Dioon argenteum

Dioon argenteum is a cycad from Mexico, endemic to the canyons of the Sierra Norte in northern Oaxaca, where it grows between 1,100 and 1,600 m (3,600 and 5,200 ft) on steep limestone slopes and cliffs. This species of the genus Dioon owes its name to the silvery down that covers its young leaves and lasts for at least a year, so that the newest crown stays silver above the older, dark green ones. Described only in 2003, it forms with Dioon purpusii and Dioon califanoi a small group of closely related species confined to the basin of the río Santo Domingo. It is listed as Vulnerable on the IUCN Red List and remains rare in cultivation.

Key facts about Dioon argenteum

  • Accepted name: Dioon argenteum T.J.Greg., Chemnick, Salas-Mor. & Vovides (2003), accepted by The World List of Cycads (version 2026.08.15), GBIF and Kew’s World Checklist of Vascular Plants, the database behind POWO (June 2026 version, published on GBIF)
  • Former names and synonyms: no synonym known
  • Family: Zamiaceae, order Cycadales
  • Common names: no English name is attested; “palma real” in Spanish, a name shared by several Dioon (Enciclovida, CONABIO)
  • Natural range: Mexico, northern Oaxaca (canyons of the Sierra Norte); four populations known in 2003
  • IUCN status: Vulnerable (VU B1ab(iii,v)), assessment by J. Chemnick and T. Gregory dated July 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 dated observation; a Canary Islands nursery places it in its “−10 °C to 0 °C” class (14 to 32 °F), without supporting observations (see Cold hardiness)
  • Light and soil: in the wild, steep limestone slopes and cliffs, between pine-oak forest and tropical dry forest; not documented in cultivation
  • Habit and adult size: trunk 0.2 to 3.8 m long and 18 to 32 cm (7 to 12.6 in) across, upright to about 3 m and then reclining; 24 to 41 leaves 85 to 184 cm long
  • In the UK, US and Australia: Montgomery Botanical Center (Florida, five accessions from 2001) and the Fairchild Tropical Botanic Garden (Miami); declared to BGCI by about ten other American gardens; no British or Australian garden listed
  • Toxicity: no data for the species; toxins known in other cycads (see Toxicity and precautions)

How to recognise Dioon argenteum

Dioon argenteum is recognised by its flat leaves, whose leaflets keep a covering of silvery down for at least a year, whereas in its relatives this down is soon lost. The original description by Timothy Gregory and his co-authors (2003) gives the following characters:

  • Trunk: 20 to 380 cm long and 18 to 32 cm across, cylindrical, covered in old leaf bases; upright to about 3 m, it usually reclines beyond that; plants branch mostly once the trunk exceeds 2 m.
  • Leaves: 24 to 41 per crown, 85 to 184 cm long and 12.4 to 21.6 cm wide, flat (rarely slightly keeled) and stiff; they appear in annual or less frequent flushes, so that three or more crowns coexist: silver for the youngest, dark green for the next ones, and dry and hanging like a skirt around the trunk for the oldest.
  • Petiole: 7 to 23 cm, without spines, tomentose to pubescent.
  • Leaflets: 92 to 135 pairs, linear to linear-lanceolate, straight, not or slightly overlapping, set at 40 to 62° to the axis, 7 to 13.3 cm long and 6 to 12 mm wide; 0 or 1 spine on the margin facing the leaf base and 1 to 5 on the other; underside silvery, then smooth and dark green; upper side light green and finely pubescent.
  • Male cones: single, upright, cylindrical, 21.5 to 42.5 cm long and 8 to 11 cm across, silvery with brown tints.
  • Female cones: single, depressed-ovoid, 29 to 60 cm long and 18 to 25.5 cm across, woolly, with blue-green, bloomy sterile scales at the base.
  • Seeds: ovoid to almost spherical, about 2.6 to 3.2 cm long, with a fleshy coat (sarcotesta) that turns orange-yellow when ripe.

The authors saw a male plant coning on a trunk only 20 cm tall and a female on a trunk of 95 cm. Their comparative table measures the angle between the two rows of leaflets in cross-section: 164° on average in Dioon argenteum (an almost flat leaf), against 122° in Dioon purpusii and 80° in Dioon califanoi. The key of Gregory, Haynes and Chemnick (2008) also uses the adult leaflets, which stay tomentose above and pubescent beneath, and that of Gutiérrez-Ortega and co-authors (2021) separates the species at the first couplet: it is the only member of its group whose leaves are silver when they emerge.

Hybrids

No hybrid of Dioon argenteum has been reported, either in the wild or in cultivation.

Possible confusion

The likely confusions are with its two close relatives from Oaxaca. Dioon califanoi has strongly keeled, V-shaped leaves, and leaflets that are glaucous once the down has gone and almost spineless. Dioon purpusii has moderately keeled leaves and a down that does not last; its leaflets, like those of Dioon argenteum, are glossy under the down and spiny. The 2008 key also separates Dioon argenteum from Dioon holmgrenii, a species of the Pacific coast of Oaxaca, by the down that persists on the adult leaflets. These characters apply to adult leaves: no source describes the seedlings, and a young plant sold without provenance remains hard to identify.

Taxonomy

Dioon argenteum is an accepted name with no synonyms. Timothy J. Gregory, Jeffrey Chemnick, Silvia Salas-Morales and Andrew P. Vovides described it in 2003 in the Botanical Journal of the Linnean Society (141: 471–476), from a female plant collected on 3 November 2001 in the Sierra Norte of Oaxaca, at about 1,100 m (holotype in the XAL herbarium at Xalapa, isotypes at F, CAS and MEXU).

The authors placed the species close to Dioon purpusii and Dioon califanoi. The three have leaves, leaflets and cones of similar dimensions, grow in the same transition zone between pine-oak forest and tropical dry forest, and occupy sections of a single river system. The authors considered treating them as subspecies of one species, but the lack of intermediate forms between uniform populations led them to recognise three species, grouped as the “Dioon purpusii complex”. They suggested that the common ancestor colonised these canyons through the valley of the río Santo Domingo less than 12,000 years ago and then diversified rapidly, a hypothesis they themselves invited others to test with genetics.

Molecular work published since points to a recent diversification of the genus: Dorsey and co-authors (2018) date the living species of Dioon to the Pleistocene. Gutiérrez-Ortega and co-authors (2021) place Dioon argenteum in the “Purpusii clade”, with Dioon califanoi, Dioon caputoi, Dioon holmgrenii, Dioon merolae, Dioon oaxacensis, Dioon planifolium, Dioon purpusii and Dioon salas-moralesiae.

Etymology and history

The epithet argenteum comes from the Latin argenteus, “silver”. The authors chose it for the silvery look of the young crown, which contrasts with the dark green, smooth older crowns; Haynes (2022) links it to the persistent silvery down of the leaves.

The species was discovered by the Sociedad para el Estudio de los Recursos Bióticos de Oaxaca (SERBO), an association that mapped the forests of Oaxaca in the 1990s. Its researchers found an unknown Dioon in the canyons of the Sierra Norte, then four separate but similar populations. The authors thank the Chinantec people of the ejido of El Carrizal, whom they describe as the guardians of the species. They deliberately left the precise localities out of the publication, to discourage poaching.

The history of its introduction into cultivation is not documented. In 2022, the cycad curator of Nong Nooch Tropical Botanical Garden, in Thailand, listed it among the Dioon grown and coning in the garden.

Common names

No English common name is attested for Dioon argenteum. The Enciclovida database of CONABIO, in Mexico, gives it the Spanish names “palma real” and “cícada”; “palma real” is also used for other Dioon, such as Dioon caputoi, Dioon purpusii and Dioon califanoi. No name specific to the species in a local language is recorded. The English name “silver dioon”, used by some websites, is not supported by any reference source.

In the wild

Dioon argenteum grows in the canyons of the Sierra Norte of Oaxaca, whose rivers join the río Santo Domingo, which flows towards the Gulf of Mexico. According to the original description, it grows mainly between 1,100 and 1,600 m, on steep limestone slopes and cliffs, in the transition zone between pine-oak forest and tropical dry forest; it is sometimes found on river alluvium in tropical dry forest too.

The associated vegetation includes a tall layer of oaks (Quercus crassifolia, Quercus magnolifolia), pines, Lysiloma divaricata, Pseudobombax ellipticum and Bursera; a middle layer of junipers (Juniperus flaccida), Senna galeottiana, Acacia angustissima and Ptelea trifoliata; and a shrub layer with Galphimia glauca, Mentzelia hispida and Agave kerchovei.

In 2003 the four known populations contained every life stage, from seedling to adult, and over several years the authors saw cones and active recruitment. They estimated the total population at about 3,000 plants. According to them, the mountains of the Sierra de Juárez and the valley of the río Cajonos separate the range of the species from that of Dioon merolae, 200 km to the east.

A cycad cone weevil, Parallocorynus gregoryi, has been collected from Dioon argenteum (O’Brien and Tang, 2015). Whether the small beetle Pharaxonotha bicolor, known from the male cones of Dioon purpusii, Dioon caputoi and Dioon califanoi, also lives on this species remains to be checked, according to Skelley and co-authors (2022). The pollinators of the species have not been studied.

Conservation status and regulation

Dioon argenteum is listed as Vulnerable (VU) on the IUCN Red List under criteria B1ab(iii,v). The assessment, by J. Chemnick and T. Gregory, is dated 3 July 2020 and was published in 2022 (The World List of Cycads, GBIF and Crossref). These criteria mean a small extent of occurrence, few or fragmented populations, and a continuing decline in habitat quality and in the number of mature plants. In 2003 the authors of the description considered that the species faced no immediate threat apart from possible habitat destruction and poaching, and already proposed this status.

Like all Dioon, it is listed in CITES Appendix II, with annotation #4, which exempts seeds in particular, and in Annex B of the European Union wildlife trade regulation (Regulation (EU) 2026/1383, in force since 29 June 2026), according to Species+. In Mexico, Enciclovida lists it as “en peligro de extinción” (category P) under the standard NOM-059-SEMARNAT-2010, a category confirmed by the text published in the Diario Oficial de la Federación on 30 December 2010. A plant offered for sale must come from cultivation and be accompanied by the required documents. Databases checked on 26 October 2026: check the regulations in force before any purchase or transport.

Growing Dioon argenteum

No published source describes the cultivation of Dioon argenteum in Europe, North America or Australia, and the advice below is deduced from its natural habitat rather than from documented trials. The species comes from well-drained limestone slopes, in a climate with a marked dry season, which points to a mineral, free-draining soil and moderate watering outside the growing season. Its presence in open woodland, between oaks and pines, does not tell whether it prefers full sun or filtered light in cultivation.

Outdoor and in-ground cultivation

The only report from a botanic garden comes from a humid tropical climate: at Nong Nooch, in Thailand, the cycad curator writes in 2022 that all the described Dioon grow and cone there, Dioon argenteum included, and that the genus seems to cope with excess rain and humidity. This observation, made in the ground in a hot climate, says nothing about how the species behaves in a cold, wet winter. In Mediterranean-type climates, a planting on a slope or a raised bed of mineral soil, in the warmest position available, follows the logic of its habitat; no such trial has been published.

Container cultivation

Specialist nurseries such as Canarius, in Tenerife, have offered two-year-old seedlings, presenting the seeds as rare and expensive. Growing it in a pot, in a very free-draining mix, allows it to be sheltered in winter for as long as its cold tolerance remains unknown.

In cultivation in the UK, US and Australia

  • United Kingdom: no British garden declares the species to BGCI PlantSearch, and no cultivation is documented.
  • United States: the Montgomery Botanical Center (Coral Gables, Florida) lists five accessions in its online catalogue, all from 2001 (20011257 to 20011261), and the Fairchild Tropical Botanic Garden (Miami) lists the species in its online catalogue. BGCI PlantSearch also lists Lotusland, the Huntington, Naples Botanical Garden, the San Diego Zoo, UC Berkeley, Descanso Gardens, the New York Botanical Garden, Pinecrest Gardens, the John Fairey Garden and the botanic garden of the University of Hawaii at Hilo. A BGCI listing is a declaration by the garden, not proof that the plant is alive today.
  • Australia: no Australian garden declares the species to BGCI.

Propagation

Sowing is the only method mentioned; no data specific to the species have been published on germination. Seeds are rare in the trade. The authors of the description note that plants branch after 2 m of trunk, but no source describes rooting these branches or separating offsets. For the general method, see the site’s article on growing cycads from seed.

Pests and diseases

No disease or pest specific to Dioon argenteum is documented in cultivation. In the wild, its cones host specialised insects, such as the weevil Parallocorynus gregoryi, but their effect on the plant has not been described. In cultivation, the risks reported for cycads in general apply: root and trunk rot in soil that stays wet, and scale insects.

Cold hardiness

Dioon argenteum has not been the subject of any dated observation of cold tolerance, in Europe or elsewhere. The figures in circulation are estimates by nurseries.

Mediterranean and European experience

No report of Dioon argenteum grown in the ground or in a pot is documented in France or in Italy. The observations published in southern Europe on the genus concern mainly Dioon edule, which is far more common, and cannot be transferred to this species. Without data, a pot kept frost-free and dry in winter remains the prudent choice, including in mild Mediterranean-type climates.

United Kingdom, United States and Australia

No dated report from a British, American or Australian grower has been found. The Tenerife nursery Canarius places the species in its “−10 °C to 0 °C” class (14 to 32 °F), without supporting observations; this is a commercial rating. In the wild the species grows between 1,100 and 1,600 m, but no climate data have been published for its sites.

Toxicity and precautions

No toxicity data have been published for Dioon argenteum. The risk is nevertheless documented in other cycads, which contain toxic glycosides (cycasin, macrozamin), from which methylazoxymethanol is derived, a toxin affecting the liver and the nervous system; poisonings are recorded in people, livestock and pets. The species should therefore be treated as potentially toxic: keep the seeds out of reach of children and animals, and wear gloves to clean them. If any part has been eaten, contact a poison centre, a doctor or a vet.

Traditional uses

No traditional use specific to Dioon argenteum has been documented. A 1997 study of the Dioon of Oaxaca reports that the leaves of these cycads decorate windows, doors and religious statues during festivals, especially at Easter, but it does not mention this species, which was described later.

Frequently asked questions

Why are the leaves of Dioon argenteum silver?

Because a silvery down covers the young leaves and lasts for at least a year. In its close relatives this down is soon lost. The newest crown therefore stays silver above the older, dark green ones, which gave the species its name.

How can it be told from Dioon purpusii and Dioon califanoi?

By the shape of the leaves and how long the down lasts. The leaves of Dioon argenteum are almost flat and stay tomentose; those of Dioon purpusii are moderately keeled and soon smooth; those of Dioon califanoi are strongly keeled, with glaucous, nearly spineless leaflets.

Is Dioon argenteum hardy?

Nobody knows. No dated trial has been published. A Canary Islands nursery places it in a “−10 °C to 0 °C” (14 to 32 °F) class, without supporting observations. In the meantime, grow it in a pot and protect it from frost and rain in winter.

How big does it get?

Its trunk can exceed 3 m in the wild, but very slowly. The trunk, 18 to 32 cm across, stays upright to about 3 m and then reclines; the leaves are 85 to 184 cm long. In cultivation, the plants available are young and small.

Is it endangered?

Yes. It is listed as Vulnerable by the IUCN (2022) and in CITES Appendix II. It is known only from four populations in the Sierra Norte of Oaxaca, estimated at about 3,000 plants in 2003.

Is it poisonous?

It should be treated as such. No data exist for the species, but other cycads contain toxins dangerous to people and animals. Keep the seeds out of reach of children and pets.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • Gregory, T.J., Chemnick, J., Salas-Morales, S. & Vovides, A.P. (2003). A new species in the genus Dioon (Zamiaceae) from north-central Oaxaca, Mexico. Botanical Journal of the Linnean Society 141(4): 471–476. doi:10.1046/j.1095-8339.2003.t01-1-00151.x [Protologue: description, habitat, comparison with Dioon purpusii and Dioon califanoi, estimated population.]
  • Gregory, T., Haynes, J. & Chemnick, J. (2008). Key to the species of Dioon. The Cycad Newsletter 31: 16–17. [Identification key based on adult leaves.]
  • Dorsey, B.L., Gregory, T.J., Sass, C. & Specht, C.D. (2018). Pleistocene diversification in an ancient lineage: a role for glacial cycles in the evolutionary history of Dioon Lindl. (Zamiaceae). American Journal of Botany 105(9): 1512–1530. doi:10.1002/ajb2.1149 [Peer-reviewed article; Pleistocene age of the species; abstract.]
  • 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, a new cycad species from Southeastern Oaxaca, Mexico. Phytotaxa 528(2): 93–110. doi:10.11646/phytotaxa.528.2.3 [Composition of the Purpusii clade and key; abstract and key (Plazi).]
  • O’Brien, C.W. & Tang, W. (2015). Revision of the New World cycad weevils of the subtribe Allocorynina. Zootaxa 3970(1): 1–87. doi:10.11646/zootaxa.3970.1.1 [Parallocorynus gregoryi collected from Dioon argenteum, according to the plate captions.]
  • Skelley, P.E., Tang, W. & Pérez-Farrera, M.A. (2022). Review of Pharaxonotha Reitter inhabiting the cycad genus Dioon Lindl., with descriptions of nine new species. Insecta Mundi 0908: 1–21. [Hosts of Pharaxonotha bicolor.]
  • Chemnick, J. & Gregory, T. (2022). Dioon argenteum. The IUCN Red List of Threatened Species 2022: e.T178880A69831083. [Category VU B1ab(iii,v); reference taken from The World List of Cycads and checked on Crossref.]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. doi:10.11646/phytotaxa.550.1.1 [Etymology of the epithet, as given by The World List of Cycads.]
  • Lindstrom, A. (2022). The cycad collection at Nong Nooch Tropical Botanical Garden, Thailand. CYCADS 6(1). [Cultivation of Dioon in a tropical climate.]
  • Chemnick, J., Gregory, T.J. & Salas-Morales, S. (1997). A revision of Dioon tomasellii (Zamiaceae) from western Mexico, a range extension of Dioon merolae, and clarification of Dioon purpusii. Phytologia 83(1): 1–6. [Uses of the Dioon of Oaxaca; full text on archive.org.]
  • 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.]