Dioon stevensonii

Dioon stevensonii is a medium-sized cycad endemic to south-western Mexico, where it grows on rocky outcrops in oak woodland and dry tropical forest in the Balsas river basin, between Guerrero and Michoacán. This species of the genus Dioon forms a trunk about 1.2 m tall and leaves around a metre long, whose young growth is covered in golden-brown down. Its populations were long included in Dioon tomasellii, a western Mexican species it resembles, until they were described as a separate species in 2009 and named after Dennis W. Stevenson, a botanist at The New York Botanical Garden. Listed as Endangered by the IUCN, it is very rare in cultivation and its cold tolerance has never been documented.

Key facts about Dioon stevensonii

  • Accepted name: Dioon stevensonii Nic.-Mor. & Vovides (2009), according to The World List of Cycads and IPNI; also accepted by Kew’s World Checklist of Vascular Plants, the database behind POWO (June 2026 version, published on GBIF)
  • Former names and synonyms: no synonym; its populations were formerly included in Dioon tomasellii
  • Family: Zamiaceae, order Cycadales
  • Common names: no English name attested; palma and palmita in its home region (see Common names)
  • Natural range: Mexico, Guerrero and Michoacán (Balsas basin), between 400 and 1,550 m according to the 2020 revision (500 to 1,200 m according to the original description)
  • IUCN status: Endangered (EN B1ab(ii,v); C2a(i)), assessment published in 2022; the original description of 2009 recommended Critically Endangered (CR C2a(ii))
  • CITES and other protection: CITES Appendix II (annotation #4: seeds exempt); Annex B in the European Union; in Mexico its populations appear in NOM-059-SEMARNAT-2010 under Dioon tomasellii, “in danger of extinction” (P)
  • Documented hardiness: not documented (see Cold hardiness)
  • Light and soil: not documented in cultivation; in the wild, rocky outcrops in oak woodland and tropical deciduous forest
  • Habit and adult size: trunk up to 1.2 m tall and 26 cm across, sometimes branching with age; up to 25 leaves, 63 to 125 cm long
  • In the UK, US and Australia: Montgomery Botanical Center (twelve accessions, nine of them from 2004), Fairchild Tropical Botanic Garden and Lotusland (BGCI); no garden documented in the UK or Australia
  • Toxicity: no analysis specific to the species; toxins documented in other cycads (see Toxicity and precautions)

How to recognise Dioon stevensonii

Dioon stevensonii is recognised by its young leaves clothed in golden-brown down and by its flat, straight or barely curved leaflets, set almost at right angles on the rachis, with one to four tiny teeth near the tip. The page of Fernando Nicolalde-Morejón, one of the authors of the species, at the University of Veracruz, and the revision of the genus published in 2020 by Hernández-Tapia, Jiménez-Ramírez and Vovides agree on the following description:

  • Habit: an aerial, cylindrical trunk about 1.2 m tall and 26 cm in diameter, sometimes branching with age;
  • Leaves: up to 25 in the crown, 60 to 125 cm long and 20 to 30 cm wide, erect to drooping; abundant golden-brown down when they emerge, yellowish later; petiole 8 to 19.5 cm long;
  • Leaflets: 50 to 85 pairs according to Nicolalde-Morejón (50 to 100 according to the revision), flat, linear to lance-shaped, rarely slightly sickle-shaped, not overlapping, 7 to 14 cm long; margin entire, with one to four tiny teeth near the tip;
  • Male cones: 20 to 25 cm long, covered in whitish down;
  • Female cones: ovoid, up to 35 cm long and 10 to 15 cm across; megasporophylls triangular;
  • Seeds: spherical to ovoid, 2.2 to 3.5 cm long, with a yellow sarcotesta when ripe.

The original description (Nicolalde-Morejón and co-authors, 2009), read in the copy held by The Cycad Society, adds detail. The trunk, up to 120 cm by 26 cm, sometimes forks in old plants and is protected by an armour of persistent leaf bases; the cataphylls are leathery and triangular, 6 to 8.5 cm long, with yellowish down. The 2 to 25 leaves, 63 to 125 cm long, are downy at every stage, golden-brown at emergence and yellowish at maturity; the unarmed petiole is 8 to 15 cm long and the rachis 55 to 110 cm. The 50 to 85 pairs of leaflets, 7 to 14 cm by 0.6 to 1.1 cm, are set at right angles about 0.5 cm apart and carry up to three fine spiny teeth (3 mm at most) on the upper margin and usually one on the lower margin. The male cone is solitary, 20 cm by 7.5 cm and densely white-woolly; the female cone is solitary, ovoid, 30 to 35 cm by 10 to 15 cm, on a stalk of 5 cm at most, with tightly overlapping, green, triangular megasporophylls 7 to 8.3 cm long whose tip is not turned back. The seeds, 2.9 to 3.3 cm long, have a yellow sarcotesta and a smooth inner shell marked with 13 to 15 radiating ridges.

Two details differ between the sources. According to the original description, the petiole and rachis of young leaves are densely covered in whitish down, then become greenish-yellow and hairless with age, rather than golden-brown; and the leaflets overlap slightly towards the leaf tip in young plants, but no longer in adults. The 2020 revision describes the megasporophylls as flat at the tip but curved elsewhere.

Hybrids

No hybrid of Dioon stevensonii has been reported.

Possible confusion

The historical confusion is with Dioon tomasellii, which grows further north-west (Durango, Jalisco, Nayarit, Sinaloa) and also has drooping leaflets. According to the 2020 revision, Dioon stevensonii has linear to lance-shaped leaflets, very rarely slightly sickle-shaped, and megasporophylls flat at the tip but curved elsewhere, whereas Dioon tomasellii has clearly sickle-shaped leaflets and entirely flat megasporophylls. The original description of 2009 contrasts them differently: in its key, Dioon tomasellii has sickle-shaped, lance-shaped leaflets that are spiny on the upper margin and megasporophylls with a turned-back tip (after De Luca and co-authors, 1984), while Dioon stevensonii has straight leaflets with one or two teeth and megasporophylls whose tip is not turned back. Origin is a great help: plants from the Balsas basin, in Guerrero and Michoacán, belong to Dioon stevensonii.

Two other species of southern Mexico resemble it. Dioon nuusaviorum, described from Guerrero in 2026, differs in its leaves, which are light green when they emerge (not golden-brown), in its overlapping to strongly overlapping leaflets, its microsporophylls with a drawn-out tip and its narrowly triangular, downy megasporophylls (Martínez-Domínguez and co-authors, 2026). Dioon holmgrenii, from Oaxaca, was compared with both species in the same study.

Taxonomy

Dioon stevensonii is accepted by The World List of Cycads, IPNI and Kew’s World Checklist of Vascular Plants (June 2026 version, published on GBIF). It was described by Fernando Nicolalde-Morejón and Andrew P. Vovides in 2009, in an article co-signed by Francisco Vergara-Silva, Jorge González-Astorga and Alejandro Espinosa de los Monteros in Systematics and Biodiversity (volume 7, issue 1, pages 73 to 79). The type was collected on 13 March 2007 at Coyuca de Catalán (Guerrero) by Nicolalde-Morejón, González-Astorga and Vergara-Silva (number 1551); the holotype is held at the XAL herbarium in Xalapa, with an isotype at MEXU.

The title of the article sums up the approach: “Reciprocal illumination of morphological characters upon a molecular hypothesis supports the proposal of a new species of cycad from Mexico”. Re-analysing nuclear DNA sequences produced for a study of the genus (Vovides and co-authors, 2007), the authors noticed that a specimen of “Dioon tomasellii” from the Balsas basin in Guerrero did not group with the other specimens of that species. Having ruled out a sequencing error, they re-examined the herbarium specimens in Xalapa (XAL and XALU) and then visited the populations. Two groups emerged: one from the type locality of Dioon tomasellii (Compostela, Nayarit), with Pánuco (Sinaloa) and El Tuito (Jalisco), and another from El Higueral (Michoacán), Arcelia, Guayameo and El Manchón (Guerrero). The differences held in plants grown for seven years under the same conditions at the Francisco Javier Clavijero botanic garden in Xalapa. The authors saw this as an unusual case of “reciprocal illumination”: morphology confirmed a molecular hypothesis, rather than the other way round.

Before 2009, plants from Guerrero and Michoacán were filed under Dioon tomasellii. Traces of this remain in the Mexican standard NOM-059-SEMARNAT-2010, which gives local names from Guerrero and Michoacán for Dioon tomasellii, and on the label of a beetle collected in 2004 from a cone in Guerrero, “ex Dioon tomasellii”, since corrected to Dioon stevensonii (Skelley and co-authors, 2022). In practice, an old plant labelled Dioon tomasellii that comes from these two states now corresponds to Dioon stevensonii.

In the 2020 revision, which follows the phylogeny of Dorsey and colleagues (2018), Dioon stevensonii belongs to the western clade, with Dioon tomasellii, Dioon sonorense and Dioon vovidesii. The 2009 description reached a different conclusion: despite its resemblance to Dioon tomasellii, its molecular analyses placed Dioon stevensonii closer to the groups of Dioon edule and Dioon spinulosum, on the Gulf of Mexico and Caribbean side. The published sources do not explain this change.

Etymology and history

The epithet honours Dennis William Stevenson, of The New York Botanical Garden, for his work on the morphology, taxonomy, systematics and genomics of cycads (original description; The World List of Cycads, after Haynes 2022). The genus name comes from the Greek dis, “two”, and ōon, “egg”, for the two seeds borne on each scale of the female cone.

In 1979 and 1980, the Naples botanists Aldo Moretti and Sergio Sabato visited populations in Guerrero, at Achotla and Arcelia, which today belong to this species; at the time they attributed them to Dioon tomasellii, which they were describing. In his account of the 1979 expedition, Moretti reports that the farmers of Achotla called the plant Palma de la Virgen and often gathered its leaves for religious occasions (Moretti, published in 2022). Plants of the species were photographed in Guerrero and Michoacán in May 2004, before it was described, during an expedition by Terrence Walters, Timothy J. Gregory, Jeffrey Chemnick and Silvia H. Salas-Morales (Montgomery Botanical Center photographs published by The World List of Cycads). No information is documented on its introduction into cultivation.

Common names

No English name is attested for Dioon stevensonii. In its home region it is called palma and palmita, according to the 2020 revision, which cites Bonta and co-authors (2019). These two generic names, meaning “palm” and “little palm”, are also used for other Mexican cycads and do not identify the species. In 1979, the farmers of Achotla called it Palma de la Virgen (“the Virgin’s palm”), a name also used for other Dioon.

In the wild

Dioon stevensonii is endemic to south-western Mexico: Guerrero and the neighbouring part of Michoacán. According to Nicolalde-Morejón, it grows on rocky outcrops in tropical deciduous forest and oak woodland in the Balsas depression, between 500 and 1,200 m. The 2020 revision widens the range to 400 to 1,550 m and describes temperate to warm, humid regions. The herbarium specimens it cites come from the municipalities of Arcelia, Coyuca de Catalán and Zirándaro (Guerrero) and Arteaga (Michoacán).

The original description details the vegetation: oak woodland and tropical deciduous forest in the sense of Rzedowski (1978), dominated by Quercus peduncularis and Quercus laurina, with a few Pinus oocarpa, Bursera submoniliformis and Inga spuria; the palm Brahea edulis forms patches along streams. The Balsas basin is made of Mesozoic igneous, metamorphic and sedimentary rocks reworked by Cenozoic volcanism. The climate is tropical, with a dry season from February to April and a rainy season from June to September, and a mean annual temperature of 22.7 to 24.7 °C (72.9 to 76.5 °F) (García 2004, cited by the authors). The exact localities were deliberately left out of the article to discourage commercial collecting.

The male cones host beetles. One species, Pharaxonotha gigantea, the largest known in its genus (about 5 mm), has been found only in a male cone of this plant, at Achotla (Guerrero), in oak woodland at about 1,380 m, together with a weevil, Parallocorynus andrewsi (Skelley and co-authors, 2022). These insects feed on cycad pollen.

Conservation status and regulation

Dioon stevensonii is listed as Endangered (EN) on the IUCN Red List under criteria B1ab(ii,v) and C2a(i): a small, fragmented range, a decline in occupied area and in the number of mature plants, and a small, declining population. The assessment, by Andrew P. Vovides and Fernando Nicolalde-Morejón, is dated July 2020 and was published in 2022 (reference e.T187845A1829767, according to The World List of Cycads and Crossref). The 2020 revision still cited an earlier Critically Endangered (CR) rating and listed collecting of plants for sale as ornamentals and habitat loss as threats. In 2009, the original description had already recommended CR C2a(ii), because no population was found in a protected area.

The species is covered by CITES Appendix II through the listing of all cycads (Species+, taxon 65534). Annotation #4, in the version in force since 5 March 2026, exempts seeds, pollen, in vitro cultures in sterile containers and cut flowers. In the European Union it is in Annex B (Regulation (EU) 2026/1383, applicable since 29 June 2026).

In Mexico, the 2010 list of the standard NOM-059-SEMARNAT-2010 does not name the species: its populations appear under Dioon tomasellii, classed “in danger of extinction” (P). The update of 14 November 2019 does not name it either and keeps Dioon tomasellii in category P. Databases checked on 26 October 2026 (Species+, GBIF, The World List of Cycads, Crossref); check the regulations in force before any purchase or import.

Growing Dioon stevensonii

No grower’s report on Dioon stevensonii has been published, and no nursery offer is documented. The original description mentions only living plants grown for about seven years under uniform conditions at the Francisco Javier Clavijero botanic garden in Xalapa (Veracruz), without describing those conditions. The growing advice found online for this species (light, watering, temperatures) is not based on any published observation. Its habitat of dry forest and oak woodland on rocks suggests a plant for very free-draining soil and a marked dry season, but this is only an inference.

Outdoor and in-ground cultivation

No outdoor planting is documented anywhere. By analogy with its habitat, a frost-free or almost frost-free climate with a dry winter, such as southern Florida or coastal southern California, is the most plausible setting, but this has not been tested in any published trial. No observation on this species has been published by the Jardin zoologique tropical at La Londe-les-Maures, in the south of France.

Container cultivation

In cooler climates, including the United Kingdom, the cautious choice is a pot in a very free-draining mix, moved into a bright, frost-free place for the winter. No growing result in a container has been reported.

In cultivation in the UK, US and Australia

  • United States: the Montgomery Botanical Center, at Coral Gables in Florida, holds twelve accessions in its online catalogue, nine of them from 2004, the year in which a Montgomery team photographed wild plants in Guerrero and Michoacán, one from 2020 and one from 2025, the year being taken from the accession number. Fairchild Tropical Botanic Garden (Miami) and Ganna Walska Lotusland (Santa Barbara, California) also declare the species to BGCI PlantSearch.
  • United Kingdom and Australia: no garden declares it to BGCI and no other source mentions it.

Propagation

No propagation data specific to Dioon stevensonii are documented. Sowing fresh seed is the normal method for cycads, and seeds are exempt from CITES permits. The branching trunks reported in old plants have not led to any report of propagation by offsets.

Pests and diseases

No disease or pest is documented for Dioon stevensonii. The cone beetles mentioned above feed on pollen and are not described as pests.

Cold hardiness

The cold hardiness of Dioon stevensonii is unknown: no measurement, no dated report and no reliable nursery estimate has been published.

Mediterranean and European experience

No growing report exists in Europe. On the French forum Fous de palmiers and the Italian forum Tropicamente, mentions of “stevensonii” concern another plant, Zamia stevensonii. The Jardin zoologique tropical at La Londe-les-Maures has published no observation on the species.

United Kingdom, United States and Australia

No cold reading has been published from these countries either. The figures of −1 to −3 °C (30 to 27 °F) or USDA zone 10a that circulate for this species are estimates without any observation behind them, and no USDA zone can be given.

Toxicity and precautions

No toxicity analysis has been published for Dioon stevensonii, but it should be treated as poisonous. Cycads contain toxic glycosides (cycasin, macrozamin), from which methylazoxymethanol is derived, a toxin affecting the liver and the nervous system. The risk is documented mainly in other cycads, for dogs that chew the seeds and for livestock. Wear gloves when handling the seeds and keep them out of reach of children and pets. If any part has been eaten, contact a poison centre, a doctor or a vet without delay.

Traditional uses

According to the 2020 revision, Dioon stevensonii has no known use other than as an ornamental, and collecting for sale is among its threats. In 1979, according to Aldo Moretti, the farmers of Achotla gathered its leaves for religious occasions.

Frequently asked questions

What is the difference between Dioon stevensonii and Dioon tomasellii?

Mainly the shape of the leaflets and of the female cone scales. Dioon stevensonii has straight or barely curved leaflets and megasporophylls that are curved except at the tip; Dioon tomasellii has clearly sickle-shaped leaflets and entirely flat megasporophylls, according to the 2020 revision. The original description contrasts instead the non-reflexed tip of the megasporophylls of Dioon stevensonii with the turned-back tip of those of Dioon tomasellii. The first grows in Guerrero and Michoacán, the second further north-west.

Why was it separated from Dioon tomasellii?

Because molecular data, then morphology, showed that the plants of Guerrero and Michoacán formed a distinct species. A specimen from Guerrero stood apart from the other Dioon tomasellii in a DNA analysis, and examination of herbarium specimens, wild populations and cultivated plants confirmed differences in the leaflets and in the tip of the female cone scales.

Is Dioon stevensonii hardy?

Nobody knows. No cold trial has been published anywhere, and the values in circulation are estimates without a verifiable basis. Treat it as frost-tender until proven otherwise.

How big does it get?

Medium-sized. In the wild the trunk reaches about 1.2 m (4 ft) and 26 cm (10 in) across, with up to 25 leaves 63 to 125 cm (2 to 4 ft) long. Old plants sometimes branch.

Can it be bought?

Rarely. No nursery offer is documented; the species is held by a few American botanic gardens. Seeds are exempt from CITES permits but plants are not, and wild-collected plants should be avoided.

Is it poisonous?

Yes, as a precaution. No analysis exists for the species, but the cycads studied contain dangerous toxins, especially in the seeds. If any part is eaten, call a poison centre or a vet.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • Nicolalde-Morejón, F., Vergara-Silva, F., González-Astorga, J., Vovides, A.P. & Espinosa de los Monteros, A. (2009). Reciprocal illumination of morphological characters upon a molecular hypothesis supports the proposal of a new species of cycad from Mexico. Systematics and Biodiversity 7(1): 73–79. doi:10.1017/S1477200008002879 [Protologue: approach, description, types, habitat, climate, key, proposed status; full text.]
  • 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, habitat, altitudes, local names, clades; treatment (Plazi).]
  • 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; open-access article.]
  • Skelley, P.E., Tang, W. & Pérez-Farrera, M.A. (2022). Review of Pharaxonotha Reitter (Coleoptera: Erotylidae: Pharaxonothinae) inhabiting the cycad genus Dioon Lindl. (Cycadales), with descriptions of nine new species and comments on P. kirschii Reitter. Insecta Mundi 0917: 1–41. doi:10.5281/zenodo.6391973 [Pharaxonotha gigantea, label “ex Dioon tomasellii”; open-access article.]
  • 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 [Four clades; abstract.]
  • Moretti, A. (2020 [2022]). Spedizioni scientifiche dell’Orto Botanico di Napoli – Messico 1979. Delpinoa 62: 7–48. [Achotla in 1979; in Italian.]
  • Vovides, A.P. & Nicolalde-Morejón, F. (2022). Dioon stevensonii. The IUCN Red List of Threatened Species 2022: e.T187845A1829767. doi:10.2305/IUCN.UK.2022-1.RLTS.T187845A1829767.en [Category EN and criteria; reference checked on The World List of Cycads and Crossref.]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. doi:10.11646/phytotaxa.550.1.1 [Etymology, 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.]