Deep in the Tomellín Canyon of Oaxaca, where columnar cacti, Agave, and Beaucarnea share the steep limestone walls with a punishing dry season and summer downpours, grows one of the most sought-after cycads in the collector world. Dioon purpusii is a tall, stately species — trunks reaching 5 m in the wild — with a spreading crown of tough, finely pinnate, grey-green leaves that have earned it the informal nickname of “fish bone cycad” among English-speaking growers. Described by Joseph Nelson Rose in 1909 from material collected in Tomellín Canyon, it is one of the earlier-described species in the genus Dioon and the namesake of the Dioon purpusii complex, a group of three closely related species (Dioon purpusii, Dioon califanoi, and Dioon argenteum) that have diversified within the canyon systems of northern Oaxaca.
Despite its Endangered status, Dioon purpusii remains relatively well protected in its wild habitat — the inaccessible canyon walls that would be its conservators — and local communities traditionally use its leaves only on Palm Sunday, leaving the plants otherwise undisturbed. For the collector, this is a species that combines genuine rarity, impressive adult stature, and the unmistakable visual character of a mature Dioon crown.
Quick Facts
| Scientific name | Dioon purpusii Rose |
| Family | Zamiaceae |
| Origin | Southern Mexico — Tomellín Canyon, La Cañada de Cuicatlán, and Sierra Mixteca, Oaxaca (with possible extension into Puebla) |
| Adult size | Trunk to 5 m tall, 30–40 cm diameter; crown spread 2–3 m |
| Hardiness | −3 to −5 °C (27 to 23 °F) / USDA zones 9b–11 |
| IUCN | Endangered (EN) — B1ab(v) |
| CITES | Appendix II (all Dioon species) |
| Cultivation difficulty | 3/5 |
Taxonomy and Nomenclature
Dioon purpusii was described by Joseph Nelson Rose in 1909 in Contributions from the United States National Herbarium (12(7): 260–261). The type specimen was collected by Rose and J.S. Rose (no. 11352) on 7 September 1906 in Tomellín Canyon, Oaxaca. The holotype is deposited at US (United States National Herbarium), with isotypes at F and NY.
The specific epithet purpusii honours Carl Albert Purpus (1851–1941), one of the leading botanical researchers in Mexico at the turn of the 20th century. Purpus was a German-born plant collector who spent decades exploring the flora of Mexico and sent thousands of specimens to European and American herbaria. He is also commemorated in the Oaxacan succulent Echeveria setosa Rose & Purpus, among many other species.
POWO lists no synonyms for Dioon purpusii, though the combination Dioon edule var. purpusii (Rose) Nance has appeared in some databases — reflecting the recurrent (and rejected) tendency to subsume smaller Dioon species into Dioon edule. This reduction has never been accepted by cycad specialists.
Dioon purpusii is the type species of the Dioon purpusii complex, a cluster of three closely related taxa from the canyon systems of northern Oaxaca: Dioon purpusii itself (from Tomellín Canyon and eastern La Cañada de Cuicatlán), Dioon califanoi (from the eastern Tehuacán valley), and Dioon argenteum (from the Sierra Norte canyons). These three species share similar leaf and cone dimensions and occur in relatively close geographic proximity within the same drainage system; they differ primarily in the degree of leaf keeling, leaflet surface texture, and the persistence of tomentum on new growth (Gregory, Chemnick & Salas-Morales, 2003). They appear to represent a recent diversification event within a complex topographic landscape.
Common names: Purpus’s Cycad, Purpus’s Dioon, Fish Bone Cycad (English); Chamal (Spanish/Mixtec).
Morphological Description
Dioon purpusii is one of the taller species in the “less robust” morphological group of Dioon. The trunk is erect, cylindrical, usually unbranched, reaching up to 5 m in height with a diameter of approximately 30–40 cm in the oldest wild specimens — dimensions that make it significantly larger at maturity than Dioon edule or Dioon califanoi. As in all Dioon species, the trunk is clothed in persistent leaf bases and a layer of protective woolly hairs. Growth is slow, and tall-trunked specimens in the wild represent centuries of development.
Leaves: Dioon purpusii holds a relatively large number of leaves in its crown, contributing to its impressive stature. Individual fronds are 80–160 cm long, straight, stiff, and — critically for identification — flat to only slightly keeled in cross-section. This is the key character separating Dioon purpusii from its close relative Dioon califanoi, which has a deeply keeled (~90° V) leaf profile. The leaf of Dioon purpusii appears essentially planar, with at most a faint V-shape. Colour is a dark, lusterless grey-green — not glossy. New leaves emerge with a transient tomentum (woolly covering) that disappears relatively quickly, revealing a glossy leaflet surface beneath — another diagnostic character, since Dioon califanoi has a glaucous, matte surface under the tomentum.
Leaflets number 150–260 per frond, are narrow (7–12 cm long, 8–10 mm wide), stiff, and closely spaced — sometimes overlapping. They are inserted in symmetrical pairs along the somewhat flattened rachis at angles close to perpendicular. A critical diagnostic feature: leaflet margins bear prickles, particularly near the tips — a character absent or minimal in Dioon califanoi. Basal leaflets are reduced to spines. The overall visual effect of the densely packed, prickle-tipped leaflets along the flat rachis has been compared to a fish skeleton, giving rise to the informal “fish bone cycad” nickname.
Cones: Dioon purpusii produces relatively large cones for the smaller morphological group. Female (seed) cones are ovoid, pale brown, 35–45 cm long and 15–20 cm in diameter — considerably larger than those of Dioon edule. Male (pollen) cones are elongated-ovoid, green to pale brown, 20–30 cm long and 7–8 cm in diameter. The beetle Pharaxonotha bicolor is in an obligatory mutualistic pollination relationship with Dioon purpusii, living and breeding in male cones and serving as the primary pollinating vector.
Seeds: relatively large for the genus at 3–4 cm × 2.5–3.5 cm, ovoid, with cream or white sarcotestas. Germination is cryptocotylar.
Similar Species and Common Confusions
| Character | Dioon purpusii | Dioon califanoi | Dioon argenteum | Dioon spinulosum |
|---|---|---|---|---|
| Adult trunk height | Up to 5 m | Up to 3 m | ~3 m | Up to 16 m |
| Leaf keel | Flat to slightly keeled | Strongly keeled (~90° V) | Flat | Flat |
| Leaflet surface | Glossy (after tomentum loss) | Glaucous, matte | Glossy | Glossy, bright green |
| Leaflet prickles | Present | Absent or very few | Present (persistent) | Strongly spinose |
| Tomentum persistence | Transient | Transient | Persistent (silvery) | Transient |
| Leaflet count/frond | 150–260 | 160–200 | Similar range | 120–240 (broader leaflets) |
| Distribution | Tomellín Canyon, La Cañada | Eastern Tehuacán valley | Sierra Norte of Oaxaca | Lowland Veracruz, Oaxaca |
| Cold hardiness | −3 to −5 °C | −5 to −8 °C | ~−3 to −5 °C | −2 to −4 °C |
| IUCN | Endangered | Endangered | Endangered | Vulnerable |
The most frequent confusion is with Dioon califanoi, which occurs in the same broader region. The three-step field identification is: (1) check leaf keel — flat or slightly keeled = Dioon purpusii, deeply keeled at ~90° = Dioon califanoi; (2) check leaflet surface — glossy = Dioon purpusii, glaucous/matte = Dioon califanoi; (3) check for leaflet prickles — present = Dioon purpusii, absent = Dioon califanoi. Dioon argenteum, the third member of the complex, is distinguished by its persistently silvery tomentum and flat (not keeled) leaves. Dioon spinulosum, a much larger lowland species with strongly spinose, broader leaflets, is unlikely to be confused at maturity but can resemble Dioon purpusii in the juvenile stage.
Distribution and Natural Habitat
Dioon purpusii is endemic to a restricted area of northern Oaxaca, with its core distribution in Tomellín Canyon near the villages of Santa Catarina and Tomellín, extending into the eastern branches of La Cañada de Cuicatlán and into the Sierra Mixteca. Five populations are currently known (Gregory et al., 2003). The species grows on steep, rocky slopes of deep gorges in dry tropical deciduous forest (bosque tropical caducifolio), mostly in the shade of trees, at elevations of 1,000–1,500 m (some reports extend to 450–900 m in certain canyon bottoms). It occurs alongside cacti, agaves, and Beaucarnea.
The area is relatively inaccessible, which has provided a degree of natural protection. Sites and plants are largely undisturbed, and populations remain rich in individuals — a contrast with some other Endangered Dioon species where wild populations are severely depleted.
Climate in the native range:
| Station / locality | Mean annual temp. | Average winter min. | Estimated historical min. | Summer max. average | Annual rainfall |
|---|---|---|---|---|---|
| Tomellín Canyon populations 1,000–1,500 m | ~20–24 °C | 10–15 °C (winter average) | Estimated 3–5 °C during cold events (frost very rare) | 28–33 °C | 500–1,000 mm (summer-dominant) |
| Oaxaca de Juárez (regional reference, 1,550 m) | 20.5 °C | 8–10 °C (Jan.) | ~2 °C | 30–31 °C (Apr.–May) | ~655 mm |
The native climate is characterised by warm temperatures year-round, a very pronounced dry season (November–April), and relatively mild winter minimums compared to the northern Sierra Madre Oriental habitats of Dioon edule and Dioon angustifolium. The Tomellín Canyon habitat is warmer and more sheltered than the higher-altitude sites of Dioon califanoi, which partly explains why Dioon purpusii is somewhat less cold-hardy in cultivation. Frost events are very rare in the canyon but not impossible during exceptional cold spells. The dry season is severe — several months with virtually no precipitation — selecting for deep-rooted, drought-adapted plants.
Conservation
Dioon purpusii is classified as Endangered (EN) on the IUCN Red List under criterion B1ab(v). All Dioon species are listed on CITES Appendix II. Despite the Endangered status, the conservation situation of Dioon purpusii is arguably less dire than that of some other Endangered Dioon species (such as Dioon caputoi): the canyon habitat is naturally inaccessible, populations remain relatively large, and local communities do not heavily exploit the plants. The primary long-term threat is habitat degradation from goat grazing and land clearing on the less steep slopes. Wild collection for the horticultural trade is a minor but persistent concern.
Local communities use the leaves of Dioon purpusii only on Palm Sunday for religious ceremonies — a traditional, low-impact use that does not significantly threaten the populations.
For buyers: always purchase nursery-propagated plants with CITES documentation. Dioon purpusii is rare in the trade and commands high prices; be wary of suspiciously cheap offerings.
Cultivation
| Hardiness | −3 to −5 °C (27 to 23 °F) / USDA zones 9b–11 |
| Light | Full sun (essential for compact growth; does not perform well in shade) |
| Soil | Very well-drained, mineral-dominant; tolerates limestone and alkaline substrates |
| Watering | Low — strongly seasonal; drought-tolerant once established; less water is better |
| Adult size | Trunk to 5 m (in centuries) × crown 2–3 m |
| Growth rate | Slow |
| Difficulty | 3/5 |
Dioon purpusii is one of the most magnificent Dioon species as a mature landscape specimen, but it is somewhat more demanding in cultivation than Dioon edule or Dioon califanoi. Geoff Stein (Dave’s Garden) notes that it is “one of the most sought after Dioons” and “a magnificent landscape specimen,” but also that it “does not perform well in shade” and “does not seem to do well in humid climates.” This species is best suited to warm temperate and dry tropical climates — inland Mediterranean, arid Southwestern US, or similar. It thrives in California, Arizona, the hotter parts of Florida and Texas, and equivalent climates worldwide.
Light: full sun is essential. Dioon purpusii does not tolerate shade well — a notable difference from Dioon edule, which is quite shade-tolerant. In low-light conditions, leaves elongate, the crown becomes lax and open, and the plant loses its characteristic architectural presence. Plant in the sunniest available position.
Soil and drainage: excellent drainage is non-negotiable. Use a predominantly mineral mix — pumice, coarse sand, gravel, limestone chips — with only modest organic matter. The native habitat is rocky canyon slopes with skeletal soils. Excess moisture retention, especially in winter, leads to root rot.
Watering: less water is better. Water regularly but sparingly during the growing season (spring through autumn), then reduce sharply or stop entirely in winter. The native habitat receives 500–1,000 mm of rain concentrated almost entirely in the summer months; replicating this seasonal cycle is the key to long-term success. Dioon purpusii is not well suited to climates with year-round wet conditions or heavy winter rainfall without excellent drainage.
Cold hardiness: moderate. Established plants tolerate brief frosts to approximately −3 to −5 °C (mid-20s °F) in well-drained soil. Junglemusic (San Diego) reports hardiness “down to the mid 20’s” (approximately −4 °C). This is less cold-tolerant than Dioon edule, Dioon angustifolium, or Dioon califanoi, reflecting the warmer, more sheltered canyon habitat. Young seedlings are significantly more vulnerable. In USDA zone 9b, outdoor year-round cultivation is feasible with excellent drainage and a sheltered position. In zone 9a, winter protection is advisable; in zone 8, this species requires overwintering indoors. As with all cycads, the distinction between juvenile (subterranean caudex, soil thermal inertia) and adult (aerial trunk, exposed growth point) cold tolerance is important.
Humidity: Dioon purpusii does not thrive in humid climates — a meaningful distinction from more adaptable species like Dioon edule. It performs best in arid to semi-arid conditions with low ambient humidity and good air movement.
Container culture: well suited to container growing, particularly in climates where outdoor cultivation year-round is not feasible. Use a gritty, mineral-dominant mix in a terracotta pot with drainage holes. Overwinter in a cool, bright, dry location (unheated greenhouse, bright garage). Full sun during the growing season.
Fertilization: light applications of balanced slow-release fertilizer during spring and summer. Avoid excess nitrogen.
Buying Advice
Availability: Dioon purpusii is rare in cultivation and commands premium prices. It is available from specialist cycad nurseries (Canarius, rarepalmseeds.com, Junglemusic, and occasional specialist sellers on PalmTalk forums). Most plants offered are small seedlings. Large specimens are virtually unavailable in the trade.
Confusion with Dioon califanoi: verify using the three-step method: (1) leaf keel — flat or slightly keeled for Dioon purpusii, deeply V-keeled for Dioon califanoi; (2) leaflet surface — glossy for Dioon purpusii, matte/glaucous for Dioon califanoi; (3) leaflet prickles — present in Dioon purpusii, absent in Dioon califanoi.
Seeds: when available, seeds of Dioon purpusii are sold at a significant premium. They are larger (3–4 cm) than those of Dioon edule and germinate under the same conditions (25–30 °C, well-draining mix, 2–12 months germination time).
Propagation
Seed: the primary method. Remove the sarcotesta (use gloves), soak cleaned seeds 24–48 hours in warm water, then sow horizontally, half-buried in a well-draining mineral mix. Maintain 25–30 °C. Keep medium barely moist. Germination is cryptocotylar — the radicle appears first, followed by the eophyll. Expect 2–12 months. Growth from seed is slow.
Offsets: mature plants may produce basal offsets, which can be separated once they have developed their own root system. This is a rare occurrence in cultivated plants and most propagation relies on seed.
Pests and Diseases
Cycad aulacaspis scale (Aulacaspis yasumatsui): the most significant documented pest of Dioon purpusii. Wild infestations have been recorded in the native habitat. In cultivation, regular inspection of leaflet undersides and early treatment with horticultural oil are essential.
Root rot: the primary cause of loss in cultivation. Entirely preventable through correct soil choice, drainage, and watering discipline — particularly in winter.
Mealybugs: occasional in sheltered conditions. Treat with isopropyl alcohol or horticultural oil.
Landscape Use
Dioon purpusii is among the most spectacular landscape cycads for warm, dry climates. Its eventual tall trunk and spreading crown of densely packed, grey-green fronds create an architectural presence unmatched by most other Dioon species except Dioon spinulosum. It is ideal as a statement specimen in xeric gardens, alongside Agave, columnar cacti, Beaucarnea, and Dasylirion — the same associates it grows with in the wild. Its requirements for full sun, dry conditions, and excellent drainage mean it is best suited to inland Mediterranean, desert, and subtropical dry climates. Avoid humid subtropical sites or shaded positions. The prickly leaflet tips require careful placement away from pedestrian traffic.
Frequently Asked Questions
Why is Dioon purpusii called the “fish bone cycad”?
The nickname refers to the visual effect of the densely spaced, narrow leaflets arranged symmetrically along the flat rachis, which resembles the skeleton of a fish. The effect is most striking when the frond is viewed from above.
Is Dioon purpusii suitable for humid climates?
No. Unlike Dioon edule, which adapts well to a wide range of climates, Dioon purpusii performs poorly in humid conditions. It is best suited to arid and semi-arid climates with low ambient humidity and good air circulation. In humid climates, it is more susceptible to fungal issues and does not develop its characteristic compact habit.
How cold-hardy is Dioon purpusii?
Moderate. Established plants survive brief frosts to approximately −3 to −5 °C in well-drained soil. This is less hardy than Dioon edule (−7 to −12 °C), Dioon angustifolium (−10 to −14 °C), or Dioon califanoi (−5 to −8 °C). USDA zone 9b and above for year-round outdoor cultivation.
How can I distinguish Dioon purpusii from Dioon califanoi?
Three key characters: (1) leaf profile — flat or slightly keeled in Dioon purpusii vs. deeply V-keeled (~90°) in Dioon califanoi; (2) leaflet surface — glossy in Dioon purpusii vs. matte/glaucous in Dioon califanoi; (3) leaflet prickles — present in Dioon purpusii, absent or very few in Dioon califanoi.
Is Dioon purpusii toxic?
Yes. Like all cycads, it contains cycasin and other toxic compounds in all parts. Paradoxically, indigenous communities historically used the seeds medicinally (ground into a paste to treat sore eyes, according to Gentry 1942), but this traditional use does not make the plant safe for consumption by humans or animals.
Authority Websites and Databases
POWO — Plants of the World Online (Kew)
https://powo.science.kew.org/taxon/80655-2
The accepted nomenclatural record for Dioon purpusii: native range confirmed as Mexico (Puebla, Oaxaca), seasonally dry tropical biome. No synonyms listed.
World List of Cycads — cycadlist.org
https://cycadlist.org/scientific_name/259
Detailed nomenclatural record: type specimen (Rose & Rose 11352, Tomellín Canyon, 7 Sep 1906), etymology (Haynes 2022), distribution, and IUCN status Endangered B1ab(v).
IUCN Red List — Dioon purpusii
https://www.iucnredlist.org/species/42133/10662541
The full Endangered assessment with population data, threat analysis, and habitat description.
Gregory, Chemnick & Salas-Morales (2003) — The Dioon purpusii complex
https://academic.oup.com/botlinnean/article/141/4/471/2433545
The key paper defining the Dioon purpusii complex and providing the morphological comparison table differentiating Dioon purpusii, Dioon califanoi, and Dioon argenteum. Includes figures of leaf cross-sections showing keel angles. Essential reference.
Rose, J. N. (1909) — protologue
https://www.biodiversitylibrary.org/item/13774#page/276/mode/1up
The original species description in Contributions from the United States National Herbarium 12(7): 260–261. Available via the Biodiversity Heritage Library.
iNaturalist
https://www.inaturalist.org/taxa/136083-Dioon-purpusii
Citizen science observations with field photographs from the Oaxacan habitat.
Tropicos — Missouri Botanical Garden
https://www.tropicos.org/name/32600025
Nomenclatural data and specimen records.
Dave’s Garden — Dioons: The Hardy Mexican Cycads
https://davesgarden.com/guides/articles/view/1981/
Practical grower’s guide by Geoff Stein. Notes Dioon purpusii as “one of the most sought after Dioons” and “a magnificent landscape specimen,” with specific comments on its sun requirements and poor performance in shade and humid climates.
Junglemusic (San Diego)
https://junglemusic.com/2024/04/dioon-purpusii-3/
Specialist cycad nursery page with habitat photographs and cultivation notes. Reports cold hardiness “down to the mid 20’s” (Fahrenheit) and describes it as “quite rare in cultivation.”
rarepalmseeds.com — Dioon purpusii
https://www.rarepalmseeds.com/dioon-purpusii
Specialist seed dealer with authenticated seeds. Brief species description: “a beautiful, rare cycad native to dry deciduous forest on the sides of steep canyons in Oaxaca, Mexico between 1000 and 1500 m.”
Bibliography
Gentry, H. S. (1942). Rio Mayo plants. A study of the flora and vegetation of the valley of the Rio Mayo, Sonora. Carnegie Institution of Washington Publication, 527, 1–328.
Gregory, T. J., Chemnick, J., Salas-Morales, S., & Vovides, A. P. (2003). A new species in the genus Dioon (Zamiaceae) from north-central Oaxaca, Mexico. Botanical Journal of the Linnean Society, 141(4), 471–476.
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.
Moretti, A., Caputo, P., Cozzolino, S., De Luca, P., Gaudio, L., Gigliano Siniscalco, G., & Stevenson, D. W. (1993). A phylogenetic analysis of Dioon (Zamiaceae). American Journal of Botany, 80, 204–214.
Norstog, K. J., & Nicholls, T. J. (1997). The Biology of the Cycads. Cornell University Press, Ithaca.
Rose, J. N. (1909). Cycadaceae. In: Studies of Mexican and Central American Plants. Contributions from the United States National Herbarium, 12(7), 260–261.
Sabato, S., & De Luca, P. (1985). Evolutionary trends in Dioon (Zamiaceae). American Journal of Botany, 72, 1353–1363.
Whitelock, L. M. (2002). The Cycads. Timber Press, Portland.