The story of Dioon merolae is a story of hidden diversity. When De Luca, Sabato, and Vázquez Torres described it in 1981 from the mountains of Chiapas, they believed they were naming a single species with a wide range stretching from Chiapas through Oaxaca. For nearly forty years, every Dioon population from the Central Depression of Chiapas to the Isthmus of Tehuantepec and the Central Valleys of Oaxaca was lumped under this one name. Then came the molecular revolution. Phylogenetic analyses (Dorsey et al., 2018; Gutiérrez-Ortega et al., 2018) revealed that what had been called Dioon merolae was in fact three genetically distinct lineages. The Oaxacan Central Valley populations were described as Dioon oaxacensis in 2020; the southeastern Oaxacan populations became Dioon salas-moralesiae in 2021. What remains as Dioon merolae sensu stricto is now restricted to its type locality in Chiapas — the populations of the Central Depression and the Sierra Madre de Chiapas. This taxonomic narrowing makes the species both more precisely defined and more threatened: a formerly wide-ranging taxon is now a Chiapas endemic.
For the collector, Dioon merolae is a beautiful, moderately large cycad with a distinctive “Venetian blind” leaflet arrangement that creates one of the most elegant crowns in the genus Dioon.
Quick Facts
| Scientific name | Dioon merolae De Luca, Sabato & Vázq.Torres |
| Family | Zamiaceae |
| Origin | Southern Mexico — Chiapas (Central Depression and Sierra Madre de Chiapas) |
| Adult size | Trunk to 3–6 m tall, 25–40 cm diameter; crown spread 2–3 m |
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 |
| IUCN | Vulnerable (VU) — but reassessment recommended following taxonomic split |
| CITES | Appendix II (all Dioon species) |
| Cultivation difficulty | 3/5 |
Taxonomy and Nomenclature
Dioon merolae was described by Paolo De Luca, Sergio Sabato, and Mario Vázquez Torres in 1981 in Brittonia (33: 179–185). The type specimen was collected in northwestern Chiapas, in the Distrito de Tonalá. The species was characterised by “linear-lanceolate leaflets arising at an acute angle from the rachis, obliquely inserted on it and imbricate” — a unique leaf architecture that immediately separated it from all other known Dioon species at the time.
The specific epithet merolae honours Aldo Merola, an Italian botanist whose contributions to cycad botany and conservation were recognised by the authors. The local common name in Chiapas is “espadaña” (sword lily), a reference to the sword-like stiffness of the leaves.
POWO currently restricts the native range of Dioon merolae to Mexico (Chiapas). This reflects the recent taxonomic revisions that have split the former broad concept of Dioon merolae into three separate species:
| Current species | Former concept | Distribution | Described |
|---|---|---|---|
| Dioon merolae s.s. | Type populations | Chiapas — Central Depression & Sierra Madre de Chiapas | De Luca, Sabato & Vázq.Torres, 1981 |
| Dioon oaxacensis | Central Valleys of Oaxaca populations | Oaxaca — Valles Centrales, arid habitats | Gutiérrez-Ortega, Pérez-Farrera & Vovides, 2020 |
| Dioon salas-moralesiae | Southeastern Oaxaca populations | Oaxaca — southeastern (Isthmus of Tehuantepec side) | Gutiérrez-Ortega et al., 2021 |
This taxonomic split has practical implications for collectors. Plants sold as “Dioon merolae” before 2020 could originate from any of the three lineages. Only plants from verified Chiapas provenance correspond to Dioon merolae sensu stricto under the current taxonomy. Dioon oaxacensis is distinguished by its flat, stiff, straight leaflets obliquely inserted to the rachis and its larger reproductive structures, growing in distinctly arid habitats. Dioon salas-moralesiae shows conspicuous morphological differences from both.
POWO lists one synonym: Dioon edule var. merolae (De Luca, Sabato & Vázq.Torres) Nance — reflecting the familiar pattern of historical lumping into Dioon edule.
Common names: Merola’s Dioon (English); Espadaña (Spanish/Chiapas).
Morphological Description
Dioon merolae is a moderately large, arborescent cycad. The trunk is cylindrical, erect or sometimes leaning with age, reaching 3 m or more in height (up to 5–6 m in the oldest wild specimens), with a diameter of 25–40 cm. As in all Dioon species, the trunk is clothed in persistent leaf bases and woolly hairs. When young, the trunk is subterranean, expanding in girth through cortex growth before beginning to produce aerial stem.
Leaves: the crown typically holds a moderate number of fronds. Individual leaves reach up to 100 cm in length (some sources report up to 120 cm), are slightly arching, and — most distinctively — slightly keeled in cross-section. The defining visual character of Dioon merolae is the leaflet arrangement: leaflets are narrow, linear-lanceolate, and set at slight alternating angles along the rachis, like the slats of a partially opened Venetian blind or a heater vent. This creates a unique three-dimensional texture visible even in young plants, quite different from the flat, symmetrically paired leaflet arrangement of Dioon edule or the deep V-keel of Dioon califanoi. Leaflets number approximately 200–240 per frond — among the highest counts in the genus. They are closely spaced, often imbricate (overlapping), and bear sparse prickles. New leaves emerge with a transient tomentum; mature leaflet surfaces are semi-glossy, dark green — not matte/glaucous (as in Dioon califanoi) and not brightly glossy (as in Dioon spinulosum). The leathery texture of mature leaves has been described as “stiff and somewhat rough to the touch, not slick-smooth like Dioon edule leaves” (Geoff Stein, Dave’s Garden).
Cones: male cones are cylindrical, woolly. Female cones are ovoid, woolly, moderate-sized for the genus. The species is dioecious. Coning is irregular and infrequent, as in all Dioon species.
Seeds: ovoid, with cream/white sarcotesta. Standard size for the smaller Dioon group.
Similar Species and Common Confusions
| Character | Dioon merolae | Dioon oaxacensis | Dioon califanoi | Dioon purpusii |
|---|---|---|---|---|
| Leaflet insertion | Acute angle, alternating (“Venetian blind”) | Oblique, stiff, straight | Oblique above rachis (deep V-keel) | ~Perpendicular, symmetrical pairs |
| Leaf keel | Slightly keeled | Slightly keeled | Strongly keeled (~90° V) | Flat to slightly keeled |
| Leaflet count/frond | 200–240 | Similar range | 160–200 | 150–260 |
| Leaflet surface | Semi-glossy, dark green | Semi-glossy | Glaucous, matte | Glossy |
| Prickles | Present, sparse | Present | Absent or very few | Present |
| Distribution | Chiapas only | Central Valleys of Oaxaca | Eastern Tehuacán valley (Oaxaca, Puebla) | Tomellín Canyon (Oaxaca) |
| Habitat | Pine-oak & tropical semi-deciduous forest | Arid zones, rocky hillsides | Dry mountain slopes | Dry tropical deciduous forest |
| Reproductive structures | Moderate-sized | Large | Moderate | Large |
The most important current confusion is with Dioon oaxacensis, which was only separated from Dioon merolae in 2020. Plants in collections labelled “Dioon merolae” that originated from Oaxacan Central Valley populations are now properly Dioon oaxacensis. Dioon oaxacensis can be distinguished by its flat, stiff, straight leaflets (vs. the angled “Venetian blind” arrangement of Dioon merolae) and its larger reproductive structures. With Dioon califanoi, the distinction lies in the keel angle (slight in Dioon merolae, deeply V-shaped in Dioon califanoi) and the leaflet texture (semi-glossy in Dioon merolae, matte/glaucous in Dioon califanoi).
Distribution and Natural Habitat
Under the current restricted circumscription, Dioon merolae sensu stricto is endemic to Chiapas, in southern Mexico. It occurs in at least 12 populations in two physiographic regions: the Central Depression of Chiapas (municipalities of Cintalapa, Jiquipilas, La Concordia, and Villaflores) and the Sierra Madre de Chiapas (municipalities of Arriaga and Tonalá, including the type locality). The altitudinal range is 600–1,200 m above sea level.
The habitat is diverse: Dioon merolae occurs both in pine-oak forest (bosque de Pinus and Quercus) and in tropical semi-deciduous forest, on soils developed over sedimentary rocks and karst limestone. Plants are typically found on steep sandstone or limestone cliffs and along thickly vegetated watercourses. In the Central Depression of Chiapas, the species grows on low limestone mounds (montículos) where the thin, skeletal soils provide excellent drainage.
Climate in the native range:
| Parameter | Central Depression of Chiapas | Sierra Madre de Chiapas (Tonalá area) |
|---|---|---|
| Altitude | 600–900 m | 800–1,200 m |
| Mean annual temperature | 22–26 °C | 20–24 °C |
| Average winter minimum | 12–16 °C | 10–14 °C |
| Estimated historical minimum | 5–8 °C (frost very rare) | 3–6 °C (occasional light frost) |
| Summer maximum average | 32–36 °C | 28–32 °C |
| Annual rainfall | 800–1,200 mm (summer-dominant) | 1,000–1,500 mm |
The climate is warm, with a well-defined dry season and summer rains. Frost events are rare in the Central Depression but can occur at the higher-altitude Sierra Madre sites. This relatively frost-free native climate explains why Dioon merolae is less cold-tolerant in cultivation than the northern mountain species Dioon edule and Dioon angustifolium.
Conservation
Dioon merolae is classified as Vulnerable (VU) on the IUCN Red List (Chemnick & Gregory, 2010). However, this assessment pre-dates the taxonomic split that removed the Oaxacan populations into Dioon oaxacensis and Dioon salas-moralesiae. The restricted circumscription of Dioon merolae to Chiapas alone significantly reduces its known extent of occurrence, and a reassessment under current taxonomy would likely result in a higher threat category.
Populations are threatened by land-use change (conversion to agriculture and pasture), fire (both natural and anthropogenic), and over-collection. In some areas, the top portion of plants is cut off and sold as a table decoration — a destructive practice that can kill the plant. The species has particular cultural significance in the Central Depression of Chiapas: the leaves (known as “espadaña”) have been used in an ancient religious ceremony by local communities, documented in detail by Pérez-Farrera & Vovides (2006). This ceremonial use, while culturally important, involves harvesting leaves from wild plants and adds to the pressures on populations.
All Dioon species are listed on CITES Appendix II.
For buyers: ensure nursery provenance. Be aware that plants sold as “Dioon merolae” may actually be Dioon oaxacensis or Dioon salas-moralesiae under current taxonomy, depending on the seed source.
Cultivation
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 |
| Light | Full sun to partial shade (full sun preferred; does not perform well in deep shade) |
| Soil | Well-drained, mineral-dominant; limestone soils tolerated |
| Watering | Moderate — seasonal; less water is better, but appreciates summer moisture |
| Adult size | Trunk to 3–6 m × crown 2–3 m |
| Growth rate | Moderate — faster than Dioon edule |
| Difficulty | 3/5 |
Dioon merolae is a moderately demanding species that performs best in warm, sunny climates with good drainage. Geoff Stein (Dave’s Garden) describes it as “a moderately fast grower,” but notes that “its cold tolerance is not too impressive” and “it does not perform well in shady locations, so it is a bit less adaptable than some of the other Dioons.” Tom Broome (2007, The Cycad Newsletter) and Charlie Beck (Palm Beach Palm & Cycad Society) have published detailed cultivation guides from Florida experience.
Light: full sun is preferred and produces the most compact, architectural crowns. Partial sun (morning sun, afternoon shade) is acceptable in very hot, arid climates. Deep shade is not tolerated — the plant becomes etiolated and loses its characteristic habit.
Soil and drainage: excellent drainage is essential. The native habitat includes both sandstone cliffs and limestone karst — both providing rapid drainage. In cultivation, a predominantly mineral mix (pumice, perlite, coarse sand, limestone chips) with modest organic matter is ideal. Avoid flat, water-collecting positions.
Watering: less water is better. Water regularly during the growing season (spring through early autumn), then reduce or cease in winter. The native climate has a marked dry season; replicating this pattern helps prevent root rot. Dioon merolae appreciates more summer moisture than the driest-habitat species like Dioon caputoi, but significantly less than the rainforest species Dioon spinulosum.
Cold hardiness: limited. Dioon merolae comes from relatively warm, frost-free to rarely frost-affected habitat. Established plants tolerate brief frosts to approximately −2 to −4 °C, but damage occurs quickly and recovery can be slow. It is significantly less cold-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. In zone 9a, winter protection is necessary. As with all cycads, soil moisture must be minimal during cold periods. The caveat on juvenile vs. adult cold tolerance applies: a young plant with a subterranean caudex may survive better than an older plant with an exposed aerial trunk, thanks to soil thermal inertia insulating the growth point.
Container culture: performs well in containers, which is the recommended approach for growers in zones cooler than 9b. Use a gritty, free-draining mix. Overwinter in a bright, warm location (minimum 8–10 °C). Full sun during the growing season.
Fertilization: responds well to balanced slow-release fertilizer during the growing season. Moderate feeder compared to the undemanding Dioon edule.
Buying Advice
Availability: Dioon merolae (sensu lato, including what is now Dioon oaxacensis) has been moderately available in the specialist cycad trade. True Dioon merolae sensu stricto from Chiapas provenance is rarer. Most plants sold as “Dioon merolae” before 2020 may be from Oaxacan populations now assigned to Dioon oaxacensis — which is not necessarily a problem for the gardener (both are attractive plants), but matters for taxonomic accuracy and collection records.
Ask for provenance: if possible, determine whether the parent seed came from Chiapas (true Dioon merolae) or from Oaxaca (Dioon oaxacensis or Dioon salas-moralesiae). For most collectors, the distinction is one of nomenclatural precision rather than practical cultivation difference, but for those who value taxonomic accuracy, it is important.
Identification check: the “Venetian blind” leaflet arrangement (narrow leaflets set at alternating angles along the rachis) is the key visual character. If the leaflets are flat, stiff, and straight rather than angled, the plant may be Dioon oaxacensis.
Propagation
Seed: the primary method. Remove the sarcotesta (use gloves), soak 24–48 hours in warm water, sow horizontally, half-buried in a well-draining mineral mix. Maintain 25–30 °C. Keep medium barely moist. Germination is cryptocotylar, typically 2–12 months. Growth from seed is moderately fast for a Dioon.
Offsets: mature plants may produce basal offsets or even branches (especially after damage to the main growing point). Offsets can be separated once well-rooted.
Pests and Diseases
Root rot: the primary threat in cultivation, as with all Dioon species. Prevention through proper drainage and winter watering discipline is the only reliable approach.
Cycad aulacaspis scale (Aulacaspis yasumatsui): documented on Dioon merolae in cultivation. Regular inspection and treatment with horticultural oil are recommended.
Eumaeus butterfly larvae: Eumaeus spp. cycad butterflies have been documented on Dioon merolae in Chiapas. In cultivation, larval damage to young leaves is usually cosmetic.
Landscape Use
Dioon merolae is an excellent collector’s cycad for warm-climate gardens. Its unique “Venetian blind” leaflet arrangement, moderate size, and attractive dark green foliage make it a compelling focal specimen or accent plant. It pairs well with other xeric or subtropical plants in a mixed planting — Agave, Dasylirion, ornamental grasses — but requires a warm, sunny, well-drained position. It is less versatile than Dioon edule (which tolerates more cold and more shade) and less suited to humid climates than Dioon spinulosum. Its optimal niche is in warm temperate to tropical dry gardens: inland Mediterranean, Southern California, southern Florida, and comparable climates.
Frequently Asked Questions
Has Dioon merolae been split into multiple species?
Yes. Recent molecular and morphological studies have revealed that what was previously called Dioon merolae actually comprised three distinct lineages. The Oaxacan Central Valley populations were described as Dioon oaxacensis (2020), and the southeastern Oaxacan populations as Dioon salas-moralesiae (2021). Dioon merolae sensu stricto is now restricted to Chiapas. Plants in collections sold before 2020 as “Dioon merolae” may belong to any of the three species, depending on seed provenance.
What distinguishes Dioon merolae from Dioon oaxacensis?
Dioon merolae has narrow leaflets set at alternating angles along the rachis (the “Venetian blind” effect), while Dioon oaxacensis has flat, stiff, straight leaflets obliquely inserted. Dioon oaxacensis also has larger reproductive structures and grows in distinctly arid habitats in the Central Valleys of Oaxaca, while Dioon merolae occurs in wetter pine-oak and semi-deciduous forest in Chiapas.
How cold-hardy is Dioon merolae?
Limited. It tolerates brief frosts to approximately −2 to −4 °C, but is significantly less cold-hardy than Dioon edule, Dioon angustifolium, or Dioon califanoi. USDA zone 9b and above for year-round outdoor cultivation.
Does Dioon merolae tolerate shade?
Not well. Unlike Dioon edule or Dioon spinulosum, which tolerate moderate to deep shade, Dioon merolae requires full sun or at least bright partial shade for good growth and compact habit. Shady conditions produce etiolated, weak plants.
Is Dioon merolae 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:80654-2
The accepted nomenclatural record: native range now restricted to Mexico (Chiapas), seasonally dry tropical biome. One synonym listed (Dioon edule var. merolae).
World List of Cycads — cycadlist.org
https://cycadlist.org/taxon.php?Taxon_ID=254
Nomenclatural record: type specimen (De Luca, Sabato & Vázquez Torres, Tonalá, Chiapas), etymology, distribution, and IUCN conservation status.
IUCN Red List — Dioon merolae
https://www.iucnredlist.org/species/42131/10659076
The Vulnerable (VU) assessment (Chemnick & Gregory, 2010). Note: this assessment pre-dates the 2020–2021 taxonomic split and covers the former broader species concept. A reassessment is needed.
Gutiérrez-Ortega et al. (2020) — Dioon oaxacensis
https://www.biotaxa.org/Phytotaxa/article/view/phytotaxa.474.1.5
The paper describing Dioon oaxacensis from the Central Valleys of Oaxaca, splitting it from Dioon merolae. Includes morphological comparison table and genetic evidence. Essential for understanding the current circumscription of Dioon merolae.
Gutiérrez-Ortega et al. (2021) — Dioon salas-moralesiae
https://phytotaxa.mapress.com/pt/article/view/phytotaxa.528.2.3
The paper describing the third lineage segregated from Dioon merolae. Completes the tripartite split and provides the morphometric analysis supporting recognition of three species.
Pérez-Farrera & Vovides (2006) — Ceremonial use of Dioon merolae
Published in Boletín de la Sociedad Mexicana de Botánica 78: 107–113. Documents the traditional “espadaña” ceremony in Chiapas communities, providing ethnobotanical context.
Lázaro-Zermeño et al. (2012) — Natural history of Dioon merolae
https://www.scielo.org.mx/…
Comprehensive natural history study covering distribution, habitat, population structure, growth, and reproduction in both Chiapas and Oaxaca populations. Essential field ecology reference.
iNaturalist
https://www.inaturalist.org/taxa/136082-Dioon-merolae
Citizen science observations. Note that observations may include Dioon oaxacensis and Dioon salas-moralesiae under the former broader concept.
Dave’s Garden — Dioons: The Hardy Mexican Cycads
https://davesgarden.com/guides/articles/view/1981/
Grower’s guide by Geoff Stein. Describes Dioon merolae as “a beautiful species with narrow leaflets that tend to be set at slight angles along the rachis like a partially set of open blinds” and notes its limited cold tolerance and shade intolerance.
Llifle — Encyclopedia of Living Forms
https://llifle.com/Encyclopedia/…
Species page with morphological description, habitat data, and cultivation notes. States altitude range 600–1,200 m and fewer than ten known locations.
Bibliography
Cabrera-Toledo, D., González-Astorga, J., & Flores-Vázquez, J. C. (2010). Fine-scale spatial genetic structure in two Mexican cycad species Dioon merolae (Zamiaceae) and Zamia loddigesii (Zamiaceae): implications for conservation. Biochemical Systematics and Ecology, 38, 851–860.
Chemnick, J., & Gregory, T. (2010). Dioon merolae. The IUCN Red List of Threatened Species 2010: e.T42131A10659076.
De Luca, P., Sabato, S., & Vázquez Torres, M. (1981). Dioon merolae (Zamiaceae), a new species from Mexico. Brittonia, 33, 179–185.
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., Pérez-Farrera, M. A., Vovides, A. P., Salas-Morales, S. H., Chemnick, J., et al. (2020). Dioon oaxacensis (Zamiaceae): a new cycad species from the arid central valleys of Oaxaca (Mexico). Phytotaxa, 474(1), 51–66.
Gutiérrez-Ortega, J. S., et al. (2021). A reassessment of Dioon merolae (Zamiaceae) leads to the description of Dioon salas-moralesiae, a new cycad species from Southeastern Oaxaca, Mexico. Phytotaxa, 528(2), 93–110.
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.
Lázaro-Zermeño, J. M., González-Espinosa, M., Mendoza, A., Martínez-Ramos, M., & Quintana-Ascencio, P. F. (2012). Historia natural de Dioon merolae (Zamiaceae) en Chiapas, México. Botanical Sciences, 90(1), 73–87.
Norstog, K. J., & Nicholls, T. J. (1997). The Biology of the Cycads. Cornell University Press, Ithaca.
Pérez-Farrera, M. A., & Vovides, A. P. (2006). The ceremonial use of the threatened “espadaña” cycad (Dioon merolae, Zamiaceae) by a community of the central depression of Chiapas, Mexico. Boletín de la Sociedad Mexicana de Botánica, 78, 107–113.
Whitelock, L. M. (2002). The Cycads. Timber Press, Portland.