In the canyon of the Río Santo Domingo, northern Oaxaca, the river carves a deep gorge through the Sierra de Oaxaca — and on the steep limestone slopes above the village of San Bartolomé Ayautla, at the transition between lowland tropical forest and the cloud-draped oak forests above, grows one of the most elusive species in the genus Dioon.
Dioon rzedowskii is the third and least known member of the “massive” clade — the group of large-fronded, tall-trunked species that also includes Dioon spinulosum and Dioon mejiae. Yet while Dioon spinulosum is one of the most widely cultivated cycads in the tropics, Dioon rzedowskii remains extremely rare in collections worldwide. Its distribution lies upstream and uphill from Dioon spinulosum in the same drainage system — the two species are contiguous but ecologically separated, one in the lowland evergreen rainforest, the other in the more open, semi-deciduous forest of the low mountains. The diagnostic character that immediately identifies this species is its broad, entire-margined leaflets that taper unequally toward the apex and deflect downward at their tips — a morphological signature unique in the genus and visible even in photographs.
Quick Facts
| Scientific name | Dioon rzedowskii De Luca, A.Moretti, Sabato & Vázq.Torres |
| Family | Zamiaceae |
| Origin | Southern Mexico — Río Santo Domingo canyon, northern Oaxaca |
| Adult size | Trunk to 5 m or more, 25–40 cm diameter; crown spread 2–3 m |
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 (estimated) |
| IUCN | Vulnerable (VU) |
| CITES | Appendix II (all Dioon species) |
| Cultivation difficulty | 3/5 |
Taxonomy and Nomenclature
Dioon rzedowskii was described by Paolo De Luca, Aldo Moretti, Sergio Sabato, and Mario Vázquez Torres in 1980 in Brittonia (32(2): 225–229). The type was collected by Vázquez Torres (no. 2300) on 22 May 1979 below the town of San Bartolomé Ayautla, in the Distrito de Teotitlán, Oaxaca. The holotype is deposited at XALU (Universidad Veracruzana, Xalapa), with isotypes at ENCB, FI, MEXU, NAP, and NY.
The specific epithet honours Dr. Jerzy Rzedowski (1926–2023), the eminent Polish-born botanist who spent his entire career in Mexico and produced the monumental Vegetación de México (1978) — arguably the most important single work on Mexican phytogeography. Rzedowski’s vegetation classification system is directly referenced in the protologue of this species to characterise its habitat. The honour is richly deserved: few individual botanists have contributed more to the understanding of Mexican plant diversity.
POWO lists no synonyms. The native range is given as Mexico (Oaxaca), in the wet tropical biome — a classification reflecting the high rainfall at the type locality (approximately 3,000 mm annually), even though the species grows in semi-deciduous rather than evergreen forest.
Dioon rzedowskii belongs to the “massive” clade of the genus alongside Dioon spinulosum and Dioon mejiae, characterised by large fronds, well-developed trunks, massive cones, and broad leaflets. Phylogenetic analysis (Moretti et al., 1993) confirms this grouping. Within the clade, the authors of the protologue noted that Dioon rzedowskii is “related to Dioon spinulosum Dyer (1883) and to Dioon mejiae Standl. & Williams (1951)” based on leaflet dimensions. It is distinguished from Dioon spinulosum by its entire leaflet margins (not spinulose throughout life), and from Dioon mejiae by its unequally tapering leaflets and more widely separated basal leaflets.
Common names: Rzedowski’s Dioon (English); no widely used Spanish common name recorded.
Morphological Description
Dioon rzedowskii is a tall, arborescent cycad. The trunk is cylindrical, erect or sometimes reclining with age, reaching 5 m or more in height and 25–40 cm in diameter. Cataphylls (scale leaves) are densely woolly (lanate), 16–18 cm long and about 2 cm wide at the base. These dimensions place it firmly in the “massive” group alongside Dioon spinulosum, though it is generally somewhat smaller in stature.
Leaves: numerous, coriaceous (leathery), ascending, flat (not keeled) in cross-section, 160 cm long or more. The petiole is semi-terete, 10–15 cm long, 3–4 cm wide at the base. The leaf is pubescent when young, becoming glabrous at maturity. Leaflets number 80 or more per side (160+ total), sub-opposite, and — most critically — have a unique morphology that defines the species:
The median leaflets are 14–19 cm long and 1.8–2.1 cm wide — among the broadest in the genus, comparable to Dioon spinulosum. They are elongate-lanceolate, with slightly revolute margins that are entirely smooth (entire) on mature leaves — juvenile plants may have spinulose margins that are lost with age. This is the key distinction from Dioon spinulosum, where the spinulose margins persist throughout the plant’s life. The leaflets are sharply pointed (acute-pungent), with approximately 30–35 veins per median leaflet. Most distinctively, the leaflets are narrowed above the base and deflected (drooping) near the tips, with an unequal taper toward the apex — one margin curves more than the other, creating an asymmetric tip that gives the leaf a distinctive, almost feathered appearance unlike any other Dioon species. Lower leaflets are more widely spaced; upper leaflets are more crowded.
Cones: large, pendant female cones consistent with the “massive” clade. Detailed cone descriptions are sparse in available literature, but the species is expected to produce cones approaching the scale of those of Dioon spinulosum — the protologue references the relationship between leaflet size and cone dimensions in this group.
Seeds: large, consistent with the massive clade. Germination is cryptocotylar.
Similar Species and Common Confusions
| Character | Dioon rzedowskii | Dioon spinulosum | Dioon mejiae |
|---|---|---|---|
| Leaflet margins | Entire (smooth) at maturity | Spinulose throughout life | Entire at maturity (as in Dioon rzedowskii) |
| Leaflet apex | Unequally tapering, deflected at tips | Symmetrically tapering | More symmetrically tapering |
| Leaflet width (median) | 1.8–2.1 cm | 1.0–2.0 cm | 1.0–1.8 cm |
| Basal leaflet spacing | Widely separated | Moderately spaced | More closely spaced |
| Petiole | Smooth, leafless | Smooth, leafless | Spines/leaves along entire length |
| Adult trunk height | 5 m+ | Up to 16 m | Up to 12 m |
| Habitat altitude | 650–850 m | 30–300 m | Low elevation (Honduras) |
| Habitat type | Tropical semi-deciduous forest (open) | Tropical evergreen rainforest (understory) | Tropical humid forest (Honduras) |
| Distribution | Río Santo Domingo canyon, Oaxaca | Lowland Veracruz/Oaxaca | Honduras (possibly Nicaragua) |
The three species of the massive clade are geographically separated. Dioon rzedowskii and Dioon spinulosum have contiguous but non-overlapping distributions in the same river drainage system: Dioon spinulosum occupies the lowlands (0–300 m) in evergreen rainforest, while Dioon rzedowskii occurs upstream at 650–850 m in more open, semi-deciduous forest. The ecological separation is sharp and consistent. Dioon mejiae is isolated in Honduras, far from the Oaxacan species.
The two-step identification from Dioon spinulosum is straightforward: (1) check leaflet margins — entire/smooth on mature plants = Dioon rzedowskii; persistently spinulose = Dioon spinulosum; (2) check leaflet tip shape — unequally tapering and deflected = Dioon rzedowskii; symmetrically tapering = Dioon spinulosum. From Dioon mejiae, the key is the petiole: smooth and leafless in Dioon rzedowskii, armed with spines/leaves along its full length in Dioon mejiae.
Distribution and Natural Habitat
Dioon rzedowskii is endemic to a restricted area of northern Oaxaca, in the canyon of the Río Santo Domingo. It is known from the vicinity of the villages of San Bartolomé Ayautla and San Pedro Teutila, in the Distrito de Teotitlán and Distrito de Cuicatlán respectively. Plants grow on steep limestone cliffs and outcrops, rooted in humus accumulated in crevices. The altitudinal range is 650–850 m (approximately 2,000–2,800 feet).
The habitat is classified as bosque tropical subcaducifolio (tropical semi-deciduous forest) following Rzedowski’s (1978) classification — in contrast to the bosque tropical perennifolio (tropical evergreen rainforest) occupied by Dioon spinulosum at lower elevations in the same drainage. A key ecological difference noted in the protologue is that Dioon rzedowskii prefers more open areas, while Dioon spinulosum is primarily an understory species. This preference for more light is relevant for cultivation.
The Río Santo Domingo canyon is an extraordinarily rich area for cycad diversity. In addition to Dioon rzedowskii and Dioon spinulosum downstream, the broader canyon system of northern Oaxaca also harbours Dioon purpusii, Dioon califanoi, Dioon caputoi, and Dioon argenteum — making this the single greatest concentration of Dioon species diversity anywhere in the world.
Climate in the native range:
| Parameter | Estimated range (Río Santo Domingo, 650–850 m) |
|---|---|
| Mean annual temperature | 20–24 °C |
| Average winter minimum | 12–16 °C |
| Estimated historical minimum | 5–8 °C (frost very rare or absent) |
| Summer maximum average | 30–34 °C |
| Annual rainfall | ~2,000–3,000 mm (high; summer-dominant with significant year-round moisture) |
The extremely high annual rainfall (~3,000 mm at the Cerro Rabón area above Ayautla) places Dioon rzedowskii in one of the wettest cycad habitats in the New World — comparable to Dioon spinulosum but at higher elevation. However, the semi-deciduous rather than evergreen forest character of the habitat, and the species’ preference for more open situations, suggest that it experiences more seasonal variation in moisture availability than Dioon spinulosum. The near-total absence of frost at these elevations in southern Oaxaca means that Dioon rzedowskii is not expected to be particularly cold-hardy in cultivation.
Conservation
Dioon rzedowskii is classified as Vulnerable (VU) on the IUCN Red List. The species has a restricted distribution confined to the Río Santo Domingo canyon and surrounding areas. Like other narrowly endemic Dioon species, it is threatened by habitat destruction (deforestation, agricultural expansion) and potential over-collection. The relatively inaccessible canyon terrain provides some natural protection, but the small total population makes the species inherently vulnerable to localised threats.
All Dioon species are listed on CITES Appendix II.
For buyers: Dioon rzedowskii is one of the rarest Dioon species in cultivation. Seeds and plants are only very sporadically available from the most specialised cycad nurseries. Ensure nursery provenance and CITES documentation for any international purchase.
Cultivation
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 (estimated) |
| Light | Full sun to bright partial shade (prefers more sun than Dioon spinulosum) |
| Soil | Well-drained, humus-rich; limestone soils tolerated |
| Watering | Moderate to regular — appreciates moisture; more water than dry-habitat Dioon |
| Adult size | Trunk to 5 m+ (in centuries) × crown 2–3 m |
| Growth rate | Moderate — similar to Dioon spinulosum |
| Difficulty | 3/5 |
Very little published cultivation experience exists for Dioon rzedowskii due to its extreme rarity in collections. The following recommendations are based on the known ecology of the species and extrapolated from experience with the closely related Dioon spinulosum.
Light: the protologue notes that Dioon rzedowskii “prefers more open areas” in the wild, in contrast to the understory habit of Dioon spinulosum. This suggests that full sun to bright partial shade is appropriate in cultivation — more sun than for Dioon spinulosum, but the high rainfall in the native habitat means it is not a full-desert-sun species like Dioon caputoi. Morning sun with afternoon shade in very hot climates is likely optimal.
Soil and drainage: in the wild, plants are rooted in humus accumulated in limestone crevices — a substrate that is both well-drained and humus-rich. In cultivation, a mix of quality compost, coarse grit, and limestone chips would approximate the natural substrate. Good drainage is essential despite the species’ tolerance for high rainfall, as the karstic limestone provides rapid runoff in habitat.
Watering: Dioon rzedowskii comes from one of the wettest Dioon habitats (~2,000–3,000 mm annual rainfall). It appreciates regular moisture during the growing season, more so than the dry-habitat species (Dioon edule, Dioon califanoi, Dioon caputoi). However, the semi-deciduous forest habitat suggests some seasonal drying is natural. Reduce watering in winter, especially in cool climates. As always, drainage must be excellent — wet feet in cool soil will kill any cycad.
Cold hardiness: limited, as expected for a species from a frost-free, mid-altitude tropical habitat. Estimated tolerance is −2 to −4 °C, similar to Dioon spinulosum. This is the least cold-hardy tier among Dioon species, along with Dioon spinulosum, Dioon mejiae, and Dioon merolae. USDA zone 9b minimum for outdoor year-round cultivation.
Container culture: recommended for growers outside the tropics and warm subtropics. Use a well-drained but moisture-retentive mix. Overwinter in a bright, warm location (minimum 8–10 °C). The species’ ultimate large size means that very large containers will eventually be needed.
Fertilization: likely responds well to regular feeding, as the natural habitat on humus-rich limestone suggests a moderate nutrient requirement — more than the desert species but less demanding than true tropical forest plants.
Buying Advice
Availability: Dioon rzedowskii is among the rarest Dioon species in cultivation worldwide. It appears only sporadically from the most specialised cycad seed dealers and nurseries. Rarepalmseeds.com has occasionally offered seeds. If you encounter it, be prepared to pay a premium — and verify the identity carefully.
Identification: the broad, entire-margined leaflets with unequal apical taper and deflected tips are diagnostic and should be visible even on well-grown seedlings. Distinguish from Dioon spinulosum (persistently spinulose margins) and from other broad-leafleted species by the unique asymmetric tip shape.
Propagation
Seed: extremely limited availability. When obtainable, treat as for Dioon spinulosum: remove sarcotesta (gloves essential), soak 24–48 hours, sow horizontally in a well-draining but moisture-retentive mix, maintain 25–30 °C. Keep medium consistently moist (more moisture than for dry-habitat species). Germination is cryptocotylar, expected in 2–12 months.
Offsets: no specific information is available, but mature plants of the massive clade do occasionally produce basal offsets.
Pests and Diseases
Root rot: the primary cultivation risk, despite the species’ adaptation to high rainfall — in habitat, the karstic limestone provides rapid drainage. Ensure excellent drainage in cultivation.
Cycad aulacaspis scale (Aulacaspis yasumatsui): likely susceptible, as are all Dioon species. Regular monitoring is recommended.
Mealybugs: possible in sheltered conditions.
Landscape Use
Dioon rzedowskii is a collector’s cycad of the highest order — its extreme rarity in cultivation makes it a prestige specimen for serious cycad collections, botanical gardens, and specialist tropical gardens. Its large, graceful fronds with distinctive drooping-tipped leaflets create an elegant crown quite different from the more common Dioon spinulosum. It is best suited to warm, humid subtropical or tropical climates with year-round warmth and good moisture — the same conditions that suit Dioon spinulosum, but with potentially better tolerance for more open, sunnier positions. For temperate gardeners, container culture with warm overwintering is the only realistic approach.
Frequently Asked Questions
How is Dioon rzedowskii different from Dioon spinulosum?
Two key characters: (1) leaflet margins — entire (smooth) at maturity in Dioon rzedowskii, persistently spinulose throughout life in Dioon spinulosum; (2) leaflet tip — unequally tapering and deflected downward in Dioon rzedowskii, symmetrically tapering in Dioon spinulosum. Ecologically, Dioon rzedowskii grows at higher elevation (650–850 m vs. 30–300 m) in more open, semi-deciduous forest.
Why is Dioon rzedowskii so rare in cultivation?
Its restricted natural distribution (a few sites in the Río Santo Domingo canyon), the inaccessibility of its habitat, and the infrequency of seed production in small, isolated populations all contribute to extremely limited seed availability. Unlike Dioon spinulosum, which has been widely propagated for decades, Dioon rzedowskii has never entered mainstream nursery production.
Is Dioon rzedowskii cold-hardy?
Minimally. As a mid-altitude tropical species from a frost-free canyon habitat, it is expected to tolerate only brief frosts to approximately −2 to −4 °C — comparable to Dioon spinulosum and less hardy than Dioon edule or Dioon angustifolium.
Who was Jerzy Rzedowski?
Jerzy Rzedowski (1926–2023) was a Polish-born botanist who spent his entire career in Mexico. He authored Vegetación de México (1978), the definitive phytogeographic reference for the country, and described numerous plant species. His vegetation classification system is directly used in the protologue of Dioon rzedowskii to characterise the species’ habitat.
Is Dioon rzedowskii toxic?
Yes. Like all cycads, it contains cycasin and other toxic compounds in all parts. Seeds, leaves, and roots 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:80657-2
The accepted nomenclatural record: native range Mexico (Oaxaca), wet tropical biome. No synonyms listed.
World List of Cycads — cycadlist.org
https://cycadlist.org/scientific_name/260
Detailed nomenclatural record: type specimen (Vázquez-Torres 2300, San Bartolomé Ayautla, 22 May 1979), etymology (Haynes 2022: honouring Jerzy Rzedowski), distribution, and IUCN status.
IUCN Red List — Dioon rzedowskii
https://www.iucnredlist.org/species/42134/10661820
The Vulnerable (VU) assessment with population trend, habitat description, and threat analysis.
De Luca, Moretti, Sabato & Vázquez Torres (1980) — original description
https://link.springer.com/article/10.2307/2806794
The protologue in Brittonia 32(2): 225–229. Full Latin diagnosis, distinguishing characters from Dioon spinulosum and Dioon mejiae, distribution map showing contiguous ranges with Dioon spinulosum, and habitat description.
World Flora Online — Dioon rzedowskii
https://www.worldfloraonline.org/taxon/wfo-0000648126
Full Latin protologue text, type specimen data, and additional herbarium records including the San Pedro Teutila collection (Steven R. Hill 1768, NY, 1973) noting “limestone cliffs and outcrops, rooted in humus in crevices.”
iNaturalist
https://www.inaturalist.org/taxa/136084-Dioon-rzedowskii
Citizen science observations — very few records, reflecting the species’ extreme rarity and restricted distribution.
Tropicos — Missouri Botanical Garden
https://www.tropicos.org/name/32600026
Nomenclatural data and specimen records.
ResearchGate — Protologue PDF
https://www.researchgate.net/publication/226733506
Full text of the original description with figures showing the diagnostic unequally tapering leaflets and distribution map.
Dave’s Garden — Dioons: The Hardy Mexican Cycads
https://davesgarden.com/guides/articles/view/1981/
General grower’s guide to all Dioon species. Notes Dioon rzedowskii as one of the three “wide-leaf” species alongside Dioon spinulosum and Dioon mejiae.
Gregory, Chemnick & Salas-Morales (2003) — Dioon argenteum and the canyon system species
https://academic.oup.com/botlinnean/article/141/4/471/2433545
Paper describing Dioon argenteum and providing context for all Oaxacan Dioon species including Dioon rzedowskii and Dioon spinulosum as downstream neighbours in the same river drainage system.
Bibliography
De Luca, P., Moretti, A., Sabato, S., & Vázquez Torres, M. (1980). Dioon rzedowskii (Zamiaceae), a new species from Mexico. Brittonia, 32(2), 225–229.
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.
Gutiérrez-Ortega, J. S., et al. (2018). Epidermal morphology and leaflet anatomy of Dioon (Zamiaceae) with comments on climate and environment. Plant Systematics and Evolution.
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.
Rzedowski, J. (1978). Vegetación de México. Limusa, Mexico City.
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.