The genus Dioon, the chestnut dioon and its relatives

The genus Dioon brings together nineteen cycads of the Americas, almost all of them Mexican: eighteen species grow in Mexico, from Sonora to Chiapas, and a single one, Dioon mejiae, in Honduras. They are palm-like plants whose trunk stays short or partly buried in some species but exceeds ten metres in Dioon spinulosum. Many grow on rocky slopes, in dry forest or among oaks; others, such as Dioon spinulosum, live in the shade of tropical forest. Their growth is very slow and their lifespan exceptional: as early as 1911, a Dioon edule in Xalapa was estimated to be 970 years old. Lindley published the genus in 1843 with the spelling Dion, still found in old books, and it was also described as Platyzamia. Two current species were long treated as varieties: Dioon angustifolium, long known as Dioon edule var. angustifolium, and Dioon sonorense, described as Dioon tomasellii var. sonorense. The whole genus is listed in CITES Appendix II, and fourteen of the fifteen species assessed by IUCN are Vulnerable or Endangered. In English-speaking gardens the best known are the chestnut dioon, Dioon edule, and the giant dioon, Dioon spinulosum.

Key facts about Dioon

  • Accepted name: Dioon Lindl., published in 1843 in Edwards’s Botanical Register 29 (misc.): 59 with the spelling Dion; the spelling Dioon is conserved
  • Number of species: 19, with no infraspecific taxa (The World List of Cycads, version 2026.08.15)
  • Former names: Dion Lindl. (1843); Platyzamia Zucc. (1846); at species level, Dioon edule var. angustifolium (now Dioon angustifolium), Dioon tomasellii var. sonorense (now Dioon sonorense), Dioon edule var. latipinnium (placed under Dioon mejiae)
  • Family: Zamiaceae, in which Dioon forms the oldest branch according to genetic analyses
  • Common names: chestnut dioon (Dioon edule) and giant dioon (Dioon spinulosum) in English; chamal, tiotamal or palma de dolores in Mexico (Dioon edule), tiusinte in Honduras (Dioon mejiae)
  • Natural range: Mexico (18 species) and Honduras (1 species); no confirmed natural occurrence in Nicaragua
  • IUCN status: 8 species Endangered (EN), 6 Vulnerable (VU), 1 Near Threatened (NT), 4 Not Evaluated (NE); all the assessments cited date from 2022
  • CITES: Appendix II since 4 February 1977, with annotation #4, which exempts seeds in particular; Annex B in the European Union
  • Documented hardiness: the University of Florida gives Dioon edule for USDA zones 8 to 11 and Dioon spinulosum for zones 9b to 11. In France, Dioon edule survived in the ground at −7 °C (19.4 °F) under passive protection (Paris suburbs, winter 2009–2010) and at −8 to −10 °C (17.6 to 14 °F) under fleece (young plant, Nîmes, February 2012), but a potted plant was lost at −7 °C in Ravenna (Italy, December 2009); a potted Dioon spinulosum took −2 °C (28.4 °F) one morning without harm (see Cold hardiness)
  • Light and soil: full sun to partial shade for Dioon edule, partial to full shade for Dioon spinulosum (University of Florida); very free-draining soil
  • Habit: trunk rarely taller than 1.2 to 1.5 m (4 to 5 ft) in Dioon edule; up to 10 to 15 m (33 to 49 ft) in Dioon spinulosum (Chamberlain, 1919)
  • Lifespan: age estimated at 970 years for a Dioon edule less than 1.5 m tall (Chamberlain, 1919), an estimate confirmed by Vovides (1990)
  • Toxicity: seeds toxic when raw, as in all cycads (see Toxicity and precautions)

Characteristics of the genus Dioon

A Dioon is recognised by its leaflets with no midrib, crossed by many parallel veins, and by its female cones, each scale of which carries two seeds, hence the name of the genus. The World List of Cycads describes it as follows:

  • Habit: dioecious, palm-like plants with a thick, cylindrical stem, above or below ground. Depending on the species, the bases of old leaves persist on the trunk or fall, leaving a smooth bark. Most species produce offsets at the base; aerial branching is rare.
  • Leaves: pinnate, spirally arranged and separated by cataphylls (leaves reduced to scales). The lowest leaflets are often reduced to spines. The petiole has no prickles and is often swollen at the base. The leaves are hairy, at least when young.
  • Leaflets: simple, often edged with spines, broad-based, not jointed, inserted near the edges of the upper side of the leaf axis. They have no midrib but many parallel veins that fork.
  • Cones: male and female cones on stalks. Each scale of the female cone ends in a flattened, upturned part and carries two ovules, rarely three, facing the cone axis.
  • Seeds: nearly round to ellipsoid, covered with a white or cream fleshy layer (sarcotesta).
Dioon merolae
Dioon merolae grown under glass at the Palmengarten in Frankfurt, Germany

The cones can be enormous. Charles J. Chamberlain, who studied the Mexican cycads in the field between 1904 and 1910, describes female cones of Dioon spinulosum more than 50 cm long and weighing 9 to 14 kg, holding 200 to 300 seeds about 4 cm long; those of Dioon edule weigh 4.5 to 6.5 kg. He also points out a feature of the genus: growth rings in the wood, which were thought to be absent in cycads. In Dioon spinulosum a ring forms with each new crown of leaves; in Dioon edule the rings mark only long resting periods.

Dioon or Cycas?

In the garden, young Dioon are sometimes mistaken for Cycas revoluta. The simplest test is the leaflet: according to the descriptions in The World List of Cycads, a Cycas leaflet has a single thick midrib, a Dioon leaflet many parallel veins and no midrib. The female organs differ too: in Cycas the fertile leaves form an open rosette at the top of the stem, which then carries on growing, whereas Dioon forms a true cone. Finally, Cycas seeds have a yellow, orange or brown fleshy coat, Dioon seeds a white or cream one. Geography also separates them: Dioon is American, Cycas Asian, African and Oceanian, and the African genus Encephalartos does not grow in the Americas either. The other American genera, Zamia and Ceratozamia, have leaflets jointed at the base, unlike Dioon.

Taxonomy and classification

Dioon is an accepted genus of the family Zamiaceae. The World List of Cycads, the reference checklist of the IUCN Cycad Specialist Group, recognises 19 species in its version of 15 August 2026, with no subspecies or varieties. Kew’s World Checklist of Vascular Plants, the database behind POWO (the site’s nomenclatural authority), accepts 18 in its June 2026 version published on GBIF: it differs on the Honduran species (see Dioon mejiae below) and does not yet include Dioon nuusaviorum, described in 2026.

From Dion to Dioon

John Lindley published the genus in 1843 as Dion, according to IPNI. The spelling Dioon, closer to the Greek etymology, is the one used by The World List of Cycads, GBIF and most works; the French TAXREF database lists it as conserved. Both spellings therefore appear in the old literature, for example Dion edule or Dion angustifolium. The genus Platyzamia, published by Zuccarini in 1846, is a synonym: its species Platyzamia rigida is now placed under Dioon edule.

The oldest branch of the Zamiaceae

Genetic data set Dioon apart within the family. The analysis of five nuclear genes published by Salas-Leiva and colleagues in 2013 (Annals of Botany) shows that Dioon is the first lineage to split off after Cycas, before Bowenia and all the other Zamiaceae. The same study estimates that the living cycad genera are all less than 12 million years old in their modern form: the lineage is ancient, but the present species are recent.

Two main groups of species

Within the genus, a study based on nuclear and chloroplast DNA (González, Vovides and Bárcenas, Systematic Botany, 2008) confirmed two main groups, which set the species of the Sierra Madre Occidental, on the Pacific side, against those of the Sierra Madre Oriental, on the Gulf of Mexico side. In 2018 Gutiérrez-Ortega and colleagues distinguished four lineages descended from a common ancestor in eastern Mexico: one diversified in south-eastern Mexico and Central America, the other three in north-eastern, southern and north-western Mexico, mostly in the Miocene, about 15 million years ago. A second study by the same team shows that moves into arid habitats happened in only one of the two main groups, and that the species of dry areas evolved adaptations of the leaf epidermis.

Recently described species, and others that changed rank

The number of species has risen sharply over the last forty years. De Luca and Sabato, mostly with Vázquez Torres, described six species between 1979 and 1984: Dioon califanoi, Dioon caputoi, Dioon rzedowskii, Dioon merolae, Dioon holmgrenii and Dioon tomasellii. Seven more have followed since 2003, the latest being Dioon nuusaviorum, published in 2026 in PhytoKeys. Two older names have changed rank. Dioon angustifolium, described by Miquel in 1847 and later reduced to a variety or subspecies of Dioon edule, was recognised again as a species in 2003 by González-Astorga and colleagues after a genetic and morphological study, a choice confirmed in 2005 and followed by The World List of Cycads. Dioon sonorense was described in 1984 as a variety of Dioon tomasellii and raised to species rank in 1998 by Chemnick, Gregory and Salas-Morales (Phytologia). For the reader, this means that a plant bought under the old variety name now belongs to a species in its own right.

Dioon mejiae or Dioon pectinatum?

The sources disagree on the Honduran species. The World List of Cycads accepts Dioon mejiae Standl. & L.O.Williams (1950) and treats Dioon pectinatum Mast. (1893), known only from an illustration in the Gardeners’ Chronicle, as a doubtful name (nomen dubium). Kew’s World Checklist of Vascular Plants, on which POWO is based, does the reverse: it accepts Dioon pectinatum, restricted to Honduras, and places Dioon mejiae in synonymy (June 2026 version, published on GBIF). The site follows The World List of Cycads and keeps the name Dioon mejiae, used by IUCN, CITES and almost all the literature.

Etymology and history of the genus

The name Dioon comes from the Greek dis, “two”, and ôon, “egg”, in allusion to the two seeds carried by each scale of the female cone (The World List of Cycads). Lindley described the genus and its first species, Dioon edule, in 1843, from a plant brought from Mexico and grown in the garden of the Horticultural Society of London, now the Royal Horticultural Society. The epithet edule means “edible”, a reference to the eating of the seeds in Mexico (Haynes, 2022). Several species names honour botanists from Naples and elsewhere in Italy who studied the genus: Luigi Califano (califanoi), Giuseppe Caputo (caputoi), Aldo Merola (merolae) and Ruggero Tomaselli (tomasellii). Dioon mejiae honours Isidoro Mejía, a doctor in Danlí, Honduras, in whose garden the type plant was collected. Dioon nuusaviorum pays tribute to the Ñuu savi people, whose name means “people of the rain”.

At the beginning of the 20th century the American botanist Charles J. Chamberlain made Dioon known in their natural habitat. In The Living Cycads (1919) he tells how he found Dioon edule in abundance at Chavarrillo, an hour by train from Xalapa (Veracruz). Dioon spinulosum, on the other hand, was known only from a few potted plants, wrongly reported from Progreso, in Yucatán, and from Córdoba. Chamberlain finally located it around 1908 near Tierra Blanca and in the mountains beyond Tuxtepec, where, in his words, “it would not be exaggerating to speak of a Dioon forest”.

Lifespan: plants a thousand years old

Yes, Dioon are among the longest-lived seed plants, and one of the rare cycads whose age has been estimated by a published method. Chamberlain counted the bases of old leaves covering the trunk and divided them by the average number of leaves produced each year. In Dioon edule, a crown of about twenty leaves forms roughly every two years, about ten leaves a year. Professor Luis Murillo had recorded every leaf produced over eleven years by the plant in his garden in Xalapa: this plant, less than 1.5 m tall, was thus estimated to be 970 years old. Chamberlain noted that the method underestimates the real age, because young plants produce very few leaves and plants sometimes rest for several years after coning.

In 1990 Andrew P. Vovides studied a wild population of Dioon edule in central Veracruz and confirmed this age estimate, according to the abstract of his article in the American Journal of Botany. He also measured how slowly the plants reproduce: one cone every 2.8 to 8.8 years in male plants, and every 10 to 52 years in female plants. Murillo’s plant has been found again in the garden of the Quinta de las Rosas in Xalapa; an article by the Universidad Veracruzana published in 2017 estimates that it is now more than a thousand years old.

Common names

In English, the University of Florida’s gardening service uses “chestnut dioon” for Dioon edule and “giant dioon” for Dioon spinulosum; otherwise the genus name, dioon, is used as a common name. The names in use in the countries of origin are Spanish:

  • Mexican Spanish: tiotamal, quiotamal and chamal for Dioon edule around Xalapa (Universidad Veracruzana, 2017); palma de santa teresita and palma de dolores for Dioon edule; coyolillo, palma de chicle and palma de dolores for Dioon spinulosum; palma de la virgen for Dioon sonorense and Dioon tomasellii; palma del sol and plumilla for Dioon holmgrenii; palma real for Dioon purpusii and Dioon caputoi (Species+)
  • Honduran Spanish: tiusinte for Dioon mejiae (Bonta et al., 2006); Species+ gives palma teosinte

Several of these names are ambiguous: palma de la virgen, palma real and palma de dolores apply to several species, and the word “palma” should not suggest that these plants are palms.

Biogeography and ecology

The genus Dioon is centred on the mountains of Mexico. Its species fall into three geographical groups, plus the Honduran species. According to The World List of Cycads:

  • North-east and Gulf of Mexico slope: Dioon edule (Hidalgo, Querétaro, San Luis Potosí, Tamaulipas, Veracruz), Dioon angustifolium (Nuevo León, Tamaulipas) and Dioon spinulosum (Veracruz, Oaxaca);
  • Southern Mexico: twelve species, ten of them in the state of Oaxaca, often over a very small area, along with Dioon merolae, also found in Chiapas, Dioon nuusaviorum in Guerrero and Dioon stevensonii in Guerrero and Michoacán;
  • North-west, Pacific slope: Dioon sonorense (Sonora, Sinaloa), Dioon vovidesii (Sonora) and Dioon tomasellii (Durango, Jalisco, Nayarit, Sinaloa);
  • Honduras: Dioon mejiae, in the departments of Colón, Olancho and Yoro.

Habitats range from moist forest to arid zones, and many are dry for part of the year. Chamberlain describes Dioon edule “in full tropical sun”, taller than the small mature oaks around it, with “wild pineapples”, cacti and drought-tolerant ferns. Vovides (1990) found it in clustered stands on thin soils and rocky outcrops in tropical deciduous forest, where prolonged drought and fire kill most seedlings. At the other extreme, Dioon spinulosum grows on limestone rocks in the shade of tall forest where Chamberlain names “Spanish cedar”, mahogany and ceiba. The type localities also show the range of altitudes: 650 to 850 m for Dioon holmgrenii, about 1,100 m for Dioon argenteum, 1,600 m for Dioon planifolium and about 2,000 m (some 6,600 ft) for Dioon caputoi.

What about Nicaragua?

Several horticultural websites and secondary databases state that Dioon mejiae also grows in Nicaragua. Neither The World List of Cycads nor Species+ includes that country: both restrict the species to Honduras. No Dioon species is therefore considered native to Nicaragua.

Traditional uses

The seeds of several Dioon have been used as food. Chamberlain reports that those of Dioon edule and Dioon spinulosum were made into flour for tortillas, and that children made toys from the hard seed coat. In Honduras, the ethnobotanical study by Bonta and colleagues (Journal of Ethnobiology, 2006) shows that tiusinte supplements the maize-and-bean diet of some 33,000 people: the seeds, harvested from the female cones, are made into tamales and tortillas, and the leaves decorate Catholic festivals. These uses rely on long traditional preparation; the raw seeds are toxic (see Toxicity and precautions).

Regulation and conservation

All Dioon species are protected under CITES, in Appendix II. According to Species+, the official CITES and EU database, the genus has been listed since 4 February 1977, under the listing of the Zamiaceae and of all cycads not in Appendix I. Species+ itemises only fourteen Dioon species, its standard reference list dating from 2013, but the genus-level listing also covers the species described since.

The listing carries annotation #4, which Species+ shows for the genus since 23 June 2010 and whose current version dates from 5 March 2026 (CoP20). For cycads, it exempts from CITES:

  • seeds, spores and pollen;
  • seedlings and tissue cultures obtained in vitro and transported in sterile containers;
  • cut flowers of artificially propagated plants.

Live plants, even young ones, therefore still need CITES permits for international trade, but seeds do not. The United Kingdom, the United States and Australia apply CITES through their own national legislation and permit systems. For readers in the European Union, the genus is in Annex B of Regulation (EC) No 338/97, with the same annotation; the list in force is set by Commission Regulation (EU) 2026/1383, applicable since 29 June 2026. In Mexico, the standard NOM-059-SEMARNAT-2010 also protects Dioon species. Before importing, selling or buying a Dioon, check its status on Species+ and with your national CITES authority, and ask for proof that the plant was propagated in cultivation. This general information does not replace the official texts. Species+ was consulted on 1 October 2026.

On the IUCN Red List, according to the references cited by The World List of Cycads, 15 species were assessed in 2022: 8 are Endangered, 6 Vulnerable and 1 Near Threatened (Dioon edule); none is Critically Endangered. The 4 species described since 2018 have not yet been assessed. The status of each species is given in the species list below. The threats cited by the studies consulted are habitat destruction, livestock farming, slash-and-burn agriculture and logging in Honduras (Bonta et al., 2006), as well as drought and fire, which kill seedlings in the wild (Vovides, 1990).

Growing Dioon

Dioon grow well as long as three rules are respected: very free-draining soil, plenty of summer warmth and a rather dry winter. They grow very slowly: on the French forum Fous de palmiers, a grower in the Ardèche reports a single leaf a year on a young Dioon edule.

Outdoor and in-ground cultivation

  • Light: Dioon edule likes full sun and tolerates partial shade; the University of Florida, by contrast, advises partial or full shade for Dioon spinulosum, an understorey species in the wild.
  • Soil: very free-draining and mineral (volcanic scoria, gravel, coarse sand). In heavy soil, plant on a mound or in a rock garden; the base of the stem must never sit in water. In growers’ reports, excess winter wet is the cause of failure most often given for plants in the ground.
  • Watering: growers advise watering in summer, while the plant is growing, letting it dry out between waterings, and keeping it almost dry in winter. Once established, Dioon stand drought well, according to the University of Florida.
  • Salt spray: the University of Florida says that Dioon edule tolerates salt well, which makes it a candidate for coastal gardens; nothing comparable is reported for Dioon spinulosum.
  • Propagation: mostly from seed, since seeds are exempt from CITES. A grower in Naples reports on the Italian forum Tropicamente that he has always obtained about 50 % germination with his Dioon edule seeds, and advises germinating them on a layer of moist perlite, covered, at about 25 to 30 °C (77 to 86 °F), before potting them up.

Container cultivation

Where winters are cold or wet, a pot is the safest way to grow Dioon, and the only sensible one for Dioon spinulosum and the rarer species. Use a deep pot and a mineral, free-draining mix, water freely in summer and keep the plant almost dry and frost-free in winter, in a cold glasshouse, conservatory or bright room. A pot does not protect the roots from cold: in Ravenna, in northern Italy, a 21-year-old Dioon edule in a pot, sheltered under a terrace, was lost after a frost of −7 °C (19.4 °F) in December 2009 (see below), whereas plants of the same species in the ground survived similar or lower temperatures in France.

Cold hardiness

The two species most often seen in cultivation behave very differently in the cold. Dioon edule is one of the hardiest cycads: the University of Florida gives it for USDA zones 8 to 11, that is, an average annual minimum of −12.2 °C (10 °F) or above. Dioon spinulosum is recommended there only for zones 9b to 11, that is, not below about −3.9 °C (25 °F). In Europe, growers’ reports point the same way, with one nuance: in Nantes, a loss of Dioon edule was put down by its owner to excess winter wet rather than to the cold itself.

Reports from Europe

In France, according to the reports below, Dioon edule gets through most winters in the ground in the Gard and the south-west; foliage damage was seen at about −6 to −7 °C (21.2 to 19.4 °F) without protection in Agen, followed by recovery. The species has also succeeded in the Paris suburbs under passive protection. The only failure reported in France was due to excess wet: in Nantes, plants died at only −4 °C (24.8 °F). For readers in mild, damp climates such as south-west England, the Atlantic reports are the closest guide, and they argue for a raised, very free-draining position. Dioon spinulosum stands a chance outdoors only in the mildest Mediterranean gardens; elsewhere, grow it in a pot brought in frost-free. These are amateur observations of indicative value, not general rules.

Dioon edule, France:

  • A grower in the Paris suburbs (Île-de-France) reports on the Fous de palmiers forum that his Dioon edule, in the ground since summer 2009, with a caudex of about 8 cm, survived −7 °C (19.4 °F) in winter 2009–2010 under passive protection, with its leaves intact. The same grower had lost a smaller Dioon angustifolium, protected the same way, at −12 °C (10.4 °F) in winter 2008–2009 (posts of April and November 2010).
  • A grower in Nîmes (Gard) reports on the same forum that his Dioon edule, an 18-month-old seedling planted in the ground in spring 2011, survived −8 to −10 °C (17.6 to 14 °F) in February 2012, with several days without thaw, protected only by P17 fleece and plastic sheeting: foliage intact, whereas his Cycas revoluta lost 80 % of its leaves (post of March 2012).
  • A grower in Agen (Lot-et-Garonne) reports on the Fous de palmiers forum that his Dioon edule, in the ground for two years and unprotected, survived −6 to −7 °C (21.2 to 19.4 °F) in winter 2009–2010: leaves burnt, then new growth in spring (posts of June and November 2010).
  • A grower in the Gard reports on the Fous de palmiers forum that his Dioon edule, planted in spring 2011, survived lows of −9 °C (15.8 °F) in winter 2011–2012 without any damage, in dry soil. He does not mention any protection (post of December 2012).
  • A grower in the Paris suburbs reports on the Fous de palmiers forum that his apple-sized Dioon edule, in the ground for its third winter under passive protection, had its crown burnt during the cold spell of February 2012, but its caudex stayed sound. He does not give the temperature (post of March 2012).
  • A grower in Nantes (Loire-Atlantique) reports on the Fous de palmiers forum that he lost the Dioon edule and Dioon califanoi he had planted in the ground in 2007, during a winter that was nonetheless mild (−4 °C, 24.8 °F, and few frost days), probably from excess wet despite good drainage. He does not mention any protection (post of January 2011).

Dioon edule, Italy:

  • A grower in Ravenna (Emilia-Romagna, Italy; USDA zone 8a/8b according to him) reports on the Tropicamente forum that his 21-year-old Dioon edule, in a pot under a terrace, against a wall and sheltered from rain, did not survive the episode of December 2009: −7 °C (19.4 °F) under the terrace (−10 °C, 14 °F, in the open), with hoar frost and a daytime maximum of −3 °C (26.6 °F). The plant looked intact in January 2010, had lost nearly all its leaves by April and was lost afterwards (posts from December 2009 to September 2012).

Dioon spinulosum:

  • A grower on the Côte Bleue (Bouches-du-Rhône, France) reports on the Fous de palmiers forum that his Dioon spinulosum, in a pot outdoors under a south-facing terrace, came through −2 °C (28.4 °F) one morning in winter 2012–2013 without after-effects (post of November 2013).

Reports from North America

In Florida, the University of Florida rates Dioon edule for USDA zones 8 to 11 and Dioon spinulosum for zones 9b to 11, and notes that the former tolerates salt and drought well once established. Further north, a small cycad nursery on the Sunshine Coast of British Columbia (Canada) reports on the UBC Botanical Garden forums, in May 2006, that a three-year-old Dioon edule planted out the previous winter lost its leaves and might not survive, while a Macrozamia communis planted at the same time stayed green; no temperature is given. In the wild, Dioon edule grows in full sun on thin, dry soils, where prolonged drought and fire are the main causes of seedling death (Vovides, 1990). No dated report from South Africa or Australia was retained for these two species.

In cultivation in the UK, US and Australia

In short: in Britain Dioon are glasshouse plants; in the United States they are grown outdoors in southern Florida and coastal California, and under glass in St. Louis, where two of the oldest cycads in an American garden still grow; in Australia, the large botanic gardens grow a few species. A species listed by a garden in the BGCI PlantSearch database (consulted in October 2026) shows that it has been declared there at some point, not that a living plant is there today.

  • United Kingdom: Kew declares Dioon edule, Dioon mejiae, Dioon merolae, Dioon purpusii, Dioon rzedowskii and Dioon spinulosum in BGCI PlantSearch. The Royal Botanic Garden Edinburgh’s Garden Explorer (updated 18 October 2026) lists Dioon edule, Dioon mejiae and Dioon spinulosum; its glasshouses are closed for restoration. The Cambridge University Botanic Garden catalogue lists Dioon edule, and Dioon spinulosum is declared in BGCI by eleven British gardens, among them the Chelsea Physic Garden, the Eden Project and RHS Wisley. No Dioon growing in the open ground in Britain is documented in a verifiable source.
  • United States: Montgomery Botanical Center (Coral Gables, Florida) holds 18 of the 19 species in its online catalogue, all but Dioon nuusaviorum; its outdoor Cycad Walk has an “oak woodland” section planted mainly with Dioon. Fairchild Tropical Botanic Garden’s Garden Explorer (updated 16 October 2026) has 19 Dioon entries. Naples Botanical Garden (Florida) received in a 2021 donation a Dioon merolae collected under permit in the early 1980s. At the Missouri Botanical Garden in St. Louis, two Dioon edule received from the Mexican commission at the 1904 World’s Fair were planted in the Climatron glasshouse in 1966 and are still alive; after hand pollination in September 2022, viable seeds were harvested in January 2024. The same garden still grows a female Dioon spinulosum donated in 1918 (Deloso et al., 2025). In California, Lotusland names Dioon oaxacensis among the notable plants of its cycad garden, where an old Dioon spinulosum was a favourite of Ganna Walska (summer 2026 newsletter); at the University of California Botanical Garden in Berkeley, a 2015 profile describes the San Francisco Bay climate as “marginal” for Dioon.
  • Australia: the Royal Botanic Garden Sydney has 7 Dioon entries in its Garden Explorer (updated 1 October 2026). At Melbourne, the Royal Botanic Gardens Victoria census places Dioon califanoi, Dioon edule and Dioon spinulosum in outdoor beds, and Dioon rzedowskii in the Tropical House. Dioon mejiae is also declared in BGCI by several Australian gardens, including Cairns and Adelaide.

Toxicity and precautions

Yes, Dioon are poisonous, and their seeds are particularly so when raw. Cycads contain liver and nerve toxins: cycasin, converted in the body into methylazoxymethanol (MAM), and a neurotoxic amino acid, BMAA. No analysis specific to the genus Dioon was consulted, but the risk is well documented in other cycads, above all for dogs that chew the seeds and for livestock that eat the leaves. The traditional eating of the seeds in Mexico and Honduras depends on long cooking or processing, which should not be copied at home. 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.

Species of the genus Dioon

Here are the 19 species accepted by The World List of Cycads (version 2026.08.15), arranged by region. For each: author and year of publication, range, meaning of the name where documented (Haynes, 2022), and IUCN status with the year of assessment. Every species has an English page on the site.

Mexico: north-east and Gulf slope

  • Dioon angustifolium Miq. (1847): Nuevo León and Tamaulipas; “narrow-leaved”, after the fine leaflets of juvenile leaves; long treated as a variety of Dioon edule; Vulnerable (VU, 2022).
  • Dioon edule Lindl. (1843): Hidalgo, Querétaro, San Luis Potosí, Tamaulipas and Veracruz; type species of the genus, “edible”; the chestnut dioon; with Dioon spinulosum, one of the two species most grown in Florida, and the hardier of the two (University of Florida); Near Threatened (NT, 2022).
  • Dioon spinulosum Dyer ex Eichler (1883): Oaxaca and Veracruz; “spiny”, after the leaflet margins; the giant dioon, the largest species in the genus, over 15 m (49 ft) in the wild (16 to 18 m, or 52 to 59 ft, according to the 2020 revision); Endangered (EN, 2022).

Mexico: south (Oaxaca, Puebla, Chiapas, Guerrero, Michoacán)

  • Dioon argenteum T.J.Greg., Chemnick, Salas-Mor. & Vovides (2003): Oaxaca, Sierra Norte; “silvery”, after the persistent down on the leaves; Vulnerable (VU, 2022).
  • Dioon califanoi De Luca & Sabato (1979): Oaxaca and Puebla; named after Luigi Califano; Endangered (EN, 2022).
  • Dioon caputoi De Luca, Sabato & Vázq.Torres (1980): Oaxaca and Puebla, at about 2,000 m; named after Giuseppe Caputo; Endangered (EN, 2022).
  • Dioon holmgrenii De Luca, Sabato & Vázq.Torres (1981): Oaxaca, type locality between Puerto Escondido and Oaxaca, at about 650 to 850 m; named after the botanist Noel H. Holmgren; Vulnerable (VU, 2022).
  • Dioon merolae De Luca, Sabato & Vázq.Torres (1981): Chiapas and Oaxaca; named after Aldo Merola; Vulnerable (VU, 2022).
  • Dioon nuusaviorum Mart.-Domínguez, Nic.-Mor. & D.W.Stev. (2026): Guerrero; named after the Ñuu savi people; Not Evaluated (NE).
  • Dioon oaxacensis Gut.Ortega, Pérez-Farr. & Vovides (2020): Oaxaca, Sierra Sur; “from Oaxaca”; Not Evaluated (NE).
  • Dioon planifolium Salas-Mor., Chemnick & T.J.Greg. (2016): Oaxaca, valley of the río Mixteco; “flat-leaved”; Vulnerable (VU, 2022).
  • Dioon purpusii Rose (1909): Oaxaca, Tomellín canyon; named after the botanist Carl A. Purpus; Endangered (EN, 2022).
  • Dioon rzedowskii De Luca, A.Moretti, Sabato & Vázq.Torres (1980): Oaxaca; named after the botanist Jerzy Rzedowski; Endangered (EN, 2022).
  • Dioon salas-moralesiae Gut.Ortega & Pérez-Farr. (2021): Oaxaca; named after the botanist Silvia H. Salas-Morales; Not Evaluated (NE).
  • Dioon stevensonii Nic.-Mor. & Vovides (2009): Guerrero and Michoacán; named after the botanist Dennis W. Stevenson; Endangered (EN, 2022).

Mexico: north-west (Pacific slope)

  • Dioon sonorense (De Luca, Sabato & Vázq.Torres) Chemnick, T.J.Greg. & Salas-Mor. (1998): Sonora and Sinaloa; formerly Dioon tomasellii var. sonorense; Endangered (EN, 2022).
  • Dioon tomasellii De Luca, Sabato & Vázq.Torres (1984): Durango, Jalisco, Nayarit and Sinaloa; named after Ruggero Tomaselli; Vulnerable (VU, 2022).
  • Dioon vovidesii Gut.Ortega & Pérez-Farr. (2018): Sonora, Sierra de Mazatán; named after the botanist Andrew P. Vovides; Not Evaluated (NE).

Honduras

  • Dioon mejiae Standl. & L.O.Williams (1950): Colón, Olancho and Yoro; the only species outside Mexico, called tiusinte; named after Isidoro Mejía; synonym: Dioon edule var. latipinnium Dyer; POWO apparently accepts Dioon pectinatum instead (see Taxonomy); Endangered (EN, 2022).

Nicaragua

No Dioon species is recognised as native to Nicaragua by The World List of Cycads or by Species+. Reports of Dioon mejiae in that country, common on horticultural websites, are unconfirmed (see Biogeography and ecology).

Frequently asked questions

How many species of Dioon are there?

Nineteen, according to The World List of Cycads (version of 15 August 2026). Eighteen grow in Mexico and one in Honduras. Only five of the present species were known before 1979; seven have been described since 2003, the latest, Dioon nuusaviorum, in 2026.

Is Dioon edule frost-hardy?

Yes, to a degree: it is clearly hardier than Dioon spinulosum, according to the University of Florida, which rates it for USDA zones 8 to 11, and to growers’ reports. In France, growers report plants in the ground surviving −7 °C (19.4 °F) near Paris under passive protection and −8 to −10 °C (17.6 to 14 °F) in Nîmes in February 2012 under simple fleece. The foliage may burn without protection, but the caudex can regrow. Losses come mainly from wet winters and poorly drained soil.

Can Dioon edule be grown outdoors in the UK?

Not on the evidence available: no Dioon growing in the open ground in Britain is documented in a verifiable source, and British gardens grow them under glass. The French Atlantic reports suggest that winter wet, rather than cold alone, is the main danger in mild, damp climates. If you try, choose a young plant, a raised, very free-draining bed in full sun and protection from winter rain; otherwise, grow it in a pot overwintered frost-free.

Can Dioon spinulosum stay outdoors in winter?

Rarely, except in the mildest gardens. The only report retained describes a potted plant unharmed after −2 °C (28.4 °F) one morning on the Mediterranean coast of France, and the University of Florida does not recommend it below USDA zone 9b, that is, about −3.9 °C (25 °F). Elsewhere, grow it in a pot and bring it in frost-free for the winter.

How old can a Dioon get?

Several centuries, and probably more than a thousand years. In 1911 a Dioon edule less than 1.5 m tall was estimated at 970 years old by counting the leaf bases on its trunk, and Vovides confirmed this estimate in 1990. The plant, found again in Xalapa, is now thought to be over a thousand years old.

Are Dion and Dioon the same genus?

Yes. Lindley published the genus in 1843 as Dion; the spelling Dioon, adopted later, is now the only one in use. Names such as Dion edule therefore refer to the same plants as Dioon edule.

Do I need a CITES permit to buy a Dioon?

It depends on what you buy. Live plants are in CITES Appendix II, so international trade in them requires documents. Seeds, on the other hand, are exempt under annotation #4. Buy plants propagated in cultivation, with proof of their origin.

Are Dioon seeds edible?

No, not raw: they are toxic, like those of all cycads. In Mexico and Honduras they are eaten after long traditional preparation, as flour for tortillas or tamales. Do not try to prepare them yourself, and call a poison centre if any are swallowed.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • Lindley, J. (1843). Dion edule. Edwards’s Botanical Register 29 (misc.): 59–60. [Publication of the genus and type species; details from IPNI and The World List of Cycads.]
  • Chamberlain, C.J. (1919). The Living Cycads. University of Chicago Press, Chicago. archive.org/details/livingcycads00cham [Field observations in Veracruz, age estimate for Dioon edule (970 years), dimensions of Dioon spinulosum, cones, growth rings, uses; early reference work.]
  • Vovides, A.P. (1990). Spatial distribution, survival, and fecundity of Dioon edule (Zamiaceae) in a tropical deciduous forest in Veracruz, Mexico, with notes on its habitat. American Journal of Botany 77(12): 1532–1543. doi:10.1002/j.1537-2197.1990.tb11394.x [Confirmation of Chamberlain’s age estimate, cone frequency, role of drought and fire; peer-reviewed article, consulted through its abstract.]
  • González, D., Vovides, A.P. & Bárcenas, C. (2008). Phylogenetic relationships of the neotropical genus Dioon (Cycadales, Zamiaceae) based on nuclear and chloroplast DNA sequence data. Systematic Botany 33(2): 229–236. doi:10.1600/036364408784571699 [Two main groups, Pacific and Gulf of Mexico; peer-reviewed article, consulted through its abstract.]
  • Salas-Leiva, D.E., Meerow, A.W., Calonje, M. et al. (2013). Phylogeny of the cycads based on multiple single-copy nuclear genes: congruence of concatenated parsimony, likelihood and species tree inference methods. Annals of Botany 112(7): 1263–1278. doi:10.1093/aob/mct192 [Dioon as the first branch of the Zamiaceae; peer-reviewed article, consulted through its abstract.]
  • Gutiérrez-Ortega, J.S., Salinas-Rodríguez, M.M., Martínez, J.F. et al. (2018). The phylogeography of the cycad genus Dioon (Zamiaceae) clarifies its Cenozoic expansion and diversification in the Mexican transition zone. Annals of Botany 121(3): 535–548. doi:10.1093/aob/mcx165 [Four lineages, origin in eastern Mexico, Miocene diversification; peer-reviewed article, consulted through its abstract.]
  • Gutiérrez-Ortega, J.S., Yamamoto, T., Vovides, A.P. et al. (2018). Aridification as a driver of biodiversity: a case study for the cycad genus Dioon (Zamiaceae). Annals of Botany 121(1): 47–60. doi:10.1093/aob/mcx123 [Moves into arid habitats in only one of the two main groups, epidermal adaptations; peer-reviewed article, consulted through its abstract.]
  • Bonta, M., Flores Pinot, O., Graham, D., Haynes, J. & Sandoval, G. (2006). Ethnobotany and conservation of tiusinte (Dioon mejiae Standl. & L.O. Williams, Zamiaceae) in northeastern Honduras. Journal of Ethnobiology 26(2): 228–257. doi:10.2993/0278-0771(2006)26[228:EACOTD]2.0.CO;2 [Food and religious uses, threats; peer-reviewed article, consulted through its abstract.]
  • Deloso, B.E., Colligan, A., Sucher, R. & Ramirez, V. (2025). Missouri Botanical Garden’s cycad collection: A journey through time. Plant Species Biology. doi:10.1111/1442-1984.12509 [Dioon edule of the 1904 World’s Fair in the Climatron, hand pollination of 2022 and seeds of 2024, Dioon spinulosum donated in 1918; open-access article.]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. [Etymology of the species names; cited by The World List of Cycads.]
  • Chemnick, J., Gregory, T. et al. (2022). Assessments of Dioon for The IUCN Red List of Threatened Species 2022 (for example Dioon edule: e.T42158A69829495; Dioon spinulosum, Vovides, Chemnick & Gregory: e.T42149A69830202; Dioon mejiae, Haynes & Bonta: e.T42184A69829967). [IUCN categories of 2022; references taken from 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: accepted species, synonyms, IUCN statuses.]