterhuis 1949).

3. Nassella manicata (E. Desv.) Barkworth
Andean Tussockgrass

Plants perennial; cespitose, not rhizomatous. Culms 40–80 cm tall, 1.5–2.5 mm thick, erect, internodes glabrous; nodes 2–3, pubescent. Sheaths glabrous; collars mostly glabrous, with tufts of hair at the sides, hairs 0.5–1 mm; ligules 0.1–0.3 mm, glabrous, truncate; blades 12–25 cm long, 1.5–2.5 mm wide, flat to convolute, pubescent. Panicles 10–20 cm, open; branches 1–3 cm, ascending to somewhat spreading, scabridulous, with 1–8 spikelets; pedicels 1–8 mm, pubescent. Glumes subequal, 10–15 mm long, 1.5–2 mm wide, narrowly lanceolate, 3-veined, keeled, keels scabrous; florets 6–8 mm long, 0.9–1.1 mm wide, terete, widest near or slightly above midlength; calluses 1.5–2.5 mm, sharp, strigose; lemmas papillose-tuberculate, constricted and purplish below the crown, midveins and exposed marginal veins pubescent over the proximal 2/3, glabrous between the veins at maturity; crowns about 1 mm long, about 0.5 mm wide, conspicuous, more or less straight-sided, purple, rims with 1–1.5 mm hairs; awns 30–50 mm, clearly twice-geniculate, terminal segment straight; anthers 3–4 mm in putatively chasmogamous florets, 0.3–0.5 mm in cleistogamous florets, both ranges sometimes present within a panicle. Caryopses about 3 mm. 2n = unknown.

Nassella manicata is native to Ecuador, Chile, Argentina, and Uruguay, growing on the foothills of the Andes Mountains. It is established in three California counties, growing in disturbed sites, including grazed meadows and old gold tailings. It has also been recorded from Mississippi; it is not known whether the Mississippi population has persisted.

Nassella manicata resembles N. leucotricha and N. pulchra. It differs from both in its shorter florets and more strongly developed crowns. It was misidentified as Nassella formicarum (Delile) Barkworth in the Jepson Manual (Barkworth 1993).

4. Nassella lepida (Hitchc.) Barkworth
Foothills Nassella, Foothills Needlegrass

Plants perennial; cespitose, not rhizomatous. Culms 35–100 cm tall, 0.7–1.2 mm thick, erect, bases sometimes decumbent, internodes glabrous, or the lower internodes pubescent just below the nodes or throughout, varying within a plant; nodes 3–4, pubescent. Sheaths glab-rous or coarsely hairy, sometimes scabridulous, margins glabrous; collars hairy, particularly towards the sides, hairs at the sides 0.2–0.5 mm; ligules 0.1–0.6 mm, glabrous, truncate to rounded; blades 12–23 cm long, 1–3.5 mm wide, flat to convolute, lax, abaxial surfaces scabridulous, adaxial surfaces coarsely hairy. Panicles 9–55 cm, open; branches 1–8 cm, ascending to spreading, with 1–6 spikelets; pedicels 1–5 mm. Glumes subequal, 5.5–15 mm long, 0.5–1 mm wide, narrowly lanceolate, glabrous, acuminate; florets 4–7 mm long, 0.5–0.7 mm wide, terete, widest near or slightly above midlength; calluses 0.4–1.6 mm, sharp, strigose; lemmas papillose, initially evenly pubescent, becoming glabrous between the veins at maturity, tapering to the crown; crowns 0.25–0.3 mm long, 0.15–0.2 mm wide, straight-sided, rims with 0.3–0.6 mm hairs; awns 12–55 mm long, 0.1–0.15 mm thick at the base, first geniculation distinct, second geniculation obscure, terminal segment cernuous; anthers 3, 2–2.5 mm, penicillate. Caryopses 3–4 mm. 2n = 34.

Nassella lepida usually grows on dry hillsides in chaparral habitats, from California into northern Mexico. It is most likely to be confused with N. cernua, but differs from that species in its shorter, thinner awns and more numerous spikelets. It occasionally hybridizes with N. pulchra.

5. Nassella pulchra (Hitchc.) Barkworth
Purple Nassella, Purple Needlegrass

Plants perennial; cespitose, not rhizomatous. Culms 35–100 cm tall, 1.8–3.1 mm thick, erect or geniculate at the lowest nodes, sometimes scabrous below the panicles, internodes mostly glabrous, lower internodes sometimes pubescent below the nodes; nodes 2–3, pubescent. Sheaths glabrous or hairy, sometimes mostly glabrous, sometimes the distal margins ciliate, varying within a plant; collars with tufts of hair at the sides, hairs 0.5–0.8 mm; ligules 0.3–1.2 mm, glabrous, truncate to rounded; blades 10–20 cm long, 0.8–3.5 mm wide, flat to convolute, abaxial surfaces glabrous or sparsely pilose. Panicles 18–60 cm, open; branches 3–9 cm, spreading, flexuous, often pilose at the axils, with 2–6 spikelets; pedicels 3–10 mm. Glumes subequal, 12–20 mm long, 1.1–2.2 mm wide, narrowly lanceolate, glabrous; florets 7.5–11.5 mm long, about 1.2 mm thick, terete; calluses 1.8–3.5 mm, sharp, strigose; lemmas papillose, evenly pubescent at maturity, constricted below the crown; crowns 0.6–1.1 mm long, 0.5–0.7 mm wide, straight-sided to slightly flared, rims with 0.8–0.9 mm hairs; awns 38–100 mm long, 0.3–0.45 mm thick at the base, strongly twice-geniculate, terminal segment straight; anthers 3.5–5.5 mm, penicillate. Caryopses 4.5–6 mm. 2n = 64.

Nassella pulchra grows in oak chaparral and grassland communities of the coast ranges and Sierra foothills of California, extending south into Mexico. It probably never formed extensive grasslands (Hamilton 1997), flourishing primarily in moderately disturbed areas. It resembles N. manicata, but has longer florets and less strongly developed crowns. Nassella pulchra and N. cernua sometimes hybridize.

6. Nassella cernua (Stebbins & Love) Barkworth
Cernuous Nassella, Nodding Needlegrass

Plants perennial; cespitose, not rhizomatous. Culms 30–100 cm tall, 1–2.2 mm thick, erect or geniculate at the basal nodes, internodes pubescent below the nodes, lowest internodes some-times pubescent throughout; nodes 2–3, glabrous. Leaves usually glaucous; sheaths mostly glabrous, throats ciliate; collars mostly glabrous, with sparse tufts of hair at the sides, hairs 1–1.6 mm; ligules 0.2–1.6 mm, glabrous, truncate to rounded; blades 3–26 cm long, 0.4–1.2 mm wide, flat to convolute, abaxial surfaces scabridulous, adaxial surfaces hairy. Panicles 15–80 cm, open, often partially enclosed at maturity; branches 1–6 cm, flexuous or cernuous, with 1–8 spikelets on the distal 1/2; pedicels 3–9 mm. Glumes glabrous, narrowly lanceolate, acuminate; lower glumes 12–22 mm long, 0.9–1.7 mm wide; upper glumes 3–4 mm shorter that the lower glumes; florets 4–9 mm long, 0.6–0.8 mm wide, terete; calluses 1.4–3.6 mm, sharp, strigose; lemmas minutely papillose, tapering to the crown, the proximal 1/4evenly pubescent, the distal 3/4 pubescent only over the veins; crowns 0.2–0.5 mm long, 0.3–0.35 mm wide, straight-sided, rims with 0.8–1.1 mm hairs; awns 50–110 mm long, 0.2–0.3 mm thick at the base, first geniculation evident, second geniculation obscure, terminal segment cernuous; anthers 3.5–5.5 mm, penicillate. Caryopses 4.5–5.5 mm. 2n = 70.

Nassella cernua grows in grasslands, chaparral, and juniper associations of the inner coast ranges of California and Baja California, Mexico. Small specimens resemble N. lepida, but have longer and thicker awns and fewer florets. Large specimens resemble N. pulchra, but have thinner awns with cernuous, rather than straight, terminal segments. It is superficially similar to Achnatherum eminens, but differs in its shorter ligules, strongly overlapping lemma margins, glabrous paleas, and geographic distribution.

7. Nassella tenuissima (Trin.) Barkworth
Fineleaved Nassella

Plants perennial; tightly ces-pitose, not rhizomatous. Culms 25–100 cm tall, 0.4–0.7(1.1) mm thick, usually erect, basal nodes sometimes geniculate, internodes mostly glabrous, pubescent just below the lower nodes; nodes 2–4, glabrous. Sheaths glabrous, even on the margins, sometimes scabridulous; collars glabrous, without tufts of hair at the sides; ligules 1–5 mm, glabrous, acute; blades 7–60 cm long, 0.2–1.5 mm wide, usually convolute, stiff, glabrous, scabridulous. Panicles 8–50 cm, loosely contracted, often partly enclosed at maturity; branches 2–8 cm, glabrous; pedicels 1–11 mm. Glumes subequal, 5–13 mm long, 0.5–1.2 mm wide, narrowly lanceolate, glabrous, aristate; florets (1.5)2.5–3 mm long, about 0.5 mm wide, widest at about midlength, somewhat laterally compressed; calluses 0.2–0.5 mm, blunt, strigose, hairs reaching to about 1/4–1/3 the length of the lemmas; lemmas finely tuberculate, rounded to the crown, midveins pubescent on the proximal 1/2; crowns 0.1–0.2 mm long, 0.2–0.25 mm wide, straight-sided, rims with hairs shorter than 0.5 mm; awns 45–100 mm, almost centric, cernuous throughout, twice-geniculate, usually both geniculations obscure; anthers 3, 1.2–1.5 mm. Caryopses about 2 mm, linear, dark brown. 2n = 40.

Nassella tenuissima grows on rocky slopes, frequently in oak or pine associations but also in open, exposed grasslands. Its native range extends from the southwestern United States into northern Mexico. It is now also established in the San Francisco Bay area, having been introduced as a garden plant. It is an attractive species, available through some horticultural outlets, but it readily escapes from cultivation into nearby disturbed sites.

8. Nassella trichotoma (Nees) Hack. ex Arechav.
Serrated Tussockgrass, Yass Tussockgrass

Plants perennial; cespitose, not rhizomatous. Culms 20–60 cm tall, about 1 mm thick, erect, internodes glabrous; nodes 2–4, pubescent. Sheaths glabrous, smooth; collars glabrous, without tufts of hair at the sides; ligules 0.5–2.5 mm, glabrous, obtuse; blades 15–45 cm long, 0.2–0.6 mm wide, convolute, stiff, scabridulous. Panicles 8–25 cm, open, lax, sparse; branches 2–6 cm, with 1–8 spikelets; pedicels 3–12 mm. Glumes subequal, 4–10 mm long, 0.9–1.2 mm wide, narrowly lanceolate, scabridulous, apices aristate; florets 1.5–2.5 mm long, 0.7–0.9 mm wide, terete, widest near the top; calluses 0.1–0.3 mm, acute, strigose, hairs reaching to midlength on the lemmas; lemmas strongly tuberculate, particularly distally, mostly glabrous, narrowing abruptly to the crown, midveins pubescent proximally; crowns about 0.5 mm long, about 0.2 mm wide, straight-sided, rims entire or irregularly lacerate, glabrous; awns 15–35 mm, eccentric, straight to twice-geniculate; anthers 3, 1–1.5 mm. Caryopses about 1.2 mm, oblong, dark brown. 2n = 36.

Nassella trichotoma is a native of South America, and has been accidentally introduced into the United States. Because it is on the U.S. Department of Agriculture’s noxious weed list, all known populations have been eliminated. New populations should be reported to the Department.

9. Nassella chilensis (Trin.) E. Desv.
Chilean Tussockgrass

Plants perennial; shortly rhizomatous, appearing cespitose, rhizomes slender, somewhat woody. Culms 30–100 cm tall, 0.4–0.7 mm thick, bases some-what bulblike, erect, geniculate and often branching intra-vaginally at the lower cauline nodes, internodes glabrous; nodes 5–8+, glabrous. Sheaths mostly glabrous, throats sometimes ciliate; collars sparsely hairy, with tufts of hair at the sides, hairs 0.5–1.3 mm; ligules 0.2–0.3 mm, truncate, usually ciliate; blades 3–10 cm long, 1–1.5 mm wide, strongly convolute, stiff, abaxial surfaces glabrous, adaxial surfaces with coarse hairs. Panicles 2–20 cm; branches 0.4–1.2 cm, with 1–4 spikelets; pedicels 0.5–4 mm. Glumes subequal, 3–4.5 mm long, 1.1–1.6 mm wide, ovate, 3-veined, glabrous or puberulent, acuminate; florets 1.6–2.2 mm long, 0.6–0.9 mm wide, obovate to oblong, terete, widest near the top; calluses 0.2–0.3 mm, obtuse, glabrous; lemmas glabrous, smooth, lustrous, transition to the crown not evident; crowns about 0.1 mm long and wide, not differing in texture from the lemmas; awns 7–10 mm, eccentric, rapidly deciduous; anthers about 1 mm or 0.3–0.4 mm, florets with longer anthers presumably chasmogamous, those with shorter anthers presumably cleistogamous. Caryopses about 1 mm. 2n = 42.

Nassella chilensis is an Andean species that was once collected from a ballast dump in Portland, Oregon. It is not established in the Flora region.

10. Nassella viridula (Trin.) Barkworth
Green Nassella, Green Needlegrass

Plants perennial; cespitose, not rhizomatous. Culms 35–120 cm, erect or geniculate basally, internodes mostly glabrous, pubescent below the lower nodes; nodes 2–3, glabrous. Sheaths mostly glabrous, margins usually ciliate; collars of basal leaves hispidulous, with tufts of hair at the sides, hairs 0.5–1.8 mm, collars of flag leaves glabrous or sparsely pubescent; ligules 0.2–1.2 mm, glabrous, truncate to rounded; blades 10–30 cm long; 1.5–3 mm wide, flat to convolute, abaxial surfaces scabridulous, glabrous, adaxial surfaces glabrous. Panicles 2.9–7.2 cm, loosely contracted; branches 1–4 cm, appressed or ascending, with 3–7 spikelets; pedicels 1–9 mm. Glumes subequal, 6.8–13 mm long, 1–2.1 mm wide, narrowly lanceolate, glabrous, apiculate; florets 3.4–5.5 mm long, 1–1.2 mm wide, terete; calluses 0.7–1.4 mm, moderately sharp, strigose; lemmas papillose, evenly pubescent, not constricted below the crown; crowns 0.4–0.5 mm long, 0.3–0.5 mm wide, not conspicuous, straight-sided, rims with 0.5–0.75 mm hairs; awns 19–32 mm, evidently twice-geniculate, terminal segment straight; anthers (0.8)2–3 mm, sometimes penicillate. Caryopses about 3.5 mm. 2n = 82, 88.

Nassella viridula grows in grasslands and open woods, frequently on sandy soils. It is the most widespread species of Nassella in North America. Its morphology, distribution, and high chromosome number suggest that it may be an alloploid between Nassella and Achnatherum. It is included in Nassella because it resembles Nassella more than Achnatherum in the characters distinguishing the two genera. It differs from Achnatherum robustum in its tightly convolute lemmas and in having glabrous to sparsely pubescent collars on its flag leaves. It differs from the hybrid with Achnatherum hymenoides, xAchnella caduca, in its less saccate glumes, shorter lemmas hairs, and shorter paleas.