Yucca

Plants perennial, acaulescent or caulescent, sometimes subscapose, sometimes arborescent, usually branching extensively, from woody, subterranean or aboveground caudices, or single stems. Leaves sessile, in rosettes on caudices or at branch ends; blade linear-lanceolate, expanded basally, usually rigid, occasionally fleshy, margins entire or denticulate, often filiferous and separating into elongated fibers, corneous, apex mostly sharp-pointed. Scape, when present, usually less than 2.5 cm diam. Inflorescences erect or rarely pendent, paniculate or racemose, sometimes paniculate proximally and racemose distally, bracteate, occasionally pubescent; bracts ascending, erect, or rarely reflexed; peduncle sometimes scapelike, sometimes extending beyond leaves, sometimes pubescent. Flowers bisexual; perianth campanulate or globose; tepals 6, similar, fleshy, distinct to or connate at base, whitish to cream or tinged slightly with green or purple, occasionally pubescent; stamens 6; filaments flattened, as wide as anthers, smooth, papillose, or granular, fleshy; pistil obovoid or oblong-cylindrical; ovary superior, usually green, 3-locular or 6-locular with false septa, 6-lobed; style white to dark green, often thick; stigmas usually 3, sometimes 1 and subcapitate, white to pale green, 1–2 mm. Fruits erect or pendent, capsular or baccate. Seeds many per locule, usually black, occasionally gray, flattened, round, rarely obovate or ovate. x = 25, 30.

Distribution

e coastal plain, se, sc, w United States, n, c, w Mexico, n Central America.

Discussion

Species ca. 35–40 (28 in the flora).

W. Trelease (1902) recognized Hesperoyucca, Clistoyucca, and Samuela as segregates from, but closely related to, Yucca. S. D. McKelvey (1938–1947) returned them to Yucca, recognizing sections Hesperoyucca and Clistoyucca, and placing Samuela in section Sarcocarpa. However, McKelvey indicated that there is ample justification for recognizing Hesperoyucca at genus level, “since a number of flower and fruit characters differ from those in all other sections.” Recent DNA evidence provides strong support for separate recognition of Hesperoyucca (M. A. Hanson 1993; D. J. Bogler 1994; D. J. Bogler and B. B. Simpson 1995, 1996; K. H. Clary 1997). Clary’s data indicate that it is more closely related to Hesperaloe than to Yucca, and that Clistoyucca and Samuela should be retained in Yucca.

Yuccas are often cultivated in many places in the United States, particularly in the south. They are not easily identified, particularly in herbarium specimens, and collectors should be certain to record information about the habits and sizes of plants, and the colors of ovaries, styles, and stigmas. If possible, field photographs of the pistil and stamens should accompany the specimens.

Selected References

Bogler, D. J., J. L. Neff, and B. B. Simpson. 1995. Multiple origins of the Yucca–yucca moth association. Proc. Natl. Acad. Sci. U.S.A. 92: 6864–6867.

Clary, K. H. 1997. Phylogeny, Character Evolution, and Biogeography of Yucca L. (Agavaceae) As Inferred from Plant Morphology and Sequences of the Internal Transcribed Spacer (ITS) Region of the Nuclear Ribosomal DNA. Ph.D. dissertation. University of Texas.

Hanson, M. A. 1993. Dispersed Unidirectional Introgression from Yucca schidigera into Y. baccata (Agavaceae). Ph.D. dissertation. Claremont Graduate School.

McKelvey, S. D. 1938–1947. Yuccas of the Southwestern United States. 2 vols. Jamaica Plain.

Lenz, L. W. and M. A. Hanson. 2000. Yuccas (Agavaceae) of the International Four Corners: Southwestern USA and northwestern Mexico. Aliso 19: 165–179.

Robbins, R. L. 1983. A Systematic Study of the Indehiscent-fruited Yuccas in the Chihuahuan Desert. Ph.D. dissertation. Texas Tech University.

Trelease, W. 1902. The Yucceae. Rep. (Annual) Missouri Bot. Gard. 13: 27–133.

Trelease, W. 1907. Additions to the genus Yucca. Rep. (Annual) Missouri Bot. Gard. 18: 225–230.

Webber, J. M. 1953. Yuccas of the Southwest. Washington. [U.S.D.A., Agric. Monogr. 17.]

Lower Taxa

Yucca aloifolia, Yucca angustissima, Yucca arkansana, Yucca baccata, Yucca baileyi, Yucca brevifolia, Yucca campestris, Yucca constricta, Yucca elata, Yucca faxoniana, Yucca filamentosa, Yucca flaccida, Yucca glauca, Yucca gloriosa, Yucca harrimaniae, Yucca intermedia, Yucca madrensis, Yucca necopina, Yucca neomexicana, Yucca pallida, Yucca reverchonii, Yucca rostrata, Yucca rupicola, Yucca schidigera, Yucca tenuistyla, Yucca thompsoniana, Yucca treculeana, Yucca utahensis

Key

1	Fruits indehiscent, pendent or occasionally erect.	> 2
1	Fruits dehiscent, erect.	> 9
2	Fruits dry and spongy or leathery.	> 3
2	Fruits fleshy and succulent.	> 4
3	Leaf blade 15–35 × 0.7–1.5 cm, rigid; fruits 6–8.5 cm; sw United States.	Yucca brevifolia
3	Leaf blade 40–100 × 3.5–6 cm, rigid or flexible; fruits 2.5–8 cm; se United States.	Yucca gloriosa
4	Tepals connate basally for 1 mm or more; pistil 2.8–8 cm.	> 5
4	Tepals distinct, or connate basally for less than 1 mm; pistil 1.5–4 cm.	> 6
5	Mature plants taller than 2.5 m; tepals 3.9–10.8 cm.	Yucca faxoniana
5	Mature plants shorter than 2.5 m; tepals 4.5–13 cm.	Yucca baccata
6	Leaf blade thin, flexible, margins entire or occasionally filiferous with slender fibers.	Yucca madrensis
6	Leaf blade thick, rigid, margins entire, denticulate, or filiferous with slender or coarse fibers.	> 7
7	Leaf margins sharply denticulate or entire, rarely filiferous with slender fibers.	Yucca aloifolia
7	Leaf margins entire, filiferous with coarse fibers.	> 8
8	Style 2–8 mm.	Yucca treculeana
8	Style 1–2 mm.	Yucca schidigera
9	Leaf margins entire or often denticulate, sometimes wavy, lemon yellow to orangish red or reddish brown; pistil 2.5–4.5 cm.	> 10
9	Leaf margins entire, filiferous or becoming so, not wavy, whitish to gray, light green, or light brown; pistil 1.5–3.8 cm.	> 14
10	Mature plants caulescent; rosettes each with more than 100 leaves.	> 11
10	Mature plants acaulescent, usually forming colonies; rosettes each with fewer than 100 leaves.	> 12
11	Mature plants 2.5–3.6 m, not including inflorescence; leaf blade 25–60 × 1.2–1.7 cm, widest considerably above middle, smooth.	Yucca rostrata
11	Mature plants 0.7–2.5 m, not including inflorescence; leaf blade 20–30(–45) × 0.7–1.2 cm, widest at or above middle, ± scabrous.	Yucca thompsoniana
12	Plants in colonies of 10–30 rosettes each; mature leaf blade straight, flat except becoming concave near apex, glaucous, margins wavy, yellow; inflorescences with wide-spreading branchlets.	Yucca pallida
12	Plants in open colonies of 2–15 rosettes each, or sometimes solitary; leaf blade flattened or twisted and concave, glaucous or not, margins yellow, dark orange, or reddish brown; inflorescences narrowly ovoid.	> 13
13	Mature leaf blade twisted, strongly concave, 1.7–4 cm at greatest width, flexible, margins orange or reddish brown.	Yucca rupicola
13	Mature leaf blade straight, slightly concave, 1–2(–2.5) cm at greatest width, rigid, margins yellow or reddish brown.	Yucca reverchonii
14	Mature plants caulescent, arborescent, with thick stems 0.6–2.5 m, rarely acaulescent; rosettes usually large.	> 15
14	Mature plants acaulescent or occasionally caulescent with stems to 1 (rarely –2) m, sometimes indistinctly arborescent; rosettes usually small.	> 16
15	Mature plants distinctly arborescent, solitary or forming small colonies; stems 1–7 per colony, erect, 1–2.5 m.	Yucca elata
15	Mature plants not distinctly arborescent, forming large colonies; stems usually 7 or more per colony, often procumbent, 0.6–1.3 m.	Yucca utahensis
16	Leaf blade usually soft and limp, rarely erect and rigid, 1–5 cm wide, proximal leaves often becoming reflexed; inflorescences paniculate.	> 17
16	Leaf blade rigid or sometimes flexible, including proximal leaves, usually less than 2 cm wide, or if wider, then inflorescences racemose and partially held within rosettes; inflorescences racemose or paniculate.	> 19
17	Tepals 5–7 cm; inflorescences glabrous.	Yucca filamentosa
17	Tepals 3–5 cm; inflorescences pubescent or glabrous.	> 18
18	Inflorescences pubescent.	Yucca flaccida
18	Inflorescences glabrous.	Yucca necopina
19	Inflorescences paniculate (and racemose distally in Y. constricta); leaf blade plano-convex or sometimes plano-keeled.	> 20
19	Inflorescences primarily racemose, occasionally paniculate proximally; leaf blade concave or concavo-convex, except for those less than 0.9 cm wide, then plano-convex or plano-keeled.	> 22
20	Distance from leaf tips to proximal inflorescence branches less than twice leaf length when fully expanded; peduncle 0.5–1 m; leaf blade 0.3–0.7(–1.5) cm wide.	Yucca campestris
20	Distance from leaf tips to proximal inflorescence branches more than twice leaf length when fully expanded; peduncle 1–2 m; leaf blade (0.3–)0.7–2 cm wide.	> 21
21	Capsules not constricted; leaf blade mostly recurving, 1–2 cm wide.	Yucca tenuistyla
21	Capsules deeply constricted near middle; leaf blade mostly linear, (0.3–)0.7–1.5 cm wide.	Yucca constricta
22	Peduncle (0.1–)0.3–2.5 m; inflorescences arising well beyond rosettes.	> 23
22	Peduncle 0.1–0.6 m; inflorescences arising within or just beyond rosettes.	> 24
23	Leaf blade usually linear, 20–80 (–150) cm, not glaucous; inflorescences (4–)8–20 dm.	Yucca angustissima
23	Leaf blade spatulate-lanceolate, 15–46 cm, rather glaucous; inflorescences 4–7 dm.	Yucca neomexicana
24	Capsules 3–5.5(–7) × 2–3 cm.	> 25
24	Capsules (3–)5–7(–9) cm × 2–4(–5) cm.	> 27
25	Peduncle shorter than 0.2 m.	Yucca harrimaniae
25	Peduncle (0.1–)0.2 m or longer.	> 26
26	Style pale green or rarely white; Colorado, New Mexico, Oklahoma panhandle.	Yucca neomexicana
26	Style dark green; Arkansas, Kansas, Missouri, Oklahoma, Texas.	Yucca arkansana
27	Leaf blade 0.7–4.3 cm wide.	> 28
27	Leaf blade 0.5–2(–2.5) cm wide.	> 29
28	Style pale to bright green; leaf blade 1.8–4.3 cm wide; Arizona, Colorado, Nevada, New Mexico, Utah.	Yucca harrimaniae
28	Style dark green; leaf blade 0.7–2.5 cm wide; Arkansas, Kansas, Missouri, Oklahoma, Texas.	Yucca arkansana
29	Style dark green.	> 30
29	Style white or pale yellowish green.	> 31
30	Inflorescences 5–10 dm; leaf blade 0.8–1.2 cm wide, rigid.	Yucca glauca
30	Inflorescences 3–6(–8) dm; leaf blade 0.7–2(–2.5) cm wide, flexible.	Yucca arkansana
31	Inflorescences 2.5–4.5(–8.5) dm; peduncle shorter than 0.2 m.	Yucca baileyi
31	Inflorescences (5–)7–13 dm; peduncle 0.1–0.5 m.	Yucca intermedia