Dioon holmgrenii

Dioon holmgrenii is a cycad from southern Mexico, endemic to the Pacific slope of the Sierra Madre del Sur in the state of Oaxaca, where it grows in oak woodland, pine-oak forest and tropical deciduous forest at roughly 450 to 1,500 m (1,500 to 4,900 ft). Described in 1981, this species of the genus Dioon slowly builds a trunk that can exceed 6 m, crowned by long, flat leaves with narrow, widely spaced leaflets armed with small marginal spines. Its very downy young leaves earned it the Mexican name “plumilla”. Listed as Vulnerable on the IUCN Red List, it remains uncommon in cultivation, and its cold tolerance is poorly known: no measured frost limit has been published.

Key facts about Dioon holmgrenii

  • Accepted name: Dioon holmgrenii De Luca, Sabato & Vázq.Torres (1981), accepted by The World List of Cycads (version 2026.08.15), GBIF, Species+ and Kew’s World Checklist of Vascular Plants, the database behind POWO (June 2026 version, published on GBIF)
  • Former names and synonyms: none
  • Family: Zamiaceae, order Cycadales
  • Common names: no traditional English name; “sun palm” and “Holmgren’s cycad” are used by Port St. Lucie Botanical Gardens in Florida; in Mexico palmera de sol, palma del sol, mirasol, plumilla and, locally, yazn-gua (see Common names)
  • Natural range: Mexico, Pacific slope of Oaxaca, at about 450 to 1,500 m (2020 revision of the genus)
  • IUCN status: Vulnerable (VU B1ab(iii)+2ab(iii)), assessment by J. Chemnick, T. Gregory and S. Salas-Morales dated August 2020 and published in 2022
  • CITES and other protection: CITES Appendix II (annotation #4); Annex B in the European Union; “in danger of extinction” (category P) in Mexico under the standard NOM-059-SEMARNAT-2010
  • Documented hardiness: no measured value; a Californian nursery rates it to about −7 to −5 °C (the “low 20s °F”), a European nursery places it in zone 9b with possible damage below 0 °C (32 °F) (see Cold hardiness)
  • Light and soil: full sun to part shade, well-drained soil, neutral to slightly alkaline (Port St. Lucie Botanical Gardens); grows well on limestone soil (Cycadales.eu)
  • Habit and adult size: trunk up to 6 m (original description) or 6 to 7 m (2020 revision), 30 to 40 cm (12 to 16 in) across; leaves 1 to 1.5 m long or more
  • In the UK, US and Australia: Montgomery Botanical Center (Florida, ten accessions, the most recent from 2018), Port St. Lucie Botanical Gardens (Florida) and some ten other American gardens; declared to BGCI by the Royal Botanic Garden Edinburgh, Royal Botanic Gardens Victoria (Melbourne) and Cairns Botanic Gardens
  • Toxicity: no data for the species; to be treated as toxic, like all cycads (see Toxicity and precautions)

How to recognise Dioon holmgrenii

Dioon holmgrenii is recognised by its flat, upright leaves carrying very many narrow, sharp leaflets, widely spaced along the axis and set almost at right angles to it, with a few small spines on the margin, mainly on the upper side. These are exactly the characters singled out in the original description by Paolo De Luca, Sergio Sabato and Mario Vázquez Torres (Brittonia, 1981). Their measurements, completed by the taxonomic revision of the genus published in 2020 by Hernández-Tapia, Jiménez-Ramírez and Vovides, are as follows:

  • Trunk: cylindrical, unbranched, up to 6 m (original description) or 6 to 7 m (revision), 30 to 40 cm across.
  • Cataphylls (the scale leaves protecting the growing point): woolly, 7 to 9 cm long in the original description, 5 to 9 cm in the revision, with a sharp tip.
  • Leaves: numerous, ascending, flat and leathery, downy when young and later hairless, 130 to 150 cm long or more (8 to 20 per crown and 100 to 150 cm according to the revision); petiole 13 to 15 cm (13 to 30 cm in the revision), woolly at the base; axis straight or slightly twisted near the tip.
  • Leaflets: 115 to 130 or more on each side in the original description (74 to 140 pairs in the revision), linear-lanceolate, sharply pointed, slightly rolled under and separated from one another; the middle leaflets 10 to 12 cm long and 7 to 9 mm wide, with about 15 veins (7 to 12.5 cm by 3 to 9 mm in the revision); spines 1.5 to 2 mm long, mostly on the upper margin, usually three or four per leaflet according to the revision (one to six depending on the plant).
  • Male cones: elongated and cylindrical, about 40 cm long and 7.5 cm across, the tips of the scales deltoid and curved upwards.
  • Female cones: ovoid to cylindrical, 30 to 50 cm long and 20 to 30 cm across, with densely woolly scales 9 to 12 cm long.
  • Seeds: almost spherical, 2.5 to 3 cm across, or ovoid, 3.5 to 4 cm long; the fleshy coat is yellow according to the revision.

The down on the young leaves is the most obvious character in cultivation: the authors of 1981 explain the local name plumilla by these “very pubescent young leaves”. The original description states that adult leaves become hairless, and the key published in 2008 in The Cycad Newsletter by Gregory, Haynes and Chemnick also describes the adult leaflets as hairless on both faces. No primary source says how long the down persists. The Californian nursery Jungle Music describes the foliage as blue-green, Port St. Lucie Botanical Gardens mentions a light lime-green petiole.

Hybrids

No natural or garden hybrid of Dioon holmgrenii has been described. Velasco-García and co-authors (2021) mention, on the basis of earlier work, ancient gene exchanges at the end of the Pleistocene between Dioon holmgrenii and its neighbours Dioon caputoi and Dioon planifolium: this is a hypothesis about the history of the populations, not hybrids observed today.

Possible confusion

With Dioon merolae

Young plants can be confused, and the resemblance has a genetic basis. Reviewing the chloroplast DNA analysis of Moretti and co-authors (1993), Chemnick, Gregory and Salas-Morales (Phytologia, 1997, issued in 1998) found only one difference between Dioon holmgrenii and Dioon merolae, fewer than between the two forms of Dioon edule, although the two plants look quite different. The original descriptions give the essential difference: in Dioon merolae the leaflets leave the axis at an acute angle, are inserted obliquely and overlap; in Dioon holmgrenii they stand almost at right angles and are spaced. The 2021 key by Gutiérrez-Ortega and co-authors adds that the leaflets of Dioon merolae are strongly bent downwards at the tip, and the 2020 revision that they carry at most one or two spines, against three or four in Dioon holmgrenii. The two species also live in different regions: Chiapas for Dioon merolae as delimited since 2021, the Pacific coast of Oaxaca for Dioon holmgrenii.

With Dioon califanoi

The 1981 authors compare the two species by their habitat, pine-oak forest, but contrast them clearly: Dioon holmgrenii grows in a moist setting at low altitude (580 to 850 m in their observations), whereas Dioon califanoi occupies dry slopes at 1,500 to 3,250 m in northern Oaxaca and Puebla. The leaves differ too: according to the 2021 key, those of Dioon califanoi are strongly keeled (V-shaped), with overlapping, obliquely inserted leaflets, whereas those of Dioon holmgrenii are flat with spaced leaflets. The 2008 key uses the same criteria.

With Dioon planifolium, Dioon caputoi and Dioon purpusii

The 2020 revision regards Dioon planifolium, from the Mixteca region of Oaxaca, as the closest species: its leaflets are flat, entire or with a single spine, and its cataphylls have long points. In the 2021 key the two species are separated at the last couplet: three or four long spines and slightly curved leaflets in Dioon holmgrenii, none to two spines and flat leaflets in Dioon planifolium. The original description already separated Dioon holmgrenii from Dioon caputoi and Dioon purpusii by its leaflets, which do not leave the axis at an acute angle, and by the deltoid, not sharply pointed, tips of the male cone scales; Dioon purpusii also has keeled leaves.

With Dioon edule

According to the original description, Dioon edule, from eastern Mexico, differs in its adult leaflets with entire, spineless margins. Young plants of both species can have slightly arched leaflets (De Luca and co-authors, 1984), which makes seedlings harder to name.

With Dioon nuusaviorum

The Guerrero populations described in 2026 as Dioon nuusaviorum were first considered similar to Dioon holmgrenii. According to their description, Dioon holmgrenii differs in its non-overlapping leaflets, generally 0.2 to 1 cm apart, with straight margins and two to five short teeth (0.15 to 0.29 cm), whereas the leaflets of Dioon nuusaviorum overlap and carry three to six long teeth.

Taxonomy

Dioon holmgrenii has never changed name and has no synonyms. It was described in 1981 in the journal Brittonia (33(4): 552–555) by Paolo De Luca and Sergio Sabato, of the University of Naples, and Mario Vázquez Torres, of the Universidad Veracruzana, as part of their study of the distribution and taxonomy of the genus in Mexico. The type is the collection Vázquez Torres 2304, made on an oak-covered slope 15 km south of San Gabriel, on the road from Puerto Escondido to Oaxaca (district of Juquila); the holotype is kept in the herbarium of the Universidad Veracruzana (given as XALU by The World List of Cycads, XALUV in the original description), with isotypes at ENCB, FI, MEXU, NAP and NY. The World List of Cycads dates this collection to 21 March 1980 at 650 to 850 m, while the 2020 revision gives 29 May 1979 and 720 m. Plants from the same small area had already been collected by Dennis Breedlove (1965), Steven R. Hill (1973) and Ray Bandar.

Gutiérrez-Ortega and co-authors (2021) place the species in the “Purpusii clade”, with Dioon argenteum, Dioon califanoi, Dioon caputoi, Dioon merolae, Dioon oaxacensis, Dioon planifolium, Dioon purpusii and Dioon salas-moralesiae. A genomic study by Dorsey, Salas-Morales and Gregory (Conservation Genetics, 2023) examined the cohesion of the populations across the range: according to its abstract, it concludes that the species once expanded and was then fragmented, each population keeping a large share of genetic variants of its own.

Etymology and history

The epithet honours Noel Herman Holmgren, taxonomist and curator at the New York Botanical Garden, for his contribution to the knowledge of American plants (original description; The World List of Cycads, after Haynes, 2022). The genus name comes from the Greek dis, “two”, and ôon, “egg”, after the two seeds borne by each scale of the female cone.

The species is one of the six Dioon described between 1979 and 1984 by the Naples team of Paolo De Luca, together with Dioon califanoi, Dioon caputoi, Dioon rzedowskii, Dioon merolae and Dioon tomasellii. In their paper the authors stress that it was then the only Dioon of Oaxaca known from the Pacific side of the Sierra Madre del Sur, the other species of the state growing in the northern mountains, separated from it by the Oaxaca plateau. The botanic garden of the University of Naples, the team’s home institution, grows the species: it appears, photographed, in the inventory of the Califano glasshouses published by Sibilio in 2019. The Gymnosperm Database also shows a female cone photographed in 1998 on a plant at the Los Angeles County Arboretum.

Common names

No traditional English name exists. The original description gives palmera de sol and plumilla, the latter “on account of its very pubescent young leaves”. The 2020 revision, citing Bonta and Osborne (2007), lists mirasol, palma sol and plumilla; Species+ gives palma del sol and plumilla, and the Mexican standard NOM-059-SEMARNAT-2010 plumilla. Bonta and co-authors (2019) use the local name yazn-gua at Santiago Textitlán, in the Sierra Sur. In English, Port St. Lucie Botanical Gardens (Florida) calls it “sun palm” or “Holmgren’s cycad”; names containing “palm” are misleading, since the plant is a cycad, not a palm.

In the wild

Dioon holmgrenii grows only in Oaxaca, on the Pacific slope of the Sierra Madre del Sur, in the Mexican Pacific Coast biogeographic province. The 2020 revision cites collections from the municipalities of San Gabriel Mixtepec, San Pedro Mixtepec, San Bartolomé Loxicha, San Pedro Juchatengo and Santa Cruz Itundujia, and places the species in tropical deciduous forest, oak woodland and pine-oak forest at 450 to 1,500 m, under warm and humid to temperate climates. The authors of 1981 found it in the understorey of a humid pine-oak forest, on slopes near ravines, in soil rich in humus.

The genetic study by Velasco-García and co-authors (Plants, 2021) gives precise data on nine populations, from Río Leche to San Bartolomé. They lie between 510 and 1,400 m, under a sub-humid tropical climate with a marked dry season, a mean annual temperature of 20 to 26 °C (68 to 79 °F) and 1,200 to 2,500 mm (47 to 98 in) of rain a year. Their areas range from 5 ha (Rancho El Limón) to almost 1,900 ha (Ocotlán), and 2 to 57% of their habitat has been cleared. The populations west of the río Verde, which are wetter, are the most genetically diverse. The authors also mention other populations, such as that of Llano de León, of about 620 ha.

Conservation status and regulation

Dioon holmgrenii is listed as Vulnerable (VU) on the IUCN Red List under criteria B1ab(iii)+2ab(iii): a restricted range and area of occupancy, fragmented or few populations, and a continuing decline in habitat quality. The assessment, by J. Chemnick, T. Gregory and S. Salas-Morales, is dated 6 August 2020 and was published in 2022 (The World List of Cycads, GBIF and Crossref). According to Velasco-García and co-authors (2021), the conversion of land to rain-fed farming and intensive grazing lowers plant density and fragments the populations; as gene flow between populations is very low, the loss of any one of them would remove part of the species’ diversity. The authors recommend community reserves and wildlife management units (UMA) allowing landowners to raise and sell plants; applications had been filed for the San Bartolomé and Ocotlán populations. Bonta and co-authors (2019) describe plants raised in a community nursery at Santiago Textitlán and replanted on the village square out of pride in this conservation programme.

The species is listed in CITES Appendix II, like the whole genus since 4 February 1977, with annotation #4, which exempts seeds and pollen in particular; international trade in live plants requires a permit. In the European Union it is listed in Annex B of Regulation (EC) No 338/97, whose current annexes are set by Regulation (EU) 2026/1383, in force since 29 June 2026, according to Species+. In Mexico the standard NOM-059-SEMARNAT-2010 lists it as “in danger of extinction” (category P), as the text published in the Diario Oficial de la Federación confirms. Databases checked on 26 October 2026: check the regulations in force before buying, and prefer seed-grown plants from cultivation.

Growing Dioon holmgrenii

Dioon holmgrenii is grown like other warm-climate Dioon: plenty of light, a soil that drains quickly and water during the warm season. Published growing advice is limited and comes from gardens and nurseries rather than trials:

  • Port St. Lucie Botanical Gardens (Florida), which grows it in its Ancient Garden, recommends full sun to part shade, a well-drained soil of any type, neutral to slightly alkaline (pH 6.0 to 7.5), regular watering without saturating the soil, and notes moderate tolerance of drought and salt, slow growth and an eventual height of up to 20 ft (6 m); the plant is frost-tolerant but does best in a frost-free site.
  • The specialist European nursery Cycadales.eu describes it as easy to grow provided it gets warmth and water in summer and protection from cold, with a fertiliser preferably slow-release, and notes that it grows very well on limestone soil.
  • The Californian nursery Jungle Music describes it as a sun-loving plant of medium growth rate, and its seed-grown stock as coming from seed received about ten years before 2023.

The sources disagree on growth rate (slow at Port St. Lucie, medium at Jungle Music), and no measurement has been published. In the wild the species receives 1,200 to 2,500 mm of rain a year and goes through a marked dry season (Velasco-García and co-authors, 2021); generous watering in the warm season and a drier soil in winter therefore suit it, which is also what the nurseries advise.

Outdoor and in-ground cultivation

Jungle Music writes that along the Californian coast the species easily takes full sun, whereas far inland it should get only part of the day’s sun. In Mediterranean-type climates, a sunny, well-drained position sheltered from cold winds follows the logic of its habitat. The only in-ground report from Europe concerns a mild stretch of the Provence coast (see Cold hardiness). No observation of Dioon holmgrenii has been published for the Jardin zoologique tropical at La Londe-les-Maures, in the Var.

Container cultivation

Young plants grow well in deep pots with a free-draining mineral mix; Jungle Music sells its stock as old pot plants. In climates with real frost, a pot that can be moved into a bright, frost-free place in winter is the prudent option until the species’ limits are known.

In cultivation in the UK, US and Australia

  • United Kingdom: the Royal Botanic Garden Edinburgh declares the species to BGCI PlantSearch; its own online Garden Explorer did not list it in October 2026, and no outdoor cultivation is documented in Britain.
  • United States: the Montgomery Botanical Center (Coral Gables, Florida) lists ten accessions in its online catalogue, including four from 2001, two from 2004 and two from 2018; Port St. Lucie Botanical Gardens grows it outdoors in its Ancient Garden, and the Fairchild Tropical Botanic Garden (Miami) lists it in its online catalogue. BGCI PlantSearch also lists Lotusland, the Huntington, the Los Angeles County Arboretum, San Diego Botanic Garden, the San Diego Zoo, Descanso Gardens, the Boyce Thompson Arboretum (Arizona), the New York Botanical Garden, the University of Hawaii at Hilo, the John Fairey Garden (Texas) and the US National Plant Germplasm System. A BGCI listing is a declaration by the garden, not proof that the plant is alive today.
  • Australia: Royal Botanic Gardens Victoria (Melbourne) and Cairns Botanic Gardens declare the species to BGCI; no Australian cultivation details have been published.

Propagation

Dioon holmgrenii is propagated from seed. Like all cycads it is dioecious: a male and a female plant are needed, and seeds from an unpollinated cone have no embryo. Plants sold in Europe and the United States are seedlings, and since seeds are exempt from CITES, they are the simplest way to obtain plants of known origin. Jungle Music adds that, like many other Dioon, the species can produce suckers at the base, which can be removed; no rooting trial has been published. No germination study specific to the species exists; the usual methods for cycads apply.

Pests and diseases

Port St. Lucie Botanical Gardens reports no pests or diseases of particular concern. As in other Dioon, the main danger in cultivation is rot of the roots and trunk in cold, waterlogged soil, and scale insects may settle on plants kept under cover in winter. No publication deals with the susceptibility of the species to the Asian cycad scale (Aulacaspis yasumatsui).

Cold hardiness

The cold tolerance of Dioon holmgrenii is not precisely known: no frost measurement has been published, either in cultivation or in the wild. Its habitat is warm all year (20 to 26 °C, or 68 to 79 °F, mean annual temperature according to Velasco-García and co-authors, 2021), which suggests little or no frost, but no source gives a minimum temperature. The only benchmarks are assessments by nurseries and gardens.

Mediterranean and European experience

A grower on the Côte Bleue (Bouches-du-Rhône, France) reports on the French Fous de palmiers forum that he planted his whole Dioon collection in the ground, Dioon holmgrenii included, and that all the plants came through a winter without protection or damage (October 2015). In February 2018 he added that his Dioon had been in the ground for several years without trouble, but that the temperature had not fallen below −1 or −2 °C (30 or 28 °F) for five years. This shows that the species can live outdoors on a mild Mediterranean coast in gentle winters, but says nothing about its reaction to a real frost. In Italy it is grown in the Califano glasshouses of the Naples botanic garden; no outdoor report is documented. For readers in comparable climates, such as coastal California or southern Australia, this European experience points to the same conclusion: a sheltered, well-drained site is promising in mild winters, but no frost threshold has been tested.

United Kingdom, United States and Australia

  • California: the nursery Jungle Music (San Diego County) rates the species cold-tolerant to about −7 to −5 °C (the “low 20s °F”); this is a commercial assessment, with no detail on plant age or damage.
  • Florida: Port St. Lucie Botanical Gardens gives USDA zones 9b to 11 and considers the plant frost-tolerant but best in a frost-free site; this zone rating is the garden’s own.
  • Europe: the nursery Cycadales.eu places it in zone 9b, with possible damage as soon as the temperature drops below 0 °C (32 °F).
  • United Kingdom and Australia: no dated report from a British or Australian grower has been found. In Britain the species would need a heated glasshouse.

Toxicity and precautions

No toxicity study has been published for Dioon holmgrenii, but it should be treated as toxic, like all cycads. Cycads contain toxic glycosides (cycasin, macrozamin), from which methylazoxymethanol is derived, a toxin affecting the liver and the nervous system; the seeds are the most dangerous part, and poisoning of dogs that chew them is well documented in other cycads. Yagi (2004) did not detect these glycosides in the adult leaves of the three Dioon he analysed (the abstract names only Dioon edule), which allows no conclusion for this species or its seeds. Wear gloves to handle the seeds and keep plants and seeds out of reach of children and animals. If any part has been eaten, contact a poison centre, a doctor or a vet without delay.

Traditional uses

The 2020 revision records only ornamental use. Bonta and co-authors (2019) nevertheless photographed at Santiago Textitlán, in the Sierra Sur of Oaxaca, in 2008, a crown of thorns made from a leaf of yazn-gua: such crowns, made prickly by the spines of the leaflets, are worn by boys in Holy Week processions. No food use is documented for the species.

Frequently asked questions

Is Dioon holmgrenii hardy?

Nobody knows precisely. No frost measurement has been published. A plant has survived several winters in the ground on the Provence coast, but without going below −1 or −2 °C (30 or 28 °F). Nurseries place it in zone 9b, with possible damage below 0 °C (32 °F), or rate it to about −7 to −5 °C (low 20s °F): these are estimates to be confirmed.

How can it be told from Dioon merolae?

By the leaflets. Those of Dioon holmgrenii are spaced, set almost at right angles and carry three or four spines; those of Dioon merolae leave the axis obliquely, overlap and bend downwards at the tip. The two species are nonetheless very close genetically.

What is the difference from Dioon califanoi?

Leaf shape and habitat. Dioon califanoi has V-shaped leaves with overlapping leaflets and grows on dry slopes above 1,500 m; Dioon holmgrenii has flat leaves with spaced leaflets and grows lower down, on the humid Pacific slope.

Why are its young leaves downy?

It is a character of the species: the young leaves are covered with hairs and later become hairless. This down gave it the Mexican name plumilla, already recorded in the 1981 description.

Is it a palm?

No. Despite names such as palmera de sol or “sun palm”, it is a cycad, a gymnosperm that bears cones, not flowers.

Is it toxic to dogs and cats?

Yes, it should be treated as toxic. No study deals with this species, but all cycads contain toxins, especially in the seeds. Remove ripe female cones if animals have access to the garden.

Is it endangered?

It is threatened. The IUCN lists it as Vulnerable (assessment published in 2022), and Mexico as in danger of extinction. Its populations, isolated from one another, are shrinking as land is cleared for farming and grazing.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • De Luca, P., Sabato, S. & Vázquez Torres, M. (1981). Dioon holmgrenii (Zamiaceae), a new species from Mexico. Brittonia 33(4): 552–555. doi:10.2307/2806763 [Original description, habitat, local names, comparison with neighbouring species; full text (copy of The Cycad Society).]
  • De Luca, P., Sabato, S. & Vázquez Torres, M. (1981). Dioon merolae (Zamiaceae), a new species from Mexico. Brittonia 33(2): 179–185. doi:10.2307/2806317 [Oblique, overlapping leaflets of Dioon merolae.]
  • De Luca, P., Sabato, S. & Vázquez Torres, M. (1984). Dion tomasellii (Zamiaceae), a new species with two varieties from western Mexico. Brittonia 36(3): 223–227. doi:10.2307/2806511 [Arched leaflets of young Dioon holmgrenii.]
  • Moretti, A., Caputo, P., Cozzolino, S., Gaudio, L., Siniscalco Gigliano, G. & Stevenson, D.W. (1993). A phylogenetic analysis of Dioon (Zamiaceae). American Journal of Botany 80: 204–214. [Chloroplast DNA analysis; cited from Chemnick et al. (1997).]
  • Chemnick, J., Gregory, T.J. & Salas-Morales, S. (1997, issued 1998). A revision of Dioon tomasellii (Zamiaceae) from western México, a range extension of D. merolae, and clarification of D. purpusii. Phytologia 83(1): 1–6. [Genetic differences, including a single one between Dioon holmgrenii and Dioon merolae; full text (Internet Archive).]
  • Gregory, T., Haynes, J. & Chemnick, J. (2008). Key to the species of Dioon. The Cycad Newsletter 31: 16–17. [Key based on adult leaves.]
  • Bonta, M., Pulido-Silva, M.T., Diego-Vargas, T., Vite-Reyes, A., Vovides, A.P. & Cibrián-Jaramillo, A. (2019). Ethnobotany of Mexican and northern Central American cycads (Zamiaceae). Journal of Ethnobiology and Ethnomedicine 15: 4. doi:10.1186/s13002-018-0282-z [Name yazn-gua, Holy Week crown of thorns, community nursery at Santiago Textitlán.]
  • Hernández-Tapia, J.E., Jiménez-Ramírez, J. & Vovides, A.P. (2020). Taxonomic revision of the genus Dioon (Zamiaceae). Phytotaxa 442(4): 267–290. doi:10.11646/phytotaxa.442.4.2 [Description, range, altitudes, habitats, common names; treatments on Plazi.]
  • Gutiérrez-Ortega, J.S., Pérez-Farrera, M.A., Chemnick, J. & Gregory, T.J. (2021). A reassessment of Dioon merolae (Zamiaceae) leads to the description of Dioon salas-moralesae. Phytotaxa 528(2): 93–110. doi:10.11646/phytotaxa.528.2.3 [Purpusii clade and key (Plazi).]
  • Velasco-García, M.V., Ramírez-Herrera, C., López-Upton, J., Valdez-Hernández, J.I., López-Sánchez, H. & López-Mata, L. (2021). Diversity and genetic structure of Dioon holmgrenii (Cycadales: Zamiaceae) in the Mexican Pacific Coast biogeographic province: implications for conservation. Plants 10(11): 2250. doi:10.3390/plants10112250 [Populations, habitat, threats, conservation.]
  • Dorsey, B.L., Salas-Morales, S.H. & Gregory, T.J. (2023). Conservation genomics of Dioon holmgrenii (Zamiaceae) reveals a history of range expansion, fragmentation, and isolation of populations. Conservation Genetics 25: 335–355. doi:10.1007/s10592-023-01569-4 [History of the populations, private alleles; abstract.]
  • Martínez-Domínguez, L., Nicolalde-Morejón, F., Stevenson, D.W., Lorea-Hernández, F.G. & Vergara-Silva, F. (2026). A striking new species of Dioon (Zamiaceae) from pine and pine-oak forest of Guerrero, Mexico. PhytoKeys 274: 229–245. doi:10.3897/phytokeys.274.173907 [Comparison with Dioon nuusaviorum.]
  • Yagi, F. (2004). Azoxyglycoside content and β-glycosidase activities in leaves of various cycads. Phytochemistry 65: 3243–3247. doi:10.1016/j.phytochem.2004.09.013 [Toxic glycosides of cycads, the case of Dioon; abstract.]
  • Chemnick, J., Gregory, T. & Salas-Morales, S. (2022). Dioon holmgrenii. The IUCN Red List of Threatened Species 2022: e.T42129A69831707. doi:10.2305/IUCN.UK.2022-1.RLTS.T42129A69831707.en [Category VU; reference checked on Crossref.]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. doi:10.11646/phytotaxa.550.1.1 [Dedicatee, as given by The World List of Cycads.]
  • Calonje, M., Stevenson, D.W. & Osborne, R. (2026). The World List of Cycads, version 2026.08.15. Montgomery Botanical Center, Coral Gables. doi:10.5281/zenodo.21958524 [Reference checklist.]