If you have grown Dioon edule or Dioon purpusii and think you know what a Dioon looks like, Dioon tomasellii will change your mind. Where most species in the genus produce stiff, erect leaves with rigid leaflets held close to the rachis, Dioon tomasellii produces long, gracefully arching leaves with markedly glossy, deflexed leaflets that hang downward from the rachis — an attitude more reminiscent of Encephalartos inopinus than of any other Dioon. This unusual leaflet posture, combined with a striking lacquer-like gloss, gives the plant an almost tropical elegance that is unique in the genus. Described in 1984 from the oak and pine-oak forests of western Mexico, Dioon tomasellii has since undergone a remarkable taxonomic dismemberment: the original concept included populations from Sonora to Guerrero, but successive revisions have carved out Dioon sonorense (1997), Dioon vovidesii (2018), and Dioon stevensonii (2009), leaving Dioon tomasellii sensu stricto restricted to the Sierra Madre Occidental of Durango, Nayarit, Jalisco, and southern Sinaloa. Even thus narrowed, it remains one of the most geographically widespread Dioon species alongside Dioon edule. In cultivation, it is a challenging and rewarding rarity: very slow, semi-deciduous, shade-preferring, and heat-demanding — the antithesis of the tough, sun-loving, drought-tolerant image most growers associate with the genus.
Quick Facts
| Scientific name | Dioon tomasellii De Luca, Sabato & Vázq.Torres |
| Family | Zamiaceae |
| Origin | Western Mexico — Durango, Nayarit, Jalisco, southern Sinaloa |
| Adult size | Trunk to 1–2 m (rarely more); crown spread 1.5–2 m |
| Hardiness | −3 to −5 °C (27 to 23 °F) / USDA zones 9b–11 (estimated) |
| IUCN | Vulnerable (VU) A2cd |
| CITES | Appendix II (all Dioon species) |
| Cultivation difficulty | 4/5 |
Taxonomy and Nomenclature
Dioon tomasellii was described by Pacifico De Luca, Salvatore Sabato, and Mario Vázquez Torres in 1984 in Brittonia (36(3): 225–227, figs. 2–5). The holotype was collected by M. Vázquez-Torres (no. 2307) on 12 June 1979 in the state of Nayarit, at Arroyo Laurel on Cerro Caratón, south of Costilla, in the Distrito de Miravalles, Municipio de Jalisco. The holotype is deposited at NAP (Naples), with isotypes at ENCB, MEXU, NY, and XALU.
The specific epithet honours Professor Ruggero Tomaselli (1920–1982), a distinguished Italian botanist who served as professor of botany at the University of Pavia and as president of the Italian Botanical Society. The choice of an Italian honoree reflects the pivotal role of the Neapolitan school of cycad taxonomy (De Luca, Sabato, and their colleagues) in the description of most Dioon species during the 1978–1984 period.
As originally described, Dioon tomasellii encompassed two varieties — var. tomasellii and var. sonorense — and a vast distribution from Sonora to Guerrero. The authors noted at the time that vegetative and reproductive characters showed “in general continuous variation patterns.” However, this assumption was based largely on conjecture rather than systematic field observation, and the species has since been progressively dismembered:
- 1997: Chemnick, Gregory, and Salas-Morales elevated var. sonorense to species rank as Dioon sonorense, based on a host of distinct morphological characters maintained even in cultivation, no continuous variation between the two taxa, and different habitats. This revision, published in Phytologia 83: 1–6, remains the fundamental reference for the narrowed circumscription of Dioon tomasellii.
- 2009: Nicolalde-Morejón, Vergara-Silva, González-Astorga, Vovides, and Espinosa de los Monteros described Dioon stevensonii from the Río Balsas basin (Michoacán, Guerrero), in a remarkable case of “reciprocal illumination”: molecular data first suggested that the Michoacán/Guerrero populations were phylogenetically distinct, then morphological re-examination confirmed leaflet contour, curvature, and megasporophyll tip differences. This further narrowed Dioon tomasellii by removing the southern populations.
- 2018: Gutiérrez-Ortega et al. described Dioon vovidesii from the northernmost populations of what had been Dioon sonorense, completing the fragmentation of the original “tomasellii” complex into four species.
Today, Dioon tomasellii sensu stricto refers only to populations in Durango, Nayarit, Jalisco, and southern Sinaloa — still a broad range by Dioon standards, but dramatically smaller than the original concept.
An interesting phylogenetic twist: despite the morphological similarity between Dioon tomasellii and Dioon stevensonii (both share deflexed, glossy leaflets), molecular analyses place Dioon stevensonii closer to the Dioon edule and Dioon spinulosum groups on the Gulf/Caribbean side. Meanwhile, trnL-F chloroplast data show that Dioon sonorense is the true sister species of Dioon tomasellii, sharing an ancestral haplotype. This is a classic case where morphological similarity misleads: the plant that looks like Dioon tomasellii (i.e. Dioon stevensonii) is not its closest relative, and its closest relative (i.e. Dioon sonorense) looks quite different.
Common names: Tomaselli’s Cycad (English); Palma de Tomaselli (Spanish, informal).
Morphological Description
Dioon tomasellii is a small to medium-sized arborescent cycad. The trunk is cylindrical, typically reaching 1–2 m in height (sometimes more in old specimens), clothed in persistent leaf bases.
Leaves: the defining character. Leaves are long and slightly arching, creating a graceful, almost weeping profile. The crown is typically sparse — few plants hold more than one or two flushes of leaves at any time, a characteristic that contributes to its semi-deciduous behaviour (discussed under Cultivation). Leaflets are the key diagnostic feature: very closely spaced, slightly curved (falcate to subfalcate), and markedly glossy — a high-gloss, lacquer-like sheen that is unique within the genus. They are lanceolate, broader than in Dioon edule or Dioon sonorense, and arranged sub-opposite along the rachis.
The most distinctive character is the leaflet deflexion: leaflets hang downward (are deflexed) from the plane of the rachis, like those of Encephalartos inopinus. This character is shared only with the sister species Dioon stevensonii among all Dioon species, and is entirely absent in the related Dioon sonorense (which has narrow, glaucous, stiffly held leaflets). Leaflet margins bear marginal spinulose denticles — small prickles typical of many Dioon species.
Cones: male cones are tomentose, cylindrical to conical. Female cones are ovoid. The species is dioecious. The cones of Dioon tomasellii are smaller than those of the “massive clade” species (Dioon spinulosum, Dioon mejiae, Dioon rzedowskii), consistent with its placement in the “edule clade” of smaller Dioon species.
Seeds: ovoid, with cream/white sarcotesta.
Similar Species and Common Confusions
The four species carved from the original “tomasellii” complex are the main sources of confusion:
| Character | Dioon tomasellii | Dioon stevensonii | Dioon sonorense |
|---|---|---|---|
| Leaflet texture | Glossy, bright green | Glossy, green | Glaucous, blue-green |
| Leaflet deflexion | Deflexed | Deflexed | Not deflexed (stiff) |
| Leaflet width | Broad, lanceolate | Broad, lanceolate | Narrow, linear |
| Leaflet shape | Falcate to subfalcate | Falcate to subfalcate | Slightly falcate |
| Megasporophyll tips | Not reflexed | Reflexed | Not reflexed |
| Phylogenetic affinity | Sister to D. sonorense | Closer to D. edule/D. spinulosum | Sister to D. tomasellii |
| Distribution | Durango, Nayarit, Jalisco, S. Sinaloa | Michoacán, Guerrero (Río Balsas) | Sonora, N. Sinaloa |
The identification key within the former “tomasellii” complex runs as follows: (1) leaflets narrow, glaucous, stiff = Dioon sonorense (or Dioon vovidesii for northernmost populations); (2) leaflets broad, glossy, deflexed → check megasporophyll tips: reflexed = Dioon stevensonii; not reflexed = Dioon tomasellii. Geography is also diagnostic: Dioon stevensonii occurs only in the Río Balsas basin (Pacific side, south of the Eje Neovolcánico), while Dioon tomasellii occurs in the Sierra Madre Occidental to the north.
Distribution and Natural Habitat
Dioon tomasellii sensu stricto is endemic to the Sierra Madre Occidental and its foothills in western Mexico. The current accepted distribution spans Durango, Nayarit, Jalisco, and southern Sinaloa, making it one of the more widely distributed Dioon species — alongside Dioon edule, the only other species with a range spanning multiple states.
The species occurs in oak forest and pine-oak forest, typically at elevations of 800–1,800 m. This places it within the Sierra Madre Occidental pine-oak forests ecoregion — one of the most biodiverse and species-rich in North America, home to globally unrivalled pine and oak diversity. The terrain is characterised by steep mountains cut through by deep barrancas (canyons), where the species typically grows on slopes and along stream courses (the type locality is at an arroyo). Associated vegetation includes Pinus oocarpa, Pinus devoniana, Quercus resinosa, Quercus crassifolia, Quercus magnolifolia, and a shrub layer with various Bursera species — reflecting the transition between pine-oak forest and tropical deciduous forest that characterises many Dioon habitats in western Mexico.
Climate in the native range:
The nearest representative weather station to the core range is Tepic, Nayarit (935 m elevation), which provides a useful baseline:
| Parameter | Tepic station (935 m) | Estimated at D. tomasellii sites (800–1,800 m) |
|---|---|---|
| Mean annual temperature | ~20 °C | 15–22 °C (cooler at higher elevations) |
| Mean January minimum | 8–10 °C | 4–10 °C |
| Historical minimum | ~4 °C (rare frost) | −1 to −3 °C (frost regular at higher elevations) |
| Mean summer maximum | 28–30 °C | 25–32 °C |
| Annual rainfall | ~1,590 mm | 800–1,600 mm (strongly summer-dominant) |
The climate is characterised by a pronounced dry season (November–May) and a heavy summer monsoon (June–October), with most annual rainfall concentrated in July–September. Winter nights at higher elevations in the Sierra Madre Occidental regularly bring frost, particularly from December to February. This seasonal cycle of drought followed by monsoon rain, with regular winter frost at elevation, is critical to understanding the species’ cultivation requirements.
Conservation
Dioon tomasellii is assessed as Vulnerable (VU) A2cd on the IUCN Red List, indicating an estimated population decline of at least 30% over the past three generations, driven by habitat degradation (criterion c) and exploitation (criterion d). Some sources list the species as Endangered, reflecting ongoing taxonomic uncertainty: whether the populations now assigned to Dioon stevensonii are included or excluded significantly affects the overall range estimate.
The principal threats are habitat destruction for agriculture and cattle ranching (clearing of oak and pine-oak forest) and overcollection for the horticultural trade. The species’ slow growth, low reproductive rate, and the demand from cycad collectors create a classic cycad conservation dilemma. All Dioon species are listed on CITES Appendix II.
Populations within the Cuenca Alimentadora del Distrito Nacional de Riego 043 Estado de Nayarit, a natural resources protection area established in 1949 to protect forests and watersheds in the southern Sierra Madre Occidental, may benefit from some degree of incidental habitat protection.
Cultivation
| Hardiness | −3 to −5 °C (27 to 23 °F) / USDA zones 9b–11 (estimated) |
| Light | Partial shade to bright shade (burns in full hot sun) |
| Soil | Well-drained, mineral-dominant; slightly acidic to neutral |
| Watering | Moderate in summer, dry in winter; less water is better in cool seasons |
| Adult size | Trunk to 1–2 m × crown 1.5–2 m |
| Growth rate | Very slow |
| Difficulty | 4/5 |
Dioon tomasellii is one of the more challenging species in the genus to cultivate successfully. It breaks several of the “rules” that growers learn from the easier species like Dioon edule.
Light: partial shade to bright shade. This is a critical departure from most Dioon species. In habitat, Dioon tomasellii grows beneath the canopy of oak-pine forest, not on exposed limestone cliffs. In cultivation in southern California, Geoff Stein reports that leaflets burn easily in hot, arid afternoon sun. The glossy leaflet surface, while beautiful, suggests adaptation to intercepting filtered light under a canopy rather than reflecting intense direct sun. Provide dappled light or morning sun with afternoon shade.
Heat: despite its shade preference, this species is very needy of heat. In southern California, it struggles near the cool coast and performs best in warmer inland locations. This combination — shade-loving but heat-demanding — narrows the range of suitable microclimates significantly and is the main reason for the 4/5 difficulty rating.
Semi-deciduous behaviour: perhaps the most unusual cultivation characteristic of this species. Plants tend to lose their entire set of leaves before producing a replacement flush, sometimes remaining leafless for weeks or months. Few plants hold more than one or two leaf flushes at any time. This “deciduous” behaviour — unprecedented in most other Dioon species — is consistent with the strongly seasonal climate in habitat, where a long dry season precedes the summer monsoon. Do not panic if your plant drops all its leaves in late winter or spring; new growth should follow when warmth and moisture increase.
Soil and drainage: well-drained, mineral-dominant soil is essential. The oak-pine forest habitat suggests a slightly acidic to neutral pH preference (unlike the limestone-adapted species of the Oaxacan canyons). A mix of pumice, coarse bark, and coarse sand works well. Avoid pure limestone substrate.
Watering: water moderately during the growing season (summer monsoon in habitat corresponds to June–October), then reduce or cease in winter. Less water is better during the cool, leafless dormant period. The summer monsoon delivers heavy rain but with good drainage on mountain slopes — replicate this by watering generously in warm weather but ensuring rapid drainage.
Cold hardiness: moderate. The pine-oak forest habitat at 800–1,800 m in the Sierra Madre Occidental regularly experiences frost, suggesting a tolerance of −3 to −5 °C for established plants. However, the species’ need for heat means that prolonged cold, grey winters are a bigger problem than occasional frost. USDA zone 9b and above, in a warm, sheltered, partially shaded position.
Container culture: recommended for most growers outside the tropics. Use a well-draining slightly acidic mix. Provide warmth and humidity during the growing season; allow to dry and cool (but not freeze) during dormancy. A bright conservatory with overhead shade is ideal.
Fertilization: light applications of balanced fertilizer during the active growing season only. Do not fertilize during the dormant leafless period.
Buying Advice
Availability: Dioon tomasellii is rare in the international trade. It is occasionally available from specialist cycad nurseries such as cycadales.eu (which has listed seedlings) and rarepalmseeds.com. Expect limited availability and moderate to high prices.
Identification: the combination of broad, glossy, deflexed leaflets on arching leaves is unmistakable among Dioon species. Even seedlings should show the characteristic glossy surface and the beginnings of the deflexed habit. If the plant has narrow, glaucous, stiff leaflets, it is more likely Dioon sonorense — a species that looks entirely different despite its former taxonomic union with Dioon tomasellii.
Beware of mislabelling: the complex taxonomic history of this species means that plants sold as “Dioon tomasellii” may in fact be Dioon sonorense, Dioon stevensonii, or even Dioon vovidesii. Confirm identification by the glossy, deflexed leaflet character. If the seller cannot specify the provenance (state of origin), exercise caution.
Propagation
Seed: propagation from seed follows the standard Dioon protocol: remove sarcotesta (wear gloves), soak 24–48 hours, sow horizontally in a well-draining mineral mix at 25–30 °C. Germination is cryptocotylar and typically requires 2–6 months, though it can be erratic. Seedlings are very slow growing; expect years before a recognisable plant develops.
Offsets: not typically produced in significant numbers.
Pests and Diseases
Root rot: the primary cultivation risk. The semi-deciduous habit means that root activity slows dramatically during the leafless dormant period; if the substrate remains wet during this time, root rot is highly likely. Reduce watering before and during leaf drop.
Cycad aulacaspis scale (Aulacaspis yasumatsui): likely susceptible, though less data exists than for commonly cultivated species.
Associated beetles: in habitat, Pharaxonotha spp. beetles have been documented in association with the cones of Dioon tomasellii in Jalisco (Navarrete-Heredia 2018), consistent with the beetle-mediated pollination syndrome universal in the genus.
Leaf scorch: excessive sun exposure causes browning and crisping of the glossy leaflets. This is a cultural problem, not a disease — move the plant to more shade.
Landscape Use
Dioon tomasellii is an exceptional collector’s cycad for those who can provide the right conditions: warm, humid, partially shaded. Its gracefully arching leaves with hanging, glossy leaflets create a tropical, almost fern-like silhouette that is completely unlike the stiff, rigid appearance of most cycads. This makes it an outstanding choice for shaded tropical gardens, ferneries, and conservatory plantings where a different cycad aesthetic is desired. The sparse crown and semi-deciduous habit mean it is not suited to formal landscape use where year-round structure is essential — but for the specialist collector, these are features, not flaws, providing seasonal rhythms rarely seen in the cycad world. Pair with tree ferns, shade-loving palms, and Zamia species for a Central American cloud forest vignette.
Frequently Asked Questions
Why does Dioon tomasellii drop all its leaves?
This semi-deciduous behaviour reflects the species’ adaptation to the strongly seasonal climate of the Sierra Madre Occidental, where a long dry season (November–May) precedes the summer monsoon. Plants typically lose their existing flush before producing new leaves once warm, moist conditions return. Do not overwater during the leafless period — the roots are less active and highly susceptible to rot.
What makes Dioon tomasellii different from Dioon sonorense?
Despite being originally described as a single species (var. tomasellii and var. sonorense), the two are visually very different. Dioon tomasellii has broad, bright green, glossy, deflexed leaflets on arching leaves; Dioon sonorense has narrow, glaucous blue-green, stiffly held leaflets. Chemnick et al. (1997) demonstrated that no pattern of continuous variation exists between the two and elevated var. sonorense to species rank.
Is Dioon tomasellii cold hardy?
Moderately. The pine-oak forest habitat at 800–1,800 m regularly experiences frost, suggesting tolerance of −3 to −5 °C. However, the species needs warm summers with heat and humidity to thrive. Prolonged cool, grey winters are more problematic than occasional frost. USDA zone 9b minimum, in a sheltered position.
How fast does Dioon tomasellii grow?
Very slowly — one of the slowest in the genus. In southern California, plants may produce only one flush of new leaves per year (or even skip a year), and stems elongate at a rate of only a few centimetres per decade. Patience is essential.
Is Dioon tomasellii toxic?
Yes. Like all cycads, it contains cycasin and other toxic glycosides in all parts (seeds, leaves, roots). All parts are poisonous to dogs, cats, livestock, and humans.
Authority Websites and Databases
POWO — Plants of the World Online (Kew)
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:315001-2
The accepted nomenclatural record. Native range: Mexico (Sinaloa to Durango and Jalisco). Seasonally dry tropical biome. No synonyms listed (former var. sonorense now under Dioon sonorense).
World List of Cycads — cycadlist.org
https://cycadlist.org/scientific_name/265
Detailed nomenclatural record: type specimen (Vázquez-Torres 2307, Nayarit, arroyo Laurel on Cerro Caratón, 12 Jun 1979, HT: NAP), etymology (Haynes 2022: honouring Prof. Ruggero Tomaselli), distribution, and IUCN status (VU A2cd).
De Luca, Sabato & Vázquez Torres (1984) — protologue
https://link.springer.com/article/10.2307/2806511
The original species description in Brittonia 36(3): 225–227. Latin diagnosis, morphological description with measurements, distribution of the two original varieties (var. tomasellii and var. sonorense), and the characterisation of “falcate to subfalcate” leaflets that remains diagnostic today.
Chemnick, Gregory & Salas-Morales (1997) — revision of Dioon tomasellii
Phytologia 83: 1–6
The landmark revision that elevated var. sonorense to species rank as Dioon sonorense, demonstrating the lack of continuous variation between the two taxa. Also includes a range extension for Dioon merolae and clarification of Dioon purpusii. Essential for understanding the current circumscription.
Nicolalde-Morejón et al. (2009) — Dioon stevensonii description
https://doi.org/10.1017/S1477200008002648
Description of Dioon stevensonii from Michoacán/Guerrero, a textbook case of “reciprocal illumination” where molecular phylogeny prompted the morphological re-examination that confirmed the new species. Fundamentally recircumscribes Dioon tomasellii by removing the Río Balsas populations.
Gutiérrez-Ortega et al. (2018) — Dioon vovidesii description
https://doi.org/10.11646/phytotaxa.369.2.4
Description of Dioon vovidesii from the northernmost Dioon populations in northwestern Mexico, further refining the circumscription of Dioon sonorense and its relationship to Dioon tomasellii.
Navarrete-Heredia (2018) — Beetles associated with Dioon tomasellii
https://doi.org/10.4289/0013-8797.120.2.458
Documentation of Pharaxonotha and other beetle species associated with Dioon tomasellii cones in Jalisco, Mexico. Published in Proceedings of the Entomological Society of Washington 120(2): 458–465. Valuable for understanding the pollination ecology of this species.
Dave’s Garden — Dioons: The Hardy Mexican Cycads (Geoff Stein)
https://davesgarden.com/guides/articles/view/1981/
The most detailed English-language cultivation account of Dioon tomasellii available online. Describes the glossy, deflexed leaflets, semi-deciduous behaviour, shade preference, heat requirement, and coastal struggles in southern California. Essential reading for growers.
cycadales.eu — Dioon tomasellii nursery listing
https://cycadales.eu/produit/dioon-tomasellii/?lang=en
European specialist nursery occasionally offering seedlings. Notes the deflexed leaflet character (“like Encephalartos inopinus”) and the sister-species relationship with Dioon stevensonii.
Dorsey et al. (2018) — Pleistocene diversification in Dioon
https://doi.org/10.1002/ajb2.1149
Molecular phylogeny placing Dioon tomasellii within the “edule clade” and providing the broader context for species relationships across the genus.
Bibliography
Chemnick, J., Gregory, T. J., & Salas-Morales, S. (1997). A revision of Dioon tomasellii (Zamiaceae) from western Mexico, a range extension of D. merolae, and clarification of D. purpusii. Phytologia, 83(1), 1–6.
De Luca, P., Sabato, S., & Vázquez Torres, M. (1984). Dioon tomasellii (Zamiaceae), a new species with two varieties from western Mexico. Brittonia, 36(3), 223–227.
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.
Gutiérrez-Ortega, J. S., et al. (2018). Species definition of Dioon sonorense (Zamiaceae, Cycadales), and description of D. vovidesii, a new cycad species from northwestern Mexico. Phytotaxa, 369(2), 107–114.
Haynes, J. L. (2022). Etymological compendium of cycad names. Phytotaxa, 550(1), 1–31.
Jones, D. L. (1993). Cycads of the World. Reed, Chatswood, NSW.
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.
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.
Norstog, K. J., & Nicholls, T. J. (1997). The Biology of the Cycads. Cornell University Press, Ithaca.
Vovides, A. P., et al. (2018). Epidermal morphology and leaflet anatomy of Dioon (Zamiaceae) with comments on climate and environment. Flora, 239, 20–44.
Whitelock, L. M. (2002). The Cycads. Timber Press, Portland.