Dioon tomasellii

Dioon tomasellii is a cycad of western Mexico, found in the states of Durango, Jalisco, Nayarit and Sinaloa, where it grows in oak and pine-oak forest and in tropical forest between about 600 and 1,900 m. This species of the genus Dioon is recognised by its arching leaves, set with sickle-shaped leaflets that droop on either side of the rachis. It was described in 1984 under the spelling Dion tomasellii, with two varieties, and has since lost part of its range: its variety sonorense became Dioon sonorense in 1998, and the plants of Guerrero and Michoacán were described in 2009 as Dioon stevensonii. Listed as Vulnerable by the IUCN and covered by CITES Appendix II, it is rare in cultivation, and its cold hardiness rests only on nursery estimates.

Key facts about Dioon tomasellii

  • Accepted name: Dioon tomasellii De Luca, Sabato & Vázq.Torres (1984), 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: Dion tomasellii De Luca, Sabato & Vázq.Torres (original spelling); Dioon tomasellii var. tomasellii; the former Dioon tomasellii var. sonorense is now Dioon sonorense
  • Family: Zamiaceae, order Cycadales
  • Common names: no English name attested; palma de la virgen, palma del diablo, palma, palmita and peinetillo in Mexico (see Common names)
  • Natural range: Mexico, Pacific slope of the west: Durango, Jalisco, Nayarit and Sinaloa, between 600 and 1,900 m
  • IUCN status: Vulnerable (VU A2cd), assessment dated July 2020 and published in 2022
  • CITES and other protection: CITES Appendix II (annotation #4: seeds exempt); Annex B in the European Union; “in danger of extinction” (P) in Mexico under NOM-059-SEMARNAT-2010, a category kept in 2019
  • Documented hardiness: no dated report; nurseries estimate it at about −4 to −7 °C (25 to 19 °F) (see Cold hardiness)
  • Light and soil: sun or part sun according to the nurseries that sell it; in the wild, oak and pine forest and tropical forest
  • Habit and adult size: trunk about 1 m tall, rarely 2 m, and 25 cm across; 10 to 15 leaves, 100 to 150 cm long
  • In the UK, US and Australia: Montgomery Botanical Center (eight accessions), Fairchild Tropical Botanic Garden, The Huntington, Lotusland, the two San Diego gardens and UC Berkeley, among others (BGCI); Royal Botanic Garden Sydney; no garden documented in the UK
  • Toxicity: no analysis specific to the species; toxins documented in other cycads (see Toxicity and precautions)

How to recognise Dioon tomasellii

Dioon tomasellii is recognised by its long, arching leaves and its many sickle-shaped leaflets, spreading to drooping, which give the crown a soft look that is unusual in Dioon. Nurseries compare this habit with that of Encephalartos inopinus, a South African cycad with hanging leaflets. The revision of the genus published in 2020 by Hernández-Tapia, Jiménez-Ramírez and Vovides gives the following description:

  • Habit: a plant 1 m tall, rarely 2 m; trunk 25 cm in diameter; cataphylls triangular and woolly, 4 to 10 cm long;
  • Leaves: 10 to 15 in the crown, 100 to 150 cm long, becoming almost hairless except on the rachis and petiole, which keep golden-brown hairs; petiole 12.5 to 20 cm long, woolly at the base;
  • Leaflets: 83 to 110 pairs, sickle-shaped with a drawn-out tip, spreading to drooping, not overlapping (sometimes slightly); the middle ones 7.5 to 17 cm long and 0.6 to 1.1 cm wide; margin entire, sometimes with one to five small teeth;
  • Male cones: cylindrical, 35 to 50 cm by 8 to 10 cm, almost stalkless;
  • Female cones: ovoid, 20 to 30 cm by 15 to 20 cm; megasporophylls elongated-triangular and entirely flat;
  • Seeds: ovoid to globose, 2.4 to 3 cm long, with a yellow sarcotesta.

The original description (De Luca and co-authors, 1984) covered a broader species, from Guerrero to Sonora, but its characters remain useful. The sickle-shaped to slightly sickle-shaped leaflets are its main feature: they keep this shape from seedling to adult, whereas the curve appears only briefly in young Dioon edule and Dioon holmgrenii. The text describes a short trunk (under 100 cm), aerial or barely above ground, rarely with suckers at the base; numerous flat, stiff, upright leaves 1 to 2 m long, felted when young and later hairless or downy; an unarmed petiole 10 to 24 cm long (6 to 35 cm at the extremes); more than 70 leaflets on each side, set at right angles, 10 to 18 cm by 4.5 to 12 mm, spiny or entire, light green or glaucous; an elongated, cylindrical male cone 25 to 50 cm by 6 to 10 cm; an ovoid female cone 20 to 30 cm by 15 to 20 cm, with lance-shaped, densely woolly megasporophylls 4 to 5 cm long and 7 to 8 cm wide at the base, the outer part usually turned back; and cream or red seeds 2 to 2.5 cm by 2.5 to 3.5 cm, with a volume of 6 to 10 cm³. In the typical variety, the only one still called by this name, the leaflets are light green and 7 to 12 mm wide.

The sources disagree on one point. The turned-back tip of the female cone scales described in 1984 contradicts the 2020 revision, which describes them as entirely flat; for the authors of 1984, this was precisely the character that separated the species from Dioon edule, whose cone is of similar size. They also noted larger seeds than those of Dioon edule (5 to 10 cm³ against 4 to 7 cm³), varying with origin: 8 to 10 cm³ in Nayarit, 6 to 8 cm³ in Guerrero and 5 to 7 cm³ in Sonora, the last two origins now belonging to Dioon stevensonii and Dioon sonorense.

Hybrids

No hybrid of Dioon tomasellii has been reported.

Possible confusion

The species most easily confused with Dioon tomasellii are those that were split from it or long included in it. According to the 2020 revision:

  • Dioon stevensonii, from Guerrero and Michoacán, has linear to lance-shaped leaflets, very rarely slightly sickle-shaped, and megasporophylls flat only at the tip; according to its original description of 2009, the tip of its megasporophylls is not turned back, whereas that of Dioon tomasellii is;
  • Dioon sonorense, from Sonora and northern Sinaloa, has oblong, trowel-shaped megasporophylls; it is a smaller plant with narrow leaflets;
  • Dioon vovidesii, from central Sonora, has triangular but not elongated megasporophylls and leaflets 0.3 to 0.66 cm wide.

Cones are therefore the surest guide, but they are rare on cultivated plants. Origin often settles the question: Dioon tomasellii is the only one of these four species found from Durango to Jalisco. Old plants labelled Dioon tomasellii without a known origin may belong to any of them.

Taxonomy

Dioon tomasellii 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 in 1984 by Paolo De Luca, Sergio Sabato and Mario Vázquez Torres in Brittonia (volume 36, issue 3), under the title “Dion tomasellii (Zamiaceae), a new species with two varieties from western Mexico”. The authors then used the spelling Dion, Lindley’s original form of 1843, since abandoned in favour of the conserved spelling Dioon. The type was collected by Mario Vázquez Torres (number 2307) on 12 June 1979 in Nayarit, on the Laurel stream at Cerro Caratón, south of Costilla, at about 950 m; the holotype is held in Naples (NAP), with duplicates at ENCB, MEXU, NY and XALU.

The concept of the species has narrowed considerably since 1984:

  • 1984: description of two varieties, var. tomasellii and var. sonorense;
  • 1992: McVaugh and Pérez de la Rosa transfer var. sonorense to Dioon edule;
  • 1998: Chemnick, Gregory and Salas-Morales raise it to species rank as Dioon sonorense (Phytologia 83(1), dated 1997, issued in 1998);
  • 2009: the populations of Guerrero and Michoacán, until then filed under Dioon tomasellii, are described as Dioon stevensonii by Nicolalde-Morejón and Vovides;
  • 2018: the northernmost populations of Dioon sonorense are in turn described as Dioon vovidesii.

For the reader, this means that a plant sold in the past as Dioon tomasellii may belong to any of these four species, depending on its origin. In the 2020 revision, which follows the phylogeny of Dorsey and colleagues (2018), the four species together form the western clade of the genus.

Etymology and history

The epithet honours Ruggero Tomaselli (1920–1982), professor of botany at the University of Pavia and a former president of the Italian Botanical Society (The World List of Cycads, after Haynes 2022). The authors of 1984 dedicated the species to his memory, for his constant support of botanical exploration in Mexico. The name belongs to a series of epithets given to Italian botanists by the team of Paolo De Luca and Sergio Sabato, such as califanoi, caputoi and merolae. The genus name comes from the Greek dis, “two”, and ōon, “egg”, for the two seeds borne on each scale of the female cone.

Before 1984, collections of Dioon from western Mexico (Nayarit, Sonora), reported by Paul C. Standley (1920) and Howard S. Gentry (1942), had been wrongly referred to Dioon purpusii or filed by Joseph N. Rose under names that were never published. The authors of 1984 chose plants from Nayarit, in the middle of the range, as the type, because they best represented the variation of the species.

The story of the description is told by Aldo Moretti, one of the Naples botanists, in his accounts of the expeditions of 1979 and 1980, published in the journal Delpinoa in 2022 and 2023. In 1979 the team examined plants at several sites on the Pacific slope: near Achotla in Guerrero, where farmers called them Palma de la Virgen (these plants now fall within Dioon stevensonii), at Cabo Corrientes, south of Puerto Vallarta in Jalisco, and at Miravalles, near Compostela in Nayarit. Ruggero Tomaselli, then director of the botanic garden of the University of Pavia, took part in the 1980 expedition with his colleague Alberto Balduzzi; he died in a road accident in 1982. Moretti explains the five-year gap between the first visit and the publication by the need to find both male and female cones: in 1980 the team returned to Miravalles, reaching it with their feet almost always in water after the autumn rains, and found plants with female cones but no male cones, while no cone at all was found at Cabo Corrientes that year.

The original description shows one-year-old seedlings raised at the Naples botanic garden from seed collected in Nayarit, and a description of the garden’s glasshouses published in 2019 lists Dioon tomasellii among the species described by Naples botanists from plants collected in Mexico and kept in the Califano glasshouses. Two articles in the Cycad Newsletter (2008) dealt with the history, ecology, ethnobotany and conservation of the species (Gregory and Osborne; Argyros), according to The World List of Cycads. No date of introduction into cultivation in English-speaking countries is documented.

Common names

No English name is attested for Dioon tomasellii; names such as “Tomaselli’s cycad” do not appear in any reference source. In Mexico, Species+ gives the Spanish names palma del diablo, palma de la virgen, palmita and peinetillo, and the 2020 revision adds palma, after Bonta and Osborne (2007). The Mexican standard NOM-059-SEMARNAT-2010 specifies regional use: palma de la virgen in Sinaloa and Durango, palma and palmita in Guerrero, Michoacán and Nayarit, as the original description of 1984 already did. These names are ambiguous: palma de la virgen (“the Virgin’s palm”) is also used for Dioon sonorense, and palma or palmita for other cycads. The names recorded in Guerrero and Michoacán now refer to Dioon stevensonii.

In the wild

Dioon tomasellii grows on the Pacific slope of western Mexico. The World List of Cycads records it in Durango, Jalisco, Nayarit and Sinaloa. The specimens cited by the 2020 revision come in particular from Tamazula (Durango), from Cabo Corrientes, Puerto Vallarta, San Sebastián del Oeste and Talpa de Allende (Jalisco), from Compostela (Nayarit) and from Concordia (Sinaloa), between 400 and 1,900 m.

According to the revision, the species grows in tropical deciduous forest, pine-oak forest, pine forest, oak forest and tropical rainforest, in warm and humid or temperate climates, between 600 and 1,900 m. The original description listed only oak forest and pine-oak forest for the typical variety. No figures for the temperatures of its habitat have been published in a verifiable source.

Its male cones host insects that feed on pollen. In Jalisco, in the Guacamayas sanctuary at Cabo Corrientes, an oak forest mixed with tropical deciduous forest, a beetle described in 2022, Pharaxonotha novoai, was found in them together with a weevil, Parallocorynus andrewsi (Skelley and co-authors, 2022). The beetles associated with the species in Jalisco were also studied by Navarrete-Heredia (2018).

Conservation status and regulation

Dioon tomasellii is listed as Vulnerable (VU) on the IUCN Red List under criterion A2cd: a population reduction of at least 30% over three generations, caused by a decline in habitat and by exploitation. The assessment, by J.D. Bösenberg, is dated 15 July 2020 and was published in 2022 (reference e.T179414474A69832173, according to The World List of Cycads and Crossref); it replaced an earlier assessment by Andrew P. Vovides and Jeffrey Chemnick, published in 2010.

The species is covered by CITES Appendix II through the listing of all cycads (Species+, taxon 65535). 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 standard NOM-059-SEMARNAT-2010 classes the species “in danger of extinction” (P), in its 2010 sense, which still included the populations of Guerrero and Michoacán. The update of 14 November 2019 keeps it in category P. Databases checked on 30 October 2026 (Species+, GBIF, The World List of Cycads, Crossref); check the regulations in force before any purchase or import, and ask for proof that the plant was raised in cultivation.

Growing Dioon tomasellii

Dioon tomasellii is grown by a few collectors and botanic gardens, notably in California and Florida, but no detailed, dated grower’s report has been published. The indications below come from specialist nurseries and should be read as such:

  • Jungle Music, a southern California nursery, describes it as a medium-sized cycad with leaves 0.9 to 1.8 m (3 to 6 ft) long and a trunk usually no taller than 1.8 m (6 ft) at maturity; along the coast it takes sun or part sun, and its foliage can turn an almost lime green in sun. The nursery considers it extremely rare and almost impossible to find at coning size;
  • Cycadales.eu, a European nursery, describes it as slow-growing, liking heat and water in summer, and recommends full sun.

Its habitat of oak and pine forest with a marked rainy season suggests free-draining soil and summer watering, which matches the nurseries’ advice. For the general principles of growing Dioon, see the genus page.

Outdoor and in-ground cultivation

The only outdoor setting documented for the species itself is the coast of southern California, where the nursery quoted above grows it in sun or part sun. The botanic gardens that hold it are in frost-free or nearly frost-free climates (southern Florida, coastal California, Hawaii, Sydney), but the available sources do not say whether their plants grow in the open. The University of California Botanical Garden at Berkeley, which also holds it, describes the climate of the San Francisco Bay Area as “marginal” for the Mexican and Central American Dioon, adding that with careful siting they have prospered and coned (Plant Collections Network, 2015). 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. Seedlings sold in Europe are small: one nursery offers plants with two leaves in a 1.4-litre pot. 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, lists eight accessions in its online catalogue. Fairchild Tropical Botanic Garden (Miami), The Huntington (San Marino), Ganna Walska Lotusland (Santa Barbara), San Diego Botanic Garden (Encinitas), the San Diego Zoo, the University of California Botanical Garden at Berkeley, Moore Farms Botanical Garden (Lake City, South Carolina) and the botanic garden of the University of Hawaii at Hilo also declare it to BGCI PlantSearch.
  • Australia: the Royal Botanic Garden Sydney lists it in its online catalogue and declares it to BGCI.
  • United Kingdom: no garden declares it to BGCI and no other source mentions it.

Two American gardens, the Arizona-Sonora Desert Museum (Tucson) and the Los Angeles County Arboretum (Arcadia), still declare plants under the old name Dioon tomasellii var. sonorense, which now corresponds to Dioon sonorense or Dioon vovidesii.

Propagation

No propagation data specific to Dioon tomasellii are documented. Sowing fresh seed is the normal method for cycads, and seeds are exempt from CITES permits; the young plants offered in Europe are seedlings. The original description mentions suckers at the base only rarely, and no propagation by offsets has been reported.

Pests and diseases

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

Cold hardiness

The cold hardiness of Dioon tomasellii has never been measured in a dated report: the only figures available are nursery estimates, between about −4 and −7 °C (25 and 19 °F).

Mediterranean and European experience

No growing report exists in Europe. On the Italian forum Tropicamente, the species appears only in purchase discussions from 2011, without any growing result. The Jardin zoologique tropical at La Londe-les-Maures has published no observation on the species. Cycadales.eu considers it “probably hardy to zone 9B with the caudex buried”, which corresponds to average annual minimums of −3.9 to −1.1 °C (25 to 30 °F); this is a commercial rating, not a measurement.

United Kingdom, United States and Australia

Jungle Music, in coastal southern California, rates it cold hardy to “the mid, perhaps low 20’s F”, that is about −4 to −7 °C (25 to 19 °F); the estimate is neither dated nor linked to a place of measurement. No cold reading has been published from the United Kingdom or Australia, and no USDA zone has been measured for the species.

Toxicity and precautions

No toxicity analysis has been published for Dioon tomasellii, 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 tomasellii has no known use other than as an ornamental. No food or ritual use specific to the species is documented; the gathering of leaves for religious occasions reported at Achotla in 1979 concerns plants now placed in Dioon stevensonii.

Frequently asked questions

Is Dioon tomasellii hardy?

Nobody knows for sure. No dated cold report has been published, and the only figures, about −4 to −7 °C (25 to 19 °F), are nursery estimates. Outside frost-free climates, growing it in a pot brought under cover in winter is the cautious choice.

What is the difference between Dioon tomasellii and Dioon sonorense?

It is clear-cut. Dioon tomasellii has broad, sickle-shaped, drooping leaflets; Dioon sonorense, long treated as one of its varieties, is smaller, with narrow leaflets and trowel-shaped megasporophylls. The two have been separate species since 1998.

Why do its leaves droop?

Because of the shape of its leaflets. They are described as sickle-shaped and spreading to drooping, which gives the crown a soft look and sets it apart from most Dioon, whose leaflets are stiffer.

How big does it get?

Medium-sized. The trunk reaches about 1 m (3 ft), rarely 2 m (6.5 ft), and 25 cm (10 in) across, with 10 to 15 leaves 1 to 1.5 m (3 to 5 ft) long. Growth is slow.

Is an old plant labelled Dioon tomasellii really this species?

Not necessarily. Until 1998 the name also covered Dioon sonorense (and therefore Dioon vovidesii), and until 2009 it covered Dioon stevensonii. The origin of the plant, when known, is the best guide: Durango, Jalisco, Nayarit and Sinaloa for Dioon tomasellii in the present sense.

Is it protected?

Yes. It is listed as Vulnerable by the IUCN, covered by CITES Appendix II and Annex B in the European Union, and classed “in danger of extinction” in Mexico. Seeds are exempt from CITES permits, but plants are not.

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

  • 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 [Protologue (name published on page 225): description, varieties and key, habitat, local names, type, etymology; full text.]
  • Chemnick, J., Gregory, T.J. & Salas-Morales, S. (1997, issued 1998). 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. [Separation of Dioon sonorense, raised to species rank; range, altitudes and rainfall; full text (archive.org).]
  • 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, habitats, altitudes, local names, clades; treatment (Plazi).]
  • 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.]
  • 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 novoai in Jalisco; open-access article.]
  • Navarrete-Heredia, J.L. (2018). Beetles (Insecta, Coleoptera) associated with Dioon tomasellii (Cycadales) in Jalisco, Mexico, with comments on others beetles found on cycads. Proceedings of the Entomological Society of Washington 120(2): 458–465. doi:10.4289/0013-8797.120.2.458 [Cone beetles; reference checked on Crossref.]
  • Moretti, A. (2020 [2022]). Spedizioni scientifiche dell’Orto Botanico di Napoli – Messico 1979. Delpinoa 62: 7–48. [Sites visited and dedication; in Italian.]
  • Moretti, A. (2021–2022 [2023]). Spedizioni scientifiche dell’Orto Botanico di Napoli – Messico 1980. Delpinoa 63–64: 5–38. [Expedition with Ruggero Tomaselli, search for cones; in Italian.]
  • Sibilio, G. (2014–2015 [2019]). The greenhouses of the Botanical Garden of Naples, Italy. Delpinoa 56–57: 69–103. [Species kept in the Califano glasshouses.]
  • Gregory, T.J. & Osborne, R. (2008). Dioon tomasellii De Luca, Sabato & Vázq. Torres: history, ecology, ethnobotany and conservation status. Cycad Newsletter 31(4): 13–14. [Reference cited by The World List of Cycads.]
  • Bonta, M. & Osborne, R. (2007). Cycads in the vernacular: a compendium of local names. Proceedings of Cycad 2005: The 7th International Conference on Cycad Biology: 143–175. doi:10.21135/893274900.010 [Local names, cited by the 2020 revision.]
  • Bösenberg, J.D. (2022). Dioon tomasellii. The IUCN Red List of Threatened Species 2022: e.T179414474A69832173. doi:10.2305/IUCN.UK.2022-1.RLTS.T179414474A69832173.en [Category VU and criterion; 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.]