Hesperoyucca (Engelmann) Trelease (Annual Rep. Missouri Bot. Gard. 4: 208, 1893). Type: Yucca whipplei Torrey. — SHH-Clade — Lit: Turner & al. (1995: ecology); Clary (2001: classification); Hochstätter (2000: 15–23, as sect. Hesperoyucca, ill. synopsis); Boeuf & al. (2009: 126–129, ill. synopsis); Powell (2013: summary pollination ecology). Distr: W USA, NW Mexico. Etym: Gr. ‘hespera’, evening; for the occurrence in W North America (i.e. in the West, where the sun disappears in the evening); and for the similarity to Yucca (Agavaceae).

  • ≡ Yucca [?] Hesperoyucca Engelmann (1871).

  • ≡ Yucca subgen. Hesperoyucca (Engelmann) Baker (1876).

Acaulescent rosette plants, monocarpic when remaining unbranched or polycarpic otherwise; Ros solitary or in colonies, sessile, sometimes stem rhizomatous, single or caespitose; L linear or rarely narrowly lanceolate, rigid and sword-like to flexible and frequently falcate, plano-convex or subtriquetrous, or keeled on both faces, 25–115 × 0.5–4 cm, ± grey-green, finely striate, base expanded to ± 4–7 × 4–7 cm, ± white to greenish, margin thin, horny, pale yellow, without fibres, mostly denticulate, terminal spine sharp; Inf terminal large panicles, 1.4–8 m with a bracteate peduncle 0.9–4.5 m long, flowering part dense, cylindrical or somewhat slenderly ellipsoid; Fl densely arranged, usually broadly expanding, pendent, campanulate or ± globose, 3.5–5 cm, very fragrant; Tep broadly lanceolate, nearly equal, 3–4.5 (−6) × 0.8–2.5 cm, white or creamy white to greenish or purple-tinged, tips generally purple, free to the base; Fil straight, linear below, tip angled, club-like, papillose, attached to the lower part of the tepals so that the stamens are pulled away from the ovary as the flower opens; Anth reniform, pollen uniquely glutinous; Ov superior, stout, 8–12 × 6–10 mm; Sty short, slender, white; Sti distinctly capitate, green towards the centre, fringed with elongated translucent papillae; Fr erect, obovoid, strictly loculicidally dehiscent, 3–5 × 1.5–4 cm; Se flat, thin, smooth, without marginal wing, 6–7 × 8 mm, dull black. — Cytology: n = 30 (Halpin & Fishbein 2013: 1007).

With the exception of Trelease (1893) and indirectly also Baker (1892), most earlier authors included Hesperoyucca within Yucca. Recent molecular studies (Bogler & Simpson 1995, Bogler & al. 1995, Bogler & Simpson 1996, Bogler & al. 2006, Smith & al. 2008) and a combined morphological/molecular study (Clary & Simpson 1995) clearly revealed a position as sister group of Hesperaloe and thus separate from Yucca. In fact, the recent molecular phylogenies of Halpin & Fishbein (2013), Archibald & al. (2015) and McKain & al. (2016) all show Hesperoyucca as sister of Hesperaloe + Schoenolirion, in an Agavaceae subclade completely distant to that which includes Yucca. A closer association with Yucca is only evident in the molecular phylogeny of Good-Avila & al. (2006) and in the morphology-based phylogeny of Hernández-Sandoval (1995). Consequently, Hesperoyucca should be treated as separate genus, morphologically clearly distinct from Yucca, as in the first edition of this handbook and in several modern floras and checklists, while some still retain it as synonym of Yuccca.

Hesperoyucca differs clearly from Yucca (data in brackets) in forming a definite bulb in the seedling stage (Webber 1953: t. 53) (vs. bulb absent), its capitate bright green and densely long-papillate stigma (vs. 3-lobed, white, papillose on the inner surface), its filaments basally attached to the tepals, usually longer than the pistil, finely papillate, and ± swollen the entire length, erect or spreading outwardly from the point of attachment at anthesis, bearing tufts of papillae at the apex (vs. filaments not attached to the tepals, usually shorter than the pistil, pubescent, distally clavate and held close to the ovary and bent outwards near the swollen apex), its cordate anthers (vs. sagittate or hastate), and its strictly loculicidally dehiscent fruits (vs. indehiscent or, if dehiscent, septicidal, occasionally also septicidal and loculicidal). The often very large inflorescences of Hesperoyucca by far exceed the inflorescence size in Yucca, and unbranched plants (“ssp. whipplei”) are monocarpic, whereas some branched plants (“ssp. caespitosa”) develop new rosettes from the leaf axils of very young plants; both features are unknown in Yucca.

Author citation and date and place of valid publication were differently interpreted over time and have been clarified by Greenhouse & Strother (2002).

Pollination: Powell (2013) published a comprehensive summary of present knowledge of the pollination of Hesperoyucca: The Yucca moth Tegeticula maculata appears to be the only pollinator, with 3 distinct geographical races. Male moths usually stay in the same inflorescence for their whole life, while females usually translocate to other inflorescences (average 53 m distance). The plants are predominantly self-sterile.

Ethnobotany: According to Hodgson (2001, 43–44, ill.), H. whipplei was formerly used as food by the local populations in its range. The base of the inflorescence as well as the “heads” left when the leaves are pruned were roasted or baked, and then tasted like a fibrous sweet potato. Young flowers were also roasted and eaten.

H. newberryi (McKelvey) Clary (Sida 19(4): 845, 2001). Type: USA, Arizona (McKelvey 4087 [A]). — Distr: USA (Arizona: Mohave & Coconino Counties: W slopes of the Colorado River Canyon); rocky granite, 400–1300 m; spring-flowering. I: Hochstätter (2015: 3, 12, 15, as Yucca).

≡ Yucca newberryi McKelvey (1947) ≡ Yucca whipplei ssp. newberryi (McKelvey) Hochstätter (2000); incl. Yucca newberryi ssp. mckelveyana Hochstätter (2015).

Monocarpic; Ros solitary; L 50–60 cm, 0.7–2 cm wide at the base in narrow-leaved forms, 2–2.5 cm in broader-leaved forms; Inf 2.8–3.2 m, peduncle 1.4–1.6 m × 10–14 cm, floriferous part in the upper ½, in outline 4–55 cm wide at the widest point; Fr woody capsules, at maturity with slight or inconspicuous placental wings.

H. whipplei (Torrey) Trelease (Annual Rep. Missouri Bot. Gard. 4: 208, 1893). Type [lecto]: USA, California (Schott s.n. [NY]). — Lit: Schaffer & Schaffer (1977: pollination ecology); Schaffer & Schaffer (1979: pollination ecology); Aker (1982: summary pollination ecology); Turner & al. (1995: 414–416, as Yucca); Powell (2013: summary pollination ecology). Distr: SW USA (SW California), NW Mexico (N Baja California, N Baja California Sur, NW Sonora: Pinacate region); coastal sage, chaparral, desert woodland, 0–1400 (−2500) m; flowers (February to) May–June. I: Greulich (2012); Hochstätter (2015: 5–10). – Figs. 1 and 2.

Fig. 1
figure 1

Hesperoyucca whipplei. (Copyright: U. Eggli)

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Fig. 2
figure 2

Hesperoyucca whipplei. (Copyright: U. Eggli)

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≡ Yucca whipplei Torrey (1859); incl. Yucca californica Groenland (1858); incl. Yucca graminifolia Alph. Wood (1868) (nom. illeg., ICN Art. 53.1); incl. Yucca engelmannii Masters (1880); incl. Yucca ortgiesiana Roezl (1880); incl. Yucca whipplei var. violacea André (1884); incl. Hesperoyucca whipplei var. graminifolia Trelease (1893) ≡ Yucca whipplei fa. graminifolia (Trelease) Voss (1895); incl. Yucca nitida C. Wright ex W. Watson (1906); incl. Yucca whipplei var. caespitosa M. E. Jones (1929) ≡ Yucca whipplei ssp. caespitosa (M. E. Jones) A. L. Haines (1942); incl. Yucca whipplei var. parishii M. E. Jones (1929) ≡ Yucca whipplei ssp. parishii (M. E. Jones) A. L. Haines (1941); incl. Yucca whipplei ssp. intermedia A. L. Haines (1941) ≡ Yucca whipplei var. intermedia (A. L. Haines) J. M. Webber (1953); incl. Yucca whipple i ssp. percursa A. L. Haines (1941) ≡ Yucca whipplei var. percursa (A. L. Haines) J. M. Webber (1953); incl. Yucca whipplei ssp. typica A. L. Haines (1941) (nom. inval., ICN Art. 24.3); incl. Yucca peninsularis McKelvey (1947) ≡ Hesperoyucca peninsularis (McKelvey) Clary (2001); incl. Yucca whipplei ssp. eremica Epling & A. L. Haines (1957); incl. Yucca whipplei ssp. rigata Afferni & Drovandi ex Afferni (2004).

Monocarpic and Ros solitary, or polycarpic and Ros caespitose and forming small to large, compact or open groups, sometimes rhizomatous, sometimes with secondary rosettes at the base or stem branching after flowering to form new rosettes; L 20–90 (−125) × 0.7–4 cm at the base; Inf 3–6 (−8) m, peduncle 0.9–3 (−4.5) m, 2.5–15 cm ∅ near the base, floriferous part 2.5–2.8 m, 1 when rosettes are solitary, 1 to many per group when rosettes form colonies; Fr woody capsules, at maturity with conspicuous placental wings.

Yucca californica is here listed as synonym with considerable doubt and would have priority if it is indeed conspecific. Within H. whipplei, Trelease (1893) recognized 2 varieties, Haines (1941) and Munz & Keck (1959) each recognized 5 subspecies, and Webber (1953) recognized 4 varieties based on growth form. In contrast, McKelvey (1938) and McKelvey (1947) as well as McKinney & Hickman (1993) argued that growth form is highly variable and recognition of any infraspecific taxa in H. whipplei is unwarranted. Moreover, wild populations often contain plants of different “varieties” (Keeley & Tufenkian 1983), and seeds from one capsule may even produce all possible growth forms (DeMason 1984). Consequently, no infraspecific taxa are recognized here.

H. peninsularis from Baja California is sunk here following Govaerts (2014+, accessed Sept. 2018). The Californian and Mexican populations show a continuous distribution (Turner & al. 1995), the differences in leaf size given by Clary (2001) are contiguous, and material from Baja California (e.g. Hodgson 9577 [DES, digital image!]) exhibits the conspicuous placental wings present in the Californian plants but absent in H. newberryi.

Forms of H. whipplei from higher altitudes may be winterhardy in protected sites outdoors in C Europe and may reach flowering size in as little as 13 years (Bolliger 1998). The frost tolerance is given as about −15 °C in C Europe (Boeuf 2007) and to about −12 °C for the USA (Irish & Irish 2000). Wolf (1935) (cited from Hodgson (2001)) reports that rosettes need 6–7 years to reach flowering size.