While most Dioon species inhabit the Atlantic drainage of Mexico — the Sierra Madre Oriental, the canyons of Oaxaca, the Gulf lowlands — Dioon holmgrenii breaks the pattern entirely. This is the Pacific coast Dioon, endemic to the Sierra Madre del Sur and the Pacific coastal plain of southwestern Oaxaca, on the rain-drenched slopes that descend toward Puerto Escondido and the Pacific Ocean. Described in 1981, it has long been overshadowed by its Atlantic-drainage relatives — Dioon edule, Dioon spinulosum, the Oaxacan canyon species — but recent conservation genomics work (Dorsey et al., 2023) has revealed a species more complex and more threatened than previously understood. At least ten localities are now known, comprising six genetically distinct, isolated populations — the result of a historical range expansion from an ancestral population in the southeast of its current range, followed by fragmentation.
For the grower, Dioon holmgrenii is a quietly impressive species: “edule-like” in general appearance but with persistently tomentose (fuzzy) new leaves, blue-green colour, faster growth than Dioon edule, and a graceful habit that makes it one of the best landscape Dioon species for warm climates.
Quick Facts
| Scientific name | Dioon holmgrenii De Luca, Sabato & Vázq.Torres |
| Family | Zamiaceae |
| Origin | Pacific coast of southern Mexico — Sierra Madre del Sur and Pacific coastal plain, southwestern Oaxaca |
| Adult size | Trunk eventually tall (taller than Dioon edule); crown spread 1.5–2.5 m |
| Hardiness | −3 to −5 °C (27 to 23 °F) / USDA zones 9a–11 (estimated) |
| IUCN | Vulnerable (VU) — B1ab(iii)+2ab(iii) |
| CITES | Appendix II (all Dioon species) |
| Cultivation difficulty | 2/5 |
Taxonomy and Nomenclature
Dioon holmgrenii was described by Paolo De Luca, Sergio Sabato, and Mario Vázquez Torres in 1981 in Brittonia (33(4): 552–554, figs. 1–2). The type specimen was collected by Vázquez Torres (no. 2304) on 21 March 1980, on a slope with Quercus, 15 km south of San Gabriel along the road from Puerto Escondido to Oaxaca, Distrito de Juquila, at 650–850 m elevation. The holotype is at XALU (Universidad Veracruzana, Xalapa), with isotypes at ENCB, FI, MEXU, NAP, and NY.
The specific epithet honours Dr. Noel Herman Holmgren, taxonomist and curator at the New York Botanical Garden — a recognition of his contributions to the study of Mexican and North American flora.
POWO lists no synonyms. The native range is given as Mexico (Oaxaca), in the seasonally dry tropical biome. Dioon holmgrenii is placed in the “edule clade” — the smaller, less robust morphological group — alongside Dioon edule, Dioon merolae, Dioon purpusii, Dioon califanoi, Dioon caputoi, and Dioon tomasellii. In the protologue, the authors note that it “differs from Dioon merolae principally in having non-imbricated leaflets along the rachis and by its normal rather than oblique leaflet insertions” (confirmed by Norstog & Nicholls, 1997). This distinction from Dioon merolae is the taxonomic anchor of the species.
Common names: Holmgren’s Dioon, Holmgren’s Cycad (English); no widely used Spanish common name recorded.
Morphological Description
Dioon holmgrenii is a medium to tall arborescent cycad. The trunk is cylindrical, erect, usually solitary (rarely branching or offsetting — a notable difference from the frequently multi-headed Dioon edule). Plants eventually grow relatively tall compared to Dioon edule, though published maximum height data are limited. The trunk is clothed in persistent leaf bases.
Leaves: the general appearance is “edule-like” — stiff, ascending fronds with narrow, evenly spaced leaflets — but several characters distinguish Dioon holmgrenii. Leaves are blue-green to grey-green. Leaflets are elongate, narrow, sharp-tipped, and — in contrast to the smooth-margined mature leaflets of Dioon edule — bear small spines (spinules) especially on the upper margin. This spinulose character is the defining morphological trait flagged in the protologue. The leaflets are non-imbricated (not overlapping) and are inserted at a normal angle to the rachis (perpendicular), not obliquely as in Dioon merolae. Leaflet teeth are absent or very rare — a distinction from the persistently spinulose Dioon spinulosum.
The most visually striking feature for growers is the persistent tomentum on new leaves. When a new flush emerges, the leaves are densely fuzzy (tomentose), and this woolly covering persists for up to a year — far longer than in Dioon edule (where the tomentum disappears quickly) or Dioon merolae (transient). This long-lasting fuzziness gives young flushes a distinctive silvery or golden-brown appearance that adds considerable ornamental interest.
Cones: moderate-sized, consistent with the edule clade. Female cones are ovoid, woolly. Male cones are cylindrical. The species is dioecious.
Seeds: ovoid, with cream/white sarcotesta. Standard size for the edule group.
Similar Species and Common Confusions
| Character | Dioon holmgrenii | Dioon edule | Dioon merolae |
|---|---|---|---|
| Leaflet insertion | Normal (perpendicular), non-imbricated | Normal (perpendicular), non-imbricated | Oblique, imbricated |
| Leaflet margin spines | Present, especially upper margin | Absent on mature leaves | Present, sparse |
| Tomentum persistence | Up to 1 year | Disappears quickly | Transient |
| Leaf colour | Blue-green | Grey-green to blue-green (varies by form) | Semi-glossy dark green |
| Offsetting habit | Usually solitary | Frequently multi-headed | Variable |
| Growth rate | Fast (for a Dioon) | Moderate | Moderate |
| Distribution | Pacific coast, SW Oaxaca | Atlantic slope, E Mexico | Chiapas |
| Shade tolerance | Poor | Good | Poor |
The most likely confusion is with Dioon edule, to which Dioon holmgrenii is superficially similar. The three reliable separation criteria are: (1) leaflet margin spines — present on upper margin in Dioon holmgrenii, absent on mature Dioon edule leaves; (2) tomentum persistence — up to a year in Dioon holmgrenii vs. quickly lost in Dioon edule; (3) geographic origin — Pacific coast of Oaxaca for Dioon holmgrenii, Atlantic slope for Dioon edule.
From Dioon merolae, the key is leaflet insertion angle: perpendicular and non-imbricated in Dioon holmgrenii, oblique and imbricated (“Venetian blind”) in Dioon merolae.
Distribution and Natural Habitat
Dioon holmgrenii is endemic to the Pacific slope of southwestern Oaxaca, Mexico — the biogeographic province known as the Mexican Pacific Coast (Costa del Pacífico Mexicano). At least nine to ten localities are currently known, distributed across the Sierra del Sur (Sierra Madre del Sur) and the adjacent Pacific coastal plain (Velasco-García et al., 2016; Dorsey et al., 2023). Named populations include Río Leche, La Lima, Ocotlán, Rancho Viejo, Rancho El Limón, Cerro Antiguo, Cieneguilla, Cerro Caballo, San Bartolomé, and Loxicha — spanning a significant stretch of the Oaxacan Pacific slope.
The type locality is on a Quercus-dominated slope at 650–850 m, along the road from Puerto Escondido to Oaxaca. Norstog & Nicholls (1997) describe the habitat as “the understory of humid, pine-oak forest on slopes of the Pacific side of the Sierra Madre.” The altitudinal range is approximately 200–1,200 m, varying among populations — some occur at relatively low coastal elevations, others in montane forest.
Climate in the native range:
| Parameter | Estimated range (Pacific slope, 200–1,200 m) |
|---|---|
| Mean annual temperature | 20–26 °C (varies with altitude) |
| Average winter minimum | 10–16 °C |
| Estimated historical minimum | 3–8 °C (frost very rare at lower elevations; occasional at higher sites) |
| Summer maximum average | 30–35 °C |
| Annual rainfall | 1,000–2,500 mm (Pacific slope receives significant summer rainfall and some winter moisture from Pacific weather systems) |
The Pacific slope of Oaxaca is wetter than many Atlantic-slope Dioon habitats but with a more pronounced dry season (November–May) than the Gulf lowlands. The “humid pine-oak forest” habitat description indicates that Dioon holmgrenii occupies a moister environment than the dry-forest species (Dioon purpusii, Dioon califanoi, Dioon caputoi), but is not a lowland rainforest species like Dioon spinulosum.
Conservation genomics: a 2023 study by Dorsey et al. using RADseq genomic data across the full known range revealed that Dioon holmgrenii comprises at least six genetically distinct, isolated populations. The study reconstructed a history of range expansion from an ancestral population in the southeast (Loxicha area), followed by fragmentation and isolation. Each population possesses a substantial percentage of private alleles — unique genetic variants not found in other populations — making every population genetically irreplaceable. Moderate genetic diversity is maintained species-wide, but the isolation between populations means that the loss of any single population would represent a permanent loss of genetic diversity. The western populations (west of the Río Verde) are interpreted as “precursor” populations with higher genetic diversity, while eastern populations are “derived” with lower diversity.
Conservation
Dioon holmgrenii is classified as Vulnerable (VU) on the IUCN Red List under criteria B1ab(iii)+2ab(iii). This assessment pre-dates the 2023 genomic study, which revealed a more complex population structure than previously understood. The discovery that each population is genetically unique and isolated raises the effective conservation concern: the species as a whole may be Vulnerable, but individual populations may be functionally Endangered or worse.
The primary threats are habitat destruction (deforestation for agriculture and livestock) and potential over-collection. Some populations (e.g., Rancho El Limón) are very small (< 5 ha) and particularly vulnerable. The patchy distribution and the absence of gene flow between populations make the species highly susceptible to local extinction events.
All Dioon species are listed on CITES Appendix II.
For buyers: Dioon holmgrenii is rare but increasingly available from specialist cycad nurseries. Seeds appear sporadically from rarepalmseeds.com and similar dealers. Ensure nursery provenance and CITES documentation.
Cultivation
| Hardiness | −3 to −5 °C (27 to 23 °F) / USDA zones 9a–11 (estimated) |
| Light | Full sun (essential — does not do well in shade) |
| Soil | Well-drained; tolerates a range of substrates including limestone |
| Watering | Moderate — seasonal; less water is better, but appreciates summer moisture |
| Adult size | Tall (taller than Dioon edule) × crown 1.5–2.5 m |
| Growth rate | Fast (for a Dioon) — one of the fastest species in the genus |
| Difficulty | 2/5 |
Geoff Stein (Dave’s Garden) describes Dioon holmgrenii as “an edule-like species from the Pacific coast of Oaxaca” that “is similar to Dioon edule in its needs.” User reports on Dave’s Garden PlantFiles note it as “one of the faster growing Dioons” that “can make a wonderful garden specimen.” This combination of edule-like ease and faster growth makes it one of the most rewarding Dioon species for the dedicated grower.
Light: full sun is essential. Despite being described from pine-oak forest understory, Dioon holmgrenii does not tend to do well in shade in cultivation (Geoff Stein). This is a meaningful difference from Dioon edule, which is very shade-tolerant. Plant in the sunniest available position.
Soil and drainage: similar to Dioon edule. Well-drained soil is the primary requirement. A range of substrates is tolerated — sandy, rocky, loamy — provided drainage is excellent. Limestone soils are acceptable.
Watering: less water is better. The Pacific slope habitat receives significant rainfall (1,000–2,500 mm) but with a dry season. In cultivation, water regularly during the growing season and reduce in winter. Dioon holmgrenii probably appreciates somewhat more moisture than the driest-habitat species like Dioon caputoi, but overwatering remains the primary risk.
Cold hardiness: moderately cold-tolerant. The Pacific slope habitat at 650–850 m experiences cool nights but very rare frost. Estimated tolerance is −3 to −5 °C for established plants in well-drained soil — comparable to Dioon purpusii or Dioon merolae, and less hardy than Dioon edule (−7 to −12 °C) or Dioon angustifolium (−10 to −14 °C). USDA zone 9a and above for year-round outdoor cultivation, with winter protection in zone 9a.
Growth rate: this is one of the key selling points of Dioon holmgrenii. Grower reports consistently describe it as one of the fastest-growing species in the genus. One Dave’s Garden user notes: “I have only had one plant which I grew from seed, but it has outpaced all the other Dioons I have grown.” This is a meaningful advantage for a genus where most species are painfully slow.
Offsetting: Dioon holmgrenii is generally solitary and not a prolific offsetter — another distinction from Dioon edule, which frequently produces basal pups. This means that propagation relies more heavily on seed than on division.
Container culture: well suited. Use a well-drained mix. Full sun during the growing season. Overwinter in a bright, cool location above 5 °C in climates below zone 9a.
Fertilization: responds well to balanced slow-release fertilizer during the growing season. The faster growth rate suggests a somewhat higher nutrient demand than Dioon edule.
Buying Advice
Availability: rare but increasing. Rarepalmseeds.com lists the species (as of 2024/2025), and specialist cycad nurseries in California, Florida, and Australia occasionally offer seedlings. Prices are moderate relative to the rarer species (Dioon caputoi, Dioon rzedowskii).
Identification: look for the persistent fuzzy tomentum on new leaves (lasting months to a year), blue-green leaf colour, spinulose upper leaflet margins, and non-imbricated leaflet arrangement. The combination of these characters separates Dioon holmgrenii from Dioon edule.
Growth advantage: for collectors seeking a faster-growing Dioon that still has the architectural elegance of the genus, Dioon holmgrenii is an excellent choice — it grows faster than Dioon edule while offering a distinct appearance.
Propagation
Seed: the primary method — especially important because this species does not offset freely. Remove the sarcotesta (gloves essential), soak 24–48 hours, sow horizontally in a well-draining mix, maintain 25–30 °C. Germination is cryptocotylar, expected in 2–12 months. Growth from seed is faster than for most other Dioon species.
Offsets: rare. Dioon holmgrenii is usually solitary and does not produce basal pups as freely as Dioon edule. When offsets do appear, they can be carefully separated.
Pests and Diseases
Root rot: the primary cultivation risk. Prevention through drainage and watering discipline.
Cycad aulacaspis scale (Aulacaspis yasumatsui): likely susceptible. Regular inspection of leaflet undersides and treatment with horticultural oil.
Mealybugs: possible in sheltered conditions.
Landscape Use
Dioon holmgrenii is a superb landscape cycad for warm-climate gardens where its faster growth rate (relative to other Dioon species) allows it to develop presence more quickly. Its blue-green foliage, persistent tomentum on new growth, and solitary, eventually tall habit make it an excellent statement specimen. It pairs well with other xeric and subtropical plants in Mediterranean-style or tropical gardens. Full sun is essential. Due to its solitary habit, it has a more vertical, single-trunked architectural profile than multi-headed Dioon edule — offering a different visual effect in the landscape. For collectors, the combination of aesthetic appeal, relative ease of cultivation, and botanical rarity makes it a compelling addition.
Frequently Asked Questions
Is Dioon holmgrenii the same as Dioon edule?
No. Although it is described as “edule-like” in general appearance, Dioon holmgrenii is a distinct species from the Pacific coast of Oaxaca (not the Atlantic slope). It differs in having spinulose leaflet margins (smooth in mature Dioon edule), persistently tomentose new leaves, a solitary habit, and faster growth rate.
Is Dioon holmgrenii really faster-growing than Dioon edule?
Yes — consistent grower reports confirm it as one of the fastest-growing species in the genus. This does not mean it is fast in absolute terms (it is still a cycad), but it outpaces Dioon edule and most other Dioon species under comparable conditions.
Does Dioon holmgrenii tolerate shade?
No — or at least not well. Despite being found in pine-oak forest, grower experience indicates that it does not perform well in shade. Full sun is recommended for best growth and compact habit.
How many populations exist in the wild?
At least nine to ten localities are known, comprising six genetically distinct populations across southwestern Oaxaca. Each population possesses unique genetic variants (private alleles), making every population irreplaceable for conservation.
Is Dioon holmgrenii 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:80652-2
The accepted nomenclatural record: native range Mexico (Oaxaca), seasonally dry tropical biome. No synonyms listed.
World List of Cycads — cycadlist.org
https://cycadlist.org/scientific_name/255
Detailed nomenclatural record: type specimen (Vázquez-Torres 2304, 15 km S of San Gabriel, Dto. Juquila, 650–850 m, 21 Mar 1980), etymology (Haynes 2022: honouring Noel Herman Holmgren), and IUCN status Vulnerable B1ab(iii)+2ab(iii).
IUCN Red List — Dioon holmgrenii
https://www.iucnredlist.org/species/42129/10660365
The Vulnerable (VU) assessment (Chemnick, Gregory & Morales, 2010). Notes limited extent of occurrence and habitat destruction. Pre-dates the 2023 genomic study.
Dorsey et al. (2023) — Conservation genomics of Dioon holmgrenii
https://link.springer.com/article/10.1007/s10592-023-01569-4
The landmark RADseq genomic study: characterises six genetically distinct populations, reconstructs a history of range expansion and fragmentation, documents private alleles in every population, and discusses implications for in situ and ex situ conservation. Essential reading.
De Luca, Sabato & Vázquez Torres (1981) — original description
https://link.springer.com/article/10.2307/2806763
The protologue in Brittonia 33(4): 552–554. Characterises the species by spinulose leaflets on the upper margin and non-imbricated, perpendicular leaflet insertion.
Velasco-García et al. (2016/2021) — Diversity and genetic structure of Dioon holmgrenii
https://pmc.ncbi.nlm.nih.gov/articles/PMC8623071/
Microsatellite study of nine populations: high genetic diversity (Ho = 0.62–0.98), heterozygote excess, and three genetic clusters corresponding to geographic groupings relative to the Río Verde.
iNaturalist
https://www.inaturalist.org/taxa/136080-Dioon-holmgrenii
Citizen science observations from the Oaxacan Pacific slope.
Dave’s Garden — Dioons: The Hardy Mexican Cycads
https://davesgarden.com/guides/articles/view/1981/
Practical grower’s guide by Geoff Stein. Notes Dioon holmgrenii as “an edule-like species from the Pacific coast of Oaxaca” that does not do well in shade, with persistently tomentose new leaves. Key cultivation reference.
Dave’s Garden — PlantFiles: Dioon holmgrenii
https://davesgarden.com/guides/pf/go/90920
User-submitted cultivation report: “one of the faster growing Dioons” with “blue-green” leaves and sharp, evenly spaced leaflets with “no teeth (or very rare teeth).”
The Gymnosperm Database — Dioon holmgrenii
https://www.conifers.org/za/Dioon_holmgrenii.php
Concise species profile. Quotes Norstog & Nicholls (1997): “differs from Dioon merolae principally in having non-imbricated leaflets along the rachis and by its normal rather than oblique leaflet insertions.”
Bibliography
De Luca, P., Sabato, S., & Vázquez Torres, M. (1981). Dioon holmgrenii (Zamiaceae), a new species from Mexico. Brittonia, 33(4), 552–555.
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.
Dorsey, B. L., Haevermans, T., Gregory, T. J., Specht, C. D., Sass, C., et al. (2023). Conservation genomics of Dioon holmgrenii (Zamiaceae) reveals a history of range expansion, fragmentation, and isolation of populations. Conservation Genetics.
González-Astorga, J., Vovides, A. P., Cabrera-Toledo, D., & Nicolalde-Morejón, F. (2008). Diversity and genetic structure of three species of Dioon Lindl. (Zamiaceae, Cycadales) from the Pacific seaboard of Mexico. Biological Journal of the Linnean Society, 94, 765–776.
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.
Norstog, K. J., & Nicholls, T. J. (1997). The Biology of the Cycads. Cornell University Press, Ithaca.
Velasco-García, M. V., et al. (2016). Population structure and genetic diversity of Dioon holmgrenii (Zamiaceae) in the Pacific coast biogeographic province of Mexico. Botanical Sciences.
Whitelock, L. M. (2002). The Cycads. Timber Press, Portland.