Yucca × quinnarjenii is one of the most distinctive entities in the genus Yucca, both for its botanical traits — a small, solitary, stemless cycad-like rosette from the sandy hills of the Arizona-New Mexico border — and for its taxonomic status, which offers an almost textbook illustration of the long-running nomenclatural debates surrounding the small acaulescent yuccas of the American Southwest. Described in 2008 by the German botanist Fritz Hochstätter as a new species, Yucca quinnarjenii, the taxon was later reclassified by Plants of the World Online (POWO) as a nothospecies — a formally named natural hybrid — under the combination Yucca × quinnarjenii Hochstätter, with the hybrid formula Yucca baileyi × Yucca intermedia. This dual reading, and the disagreement that underlies it, is inseparable from the very identity of the taxon: it is impossible to write honestly about Yucca × quinnarjenii without making both layers explicit, which is why they take centre stage in this account.
How to recognise Yucca × quinnarjenii
Yucca × quinnarjenii is a small Yucca with a strictly solitary, acaulescent habit: no offsets, no clumps, no emergent stem above ground. The single rosette grows from a stout subterranean caudex and typically bears 30 to 60 leaves arranged in a tight spiral.
The leaves, rigid (in contrast to most other small yuccas of the same region), reach up to 40 cm in length with a width of about 0.8 cm at the base. They are flat to slightly convex in cross-section, with entire margins fringed by curling white fibres that peel finely away, and end in a short but well-defined dark-brown apical spine. Foliage colour is matt green, without strong glaucous bloom. The combination of solitary growth, absence of stem, rigid leaves and modest length sets the taxon apart from the clumping, soft-leaved yuccas that dominate the same region.
The inflorescence — short, only 0.4 to 0.6 m tall — is one of the most diagnostic features. It emerges from within the rosette and barely overtops the foliage, whereas in Yucca angustissima the scape can reach 2 m and in Yucca baileyi about 0.8 m. It is a dense, raceme-like spike, sparsely branched or unbranched. The flowers, pendulous and globular-bell-shaped, are 40 to 60 mm long and display the six creamy-white tepals typical of the genus. Flowering occurs from April to July.
The fruit is a dry, dehiscent capsule (the diagnostic trait of section Chaenocarpa), held erect, with three locules containing flat, disc-shaped black seeds.
Hybrids — or rather: a taxon that is itself a putative hybrid
Unlike entries devoted to true species, this section does not deal with hybrids derived from Yucca × quinnarjenii — none have been recorded, in the wild or in cultivation — but with the putative hybrid nature of the taxon itself.
According to the position retained by POWO (the nomenclatural authority followed here), Yucca × quinnarjenii is a natural hybrid between:
- Yucca baileyi Wooton & Standl.: an acaulescent or short-stemmed Yucca with a clumping habit, flexible leaves 20 to 60 cm long, an inflorescence reaching about 0.8 m, distributed across the Arizona-Colorado-New Mexico border region on sandy and gravelly soils.
- Yucca intermedia McKelvey: an acaulescent, clump-forming Yucca of central and northern New Mexico, with slightly flexible leaves less than 1 cm wide and an inflorescence 0.7 to 1.3 m tall emerging from within the rosette. Yucca × quinnarjenii would inherit from this parent the short scape that begins inside the foliage.
This hybridogenic reading, however, runs into a problem of internal consistency that is examined in detail in the Taxonomy section below: Hochstätter, the author of Yucca × quinnarjenii, does not himself accept Yucca intermedia as a distinct species. He has published the combination Yucca baileyi subsp. intermedia (McKelvey) Hochstätter, treating it as a subspecies of Yucca baileyi. From his standpoint, speaking of a hybrid between Yucca baileyi and Yucca intermedia amounts to speaking of a hybrid between a species and one of its own subspecies — a taxonomically incoherent statement. The POWO hybrid formula is therefore consistent with the POWO view of the parents but at odds with the view of the very author who described the hybrid taxon.
No published molecular study has yet confirmed the hybrid nature of Yucca × quinnarjenii, nor pinned down its parents. The POWO reading rests on a posteriori morphological reasoning, not on genetic data.
Confusion
Yucca × quinnarjenii can be confused with several small yuccas of the Chaenocarpa–Elatae complex of the American Southwest. The distinguishing characters are habit (solitary versus clumping), leaf rigidity, leaf length and scape height.
Confusion with Yucca baileyi Wooton & Standl. Yucca baileyi is clumping — forming dense tufts of multiple rosettes through basal division — with flexible rather than rigid leaves, reaching 60 cm long, and a taller scape (up to 0.8 m, sometimes more). The solitary habit and rigid foliage of Yucca × quinnarjenii are the most reliable separators.
Confusion with Yucca intermedia McKelvey. Yucca intermedia is also clumping, with thin (less than 1 cm wide) and slightly flexible leaves, and a scape 0.7 to 1.3 m tall — taller than that of Yucca × quinnarjenii. The scape emerges from within the rosette as in Yucca × quinnarjenii, which can prompt confusion in photographs. Habit and leaf rigidity again resolve the question.
Confusion with Yucca angustissima Engelm. ex Trel. Yucca angustissima is clumping, with flexible, narrow, long leaves (25-45 cm) and a much taller scape (up to 2 m, sometimes more). Overall, it is a substantially larger plant.
Confusion with Yucca harrimaniae Trel. Yucca harrimaniae is a small, solitary or weakly clumping Yucca of the Colorado Plateau, with rigid leaves, but typically of a more compact habit, with broader and shorter leaves, and a scape that clearly overtops the foliage. Its geographical range lies further north.
Confusion with Yucca neomexicana Wooton & Standl. Yucca neomexicana is clumping, with flexible leaves and a tall scape. It occupies northeastern New Mexico and southeastern Colorado, beyond the range of Yucca × quinnarjenii.
In European cultivation, plants circulating under the name Yucca × quinnarjenii are vanishingly rare and nearly all originate from seed distributed by Hochstätter or by a handful of specialist nurseries. Any plant acquired without proper provenance should be treated with caution: as with any rare Yucca, the temptation to attach the rare name to unidentified material is well documented in the trade.
Taxonomy
The taxonomic status of Yucca × quinnarjenii is the most delicate — and the most instructive — point in this account.
Original description (2008). The taxon was described by Fritz Hochstätter in 2008, under the name Yucca quinnarjenii Hochstätter (without the multiplication sign, hence as a true species), published in the author’s Self-Reference Publication (SRP) and later re-illustrated and re-discussed in Cactus Aventures International in 2021. Hochstätter assigns it to section Chaenocarpa Engelmann, series Elatae Hochstätter, and presents it as an endemic species strictly localised to a narrow strip of the Arizona-New Mexico border at 1950 m elevation. The type locality, identified by collection number fh 1188.76.0 (June 2008), is deliberately not disclosed by the author — a measure intended to protect the wild population from collection pressure. The specific epithet quinnarjenii honours his grandson Quinn Arjen.
Reclassification by POWO as a nothospecies. Plants of the World Online subsequently reclassified the taxon as a formally named natural hybrid: Yucca × quinnarjenii Hochstätter, with the hybrid formula Yucca baileyi × Yucca intermedia. This reading has been adopted by World Flora Online and by Wikipedia. POWO justifies it on a posteriori morphological grounds: the characters of Yucca × quinnarjenii — solitary acaulescent habit, rigid leaves intermediate between the two proposed parents, short inflorescence emerging within the rosette — could correspond to a combination of parental traits. No molecular evidence currently supports this interpretation.
The crux: is Yucca intermedia a species or a subspecies? The POWO hybrid formula stands or falls on the acceptance of Yucca intermedia McKelvey as a species distinct from Yucca baileyi. That acceptance is itself contested. The positions can be summarised as follows:
- Susan Delano McKelvey (1947), in her reference work Yuccas of the Southwestern United States, describes Yucca intermedia as a true species, distinct from Yucca baileyi.
- James L. Reveal (1977) demotes it to varietal rank: Yucca baileyi var. intermedia (McKelvey) Reveal.
- Fritz Hochstätter treats it as a subspecies: Yucca baileyi subsp. intermedia (McKelvey) Hochstätter.
- POWO and the Flora of North America currently accept Yucca intermedia as a true species, following McKelvey.
The direct consequence is that under Hochstätter’s framework — in which Yucca intermedia is no more than a subordinate taxon of Yucca baileyi — speaking of a hybrid between Yucca baileyi and Yucca intermedia is to speak of a hybrid between a species and one of its own subspecies, which carries no nomenclatural meaning. In that framework, Yucca quinnarjenii would be a true species in its own right, as Hochstätter originally proposed. Under the POWO and McKelvey framework — in which Yucca intermedia is a separate species — the hybrid formula becomes coherent, but it requires a double commitment: accepting both the Yucca baileyi/Yucca intermedia split and the hybrid origin of Yucca quinnarjenii from those two parents.
Editorial position adopted here. This article follows POWO nomenclature, the editorial reference, while explicitly flagging that the position is not universally accepted and that the descriptor of the taxon — Hochstätter — does not subscribe to it. The form Yucca × quinnarjenii is used for consistency with POWO, but the reader should be aware that the form Yucca quinnarjenii (without the multiplication sign) appears in part of the literature, especially in the descriptor’s own publications.
Systematic position according to POWO:
- Kingdom: Plantae
- Division: Tracheophyta
- Class: Equisetopsida (sensu APG)
- Order: Asparagales
- Family: Asparagaceae
- Subfamily: Agavoideae
- Genus: Yucca L.
- Nothospecies: Yucca × quinnarjenii Hochstätter
- IPNI identifier: 77254022-1
Yucca × quinnarjenii as a semi-arid plateau xerophyte
Yucca × quinnarjenii shows the full set of xerophytic adaptations characteristic of the small North American yuccas of semi-arid plateaus: thick, waxy-cuticled leaves; facultative CAM photosynthesis (documented in some populations of related taxa); deep taproots reaching shallow groundwater; a subterranean caudex acting as both water and starch reserve; and tolerance of large diurnal thermal swings.
The known distribution lies at 1950 m in a continental semi-arid climate marked by hot summers (with daytime peaks of 30-35 °C), cool summer nights, cold winters with regular frost and possible snowfall, and modest annual precipitation (roughly 300-450 mm depending on the locality), with a summer maximum tied to the southwestern monsoon (July-August). Soils are sandy, free-draining, weakly developed on sedimentary substrates.
It is in this climatic envelope that the hardiness reported by Hochstätter must be read: the taxon withstands very low winter temperatures, down to -20 °C, provided the substrate remains dry and the plant is in full vegetative dormancy. This level of cold tolerance is unsurprising for the small yuccas of the Chaenocarpa–Elatae complex and reflects adaptation to a winter climate that is both cold and dry.
Yucca × quinnarjenii in the wild
The known range of Yucca × quinnarjenii is exceptionally narrow: a thin strip of sandy hills along the Arizona-New Mexico border at about 1950 m elevation. The type locality, under collection number fh 1188.76.0, has been deliberately withheld from the public record by Hochstätter. Collection pressure on rare North American yuccas — which has grown noticeably with the rise of the specialist horticultural market — fully justifies that reserve.
The typical habitat consists of sandy, free-draining hills in open country, without dense tree cover, with associated vegetation typical of the transition between piñon-juniper woodland and semi-arid grassland: Pinus edulis, Juniperus monosperma, Bouteloua spp., Hilaria jamesii, Ephedra spp., and several other small yuccas of section Chaenocarpa.
Conservation status. Yucca × quinnarjenii has not been assessed by the IUCN. As a nothospecies with a very restricted range, it falls outside most formal conservation frameworks, which generally apply to true species. The principal threat is not direct habitat destruction (the area is sparsely visited and of limited economic interest) but illegal collection by specialist amateurs, which is precisely why the type locality is kept confidential. Total population size is unknown, almost certainly small.
A biogeographical reflection. The presence of a morphologically intermediate taxon between Yucca baileyi and Yucca intermedia in a defined geographical area legitimately raises an ecological question: is this a hybrid zone between two distinct species (POWO reading), a local form within a single specific complex (Hochstätter reading, in which Yucca intermedia is a subspecies of Yucca baileyi), or an ancestral parent species from which both Yucca baileyi and Yucca intermedia have diverged? No definitive answer is currently available. A dedicated molecular study would be needed to resolve the question.
Outdoor / in-ground cultivation
Yucca × quinnarjenii is grown almost exclusively by specialist collectors and a small number of botanical gardens. It has not entered the mainstream horticultural market. The cultivation indications below are synthesised from Hochstätter’s observations on wild and cultivated material and, by analogy, from accumulated experience with other small yuccas of the Chaenocarpa complex.
Site selection. A south-facing, fully exposed position with the maximum sunshine available is essential. Air movement is desirable; stagnant pockets retaining winter humidity are to be avoided. Sloping ground is preferable to flat ground, both for surface drainage and for soil aeration.
Soil preparation. The soil must be strongly mineral: a workable mix is roughly 60 % sharp sand or fine gravel, 25 % coarse pumice or perlite-grade material, and 15 % lean loam. In heavy native soils, dig a generous planting hole (60 cm depth, 60 cm width) and replace the excavated material with the mineral mix. Surface mulching with light-coloured gravel further protects the caudex from rainfall splash and reflects excess heat from the foliage in summer.
Climate suitability. In dry continental climates, where winters are cold but rainfall is modest and snow cover dry rather than slushy, Yucca × quinnarjenii is in its element and in-ground cultivation is the natural option. In Mediterranean climates with mild but wet winters, hardiness is not the limiting factor — the hardiness rating is well above local minimum temperatures — but persistent winter humidity sharply increases the risk of caudex rot. In oceanic climates (humid temperate winters with mild minima), the combination of mild but wet conditions prevents proper dormancy and is generally unfavourable.
Winter rain protection. Where in-ground cultivation is attempted in regions with significant winter rainfall, a temporary cover (transparent rain hat, glass pane elevated above the rosette) installed from late autumn to early spring is highly effective. The cover should not seal the plant in: ventilation at the sides remains essential.
Watering after planting. Water moderately during the first growing season to support root establishment, then reduce sharply. Established plants need very little supplementary water in any climate where natural rainfall reaches even modest levels.
Container cultivation
For most growers, especially in oceanic and humid Mediterranean climates, container cultivation is the safer route.
Pot choice. A deep pot to accommodate the taproot is essential — clay or terracotta is preferred to plastic, as lateral evaporation through the porous wall reduces the risk of waterlogging at the root zone. Diameter is less critical than depth.
Substrate. A purely mineral mix is recommended: 50 % coarse sand or 4-8 mm granite grit, 30 % coarse pumice or perlite, 20 % lean sandy loam. Composts and peat-based substrates should be avoided; their water retention and progressive acidification are both problematic for this taxon.
Light and temperature in containers. Full sun whenever the plant is outdoors. Optimum growth occurs between 18 and 30 °C; brief summer peaks of 35-40 °C are tolerated without damage if airflow is good.
Watering rhythm. Water generously in the growing season but only after the substrate has dried right through — a cycle of 10 to 20 days depending on heat and pot volume. From October through March, withhold water entirely or restrict it to occasional light moistening if the pot has dried bone-dry. The combination of cold temperatures and wet substrate is the principal cause of mortality in European cultivation.
Repotting. Rare — every four to six years, at the start of the growing season, taking great care not to disturb the rootball. The fragility of the taproot and lateral roots during repotting is a real concern.
Overwintering. A frost-free or lightly heated greenhouse, or a covered terrace fully shielded from rain, suffices in most European climates. Heated protection is rarely necessary given the taxon’s hardiness; what matters is keeping the substrate dry rather than keeping the air warm.
Feeding. Restrained. A balanced mineral fertiliser at low dose (NPK 6-6-6, 1 g/L) one or two times during the growing season is enough. Like most plateau yuccas, Yucca × quinnarjenii is adapted to lean soils and reacts poorly to overfeeding, especially excess nitrogen, which causes abnormal stretching of the foliage.
Propagation
Seed. This is the principal — and for the amateur, essentially the only — practicable route. Where seed is available, germination is generally good after a few weeks of cold stratification simulating the winter of the native range, followed by sowing on a free-draining mineral substrate at 20-25 °C in full light. Germination occurs within three to eight weeks, sometimes longer. Subsequent growth is slow, as in all small plateau yuccas: several years to reach a rosette of significant size, and probably a decade or more to first flowering.
Seed of Yucca × quinnarjenii is rare on the market and circulates almost exclusively through specialist amateur networks (botanical garden Index Seminum, collector exchanges, direct contact with Hochstätter). For any taxon of putatively hybrid origin, character stability in seedlings is not guaranteed: if the hybrid status is confirmed, seed from selfing or backcrossing with one of the proposed parents could yield variable progeny ranging across the parental phenotype space.
Division. The strictly solitary character of Yucca × quinnarjenii — the rosette produces no basal offsets — rules out division. This is one of the very traits that distinguish it from every clumping Yucca of the same region.
Stem or rosette cuttings. Not applicable: there is no aerial trunk, no offshoots, and the subterranean caudex is not amenable to division.
Pests and diseases
The phytosanitary issues affecting Yucca × quinnarjenii in cultivation are the generic ones of small plateau yuccas grown outside their native range.
Caudex rot. By far the leading cause of mortality. Triggered by Phytophthora spp., Fusarium spp., or Pythium spp., it sets in during cool, wet periods when the substrate stays saturated for extended spells. Symptoms include softening at the collar, yellowing and wilting of the rosette, and a fermentation odour. Once the rot has reached the caudex, the prognosis is very poor. Prevention rests on an ultra-draining substrate, strictly seasonal watering, rain protection in winter, and end-of-winter inspection.
Mealybugs. Mealybugs (Pseudococcus spp., Planococcus citri) and, more rarely, scale insects can colonise the centre of the rosette. White oil or insecticidal soap treatment, with regular inspection.
Yucca weevil (Scyphophorus acupunctatus). This Mexican yucca weevil is established across the American continent and has been introduced into Mediterranean Europe over the past decade. It targets primarily the large stemmed yuccas (Yucca elephantipes, Yucca aloifolia) but can attack small acaulescent yuccas under heavy local pressure. Watch for entry holes at the collar and suspicious oozing.
Aphids. Occasionally appear on the developing inflorescence. Not a serious concern; treat with insecticidal soap if populations build up.
Hardiness
Yucca × quinnarjenii is one of the hardiest yuccas available in cultivation, with a frost tolerance reported by Hochstätter at -20 °C in dry conditions.
This high hardiness rating must, however, be qualified by climate context:
- Continental dry climate (central and eastern Europe, the high plateaus of certain Mediterranean ranges, semi-arid intermountain regions in North America): the -20 °C rating is realistic, provided drainage is excellent and the plant has fully entered winter dormancy. In-ground cultivation is feasible.
- Wet Mediterranean climate (southern French coast, parts of the Iberian Mediterranean, coastal Italy): hardiness is not the limiting factor — the rating sits well above local winter minima — but persistent winter humidity sharply elevates the risk of caudex rot. Container culture with rain protection is the recommended approach. Where in-ground cultivation is attempted, a fully south-facing, well-ventilated site and a strictly mineral soil are non-negotiable.
- Oceanic humid climate (Atlantic seaboard, parts of the British Isles, the Pacific Northwest): the unfavourable combination of mild but wet winters prevents proper dormancy. Container culture under a ventilated cold greenhouse is far safer than in-ground planting.
- Semi-continental humid climate (parts of central Europe, the Great Lakes region of North America): in-ground cultivation is possible with extreme drainage and winter rain protection (transparent overhead cover, winter tent), but container culture remains the simpler option.
For comparison, related yuccas of the same complex (Yucca baileyi, Yucca harrimaniae, Yucca neomexicana) have been grown in the open ground in gardens in Poland, Germany and the Czech Republic, with confirmed hardiness down to about -25 °C in well-drained soils. Yucca × quinnarjenii should, in principle, tolerate comparable conditions.
Uses
Yucca × quinnarjenii has no documented economic use. It is not used for food, traditional medicine, fibre or saponin extraction. Its very low abundance and very recent description rule out any historical role.
Horticultural use. Yucca × quinnarjenii is a specialist collection plant, sought by enthusiasts of small hardy plateau yuccas and by collectors interested in taxonomically intricate taxa. Its commercial diffusion in Europe is extremely limited: a handful of nurseries specialising in xerophytes, occasional collector exchanges, and seed circulation through amateur networks or Index Seminum. The taxon is not suited to ornamental landscape use: it is too small, too slow, too rare and too botanist-oriented to interest the mainstream garden market.
Ex situ conservation. Given the highly localised wild population and the latent collection pressure, the cultivation of seed-derived material (rather than wild-collected plants) by private collectors and institutions makes a modest contribution to safeguarding the genetic legacy of the taxon. Botanical gardens and serious amateurs acquiring stock should rely only on documented and ethical sources.
FAQ
Why is the name written with a multiplication sign (×)? In botanical nomenclature, the “×” sign indicates a formally named hybrid (a nothospecies). This is the position taken by Plants of the World Online and adopted in this article. The original descriptor, Fritz Hochstätter, however, published the taxon in 2008 without that sign, as a true species: Yucca quinnarjenii. Both forms coexist in the literature.
Is it really a hybrid species? The question is not scientifically settled. No published molecular study has confirmed either the hybrid nature of the taxon or the precise identity of its parents. The hybridogenic interpretation by POWO is grounded in a posteriori morphological reasoning. Hochstätter’s original position recognised a true species in its own right. A dedicated genetic study would be needed to reach a firm conclusion.
Where can it be obtained in Europe? This is an exceptionally rare taxon on the market. The most reliable sources are the Index Seminum of European botanical gardens (which occasionally distribute seed), exchanges among specialist collectors, and direct contact with a few central-European nurseries specialising in cold-hardy yuccas. Any mainstream commercial offer under this name should be approached with caution — the likelihood of mislabelling is high.
Can it be planted in the open ground in the Mediterranean basin? In principle yes — the hardiness rating (-20 °C) is more than adequate. The actual constraint is winter humidity rather than cold: rot, not frost, is the killer. A fully mineral soil, a south-facing, well-ventilated position, and overhead protection against winter rainfall are all advisable. Container culture with sheltered overwintering remains the safer route on humid coasts.
Why use a grandchild’s name in a botanical description? The International Code of Nomenclature allows honorific specific epithets provided certain rules of Latin formation are observed. Using personal family names is not exceptional in the history of plant systematics, and Hochstätter has stated explicitly that quinnarjenii honours his grandson Quinn Arjen. It is a personal dedication, with no bearing on the validity of the name.
Is it toxic? Like all yuccas, it contains steroidal saponins throughout the plant. These compounds irritate the digestive tract if ingested and can cause symptoms in domestic animals. The apical leaf spines can also inflict deep punctures — wear gloves and long sleeves when handling.
Authority sites
Plants of the World Online (POWO) — taxonomic reference page and the nomenclatural authority retained: https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77254022-1
International Plant Names Index (IPNI) — nomenclatural identifier 77254022-1: https://www.ipni.org/n/77254022-1
FH-IRT (Fritz Hochstätter Institute for Research on Taxonomy) — page maintained by the original descriptor, with photographs and comparative description: https://fhirt.org/yucca/y_quinnarjenii.php
Cactus Aventures International — illustrated re-edition of the original description (volume 33, issue 1, October 2021): https://cactus-aventures.com/Articles%20of%20interest/Other%20Succulents%20PDF’s/Yucca%20quinnarjenii,%20a%20new%20endemic%20species%20CAI-ENG-D-1-2021-3.pdf
Wikipedia (English) — encyclopedic synthesis of the Yucca genus, including the taxon as a nothospecies: https://en.wikipedia.org/wiki/List_of_Yucca_species
Flora of North America (FNA) — reference treatment of the Yucca genus in North America, useful for the framework of the Chaenocarpa complex: https://floranorthamerica.org/Yucca
Bibliography
Hochstätter, F. (2002). Yucca II (Agavaceae) — In the Southwestern and Eastern USA. Self-published, Mannheim, 191 p. [Reference monograph on the yuccas of the southwestern and eastern United States; framework of the Chaenocarpa–Elatae complex.]
Hochstätter, F. (2008). Yucca quinnarjenii sp. nov. Self-Reference Publication (SRP), Mannheim. [Original description of the taxon as a true species; type fh 1188.76.0, June 2008.]
Hochstätter, F. (2015, publ. April 2016). Yucca L., Hesperaloe Engelmann. Self-published, Mannheim, 262 p. [Author’s nomenclatural synthesis, including the lumper position on Yucca baileyi/Yucca intermedia.]
Hochstätter, F. (2021). Yucca quinnarjenii, a new endemic species from the south-western United States. Cactus Aventures International 33(1). [Illustrated re-edition of the original description; bilingual English-German text.]
McKelvey, S.D. (1947). Yuccas of the Southwestern United States, Part 2. The Arnold Arboretum of Harvard University, Jamaica Plain (Massachusetts), 192 p. [Original description of Yucca intermedia on pages 116-120, plates 44-47; reference treatment of North American acaulescent yuccas.]
POWO (online, accessed 2026). Yucca × quinnarjenii Hochstätter. Plants of the World Online, Royal Botanic Gardens, Kew. [Taxonomic position adopted here; reclassification as a nothospecies with hybrid formula Yucca baileyi × Yucca intermedia.]
Reveal, J.L. (1977). Yucca baileyi var. intermedia (McKelvey) Reveal. In Cronquist, A. et al. (eds), Intermountain Flora 6: 532. The New York Botanical Garden, Bronx. [Intermediate position treating Yucca intermedia as a variety of Yucca baileyi.]
Trelease, W. (1902). The Yucceae. Annual Report of the Missouri Botanical Garden 13: 27-133. [Historical framework for Yucca systematics and recognition of sections Chaenocarpa, Sarcocarpa and Hesperoyucca.]