Dioon merolae

Dioon merolae is a cycad from southern Mexico, best known from Chiapas, where its leaves, called espadaña, decorate church altars every year for the feast of the Holy Cross. This species of the genus Dioon grows on rocky hills in dry forest or pine-oak forest and, over centuries, builds trunks of 3 to 5 m that often end up lying on the ground. It is recognised by its crowded, overlapping leaflets bent downwards at the tip, which give the leaf the outline of a bird in flight. Listed as Vulnerable on the IUCN Red List, it is a collectors’ favourite, but it grows slowly and suffers from frost; since 2020 part of what was sold as Dioon merolae has been described as separate species.

Key facts about Dioon merolae

  • Accepted name: Dioon merolae De Luca, Sabato & Vázq.Torres (1981), accepted by The World List of Cycads (version 2026.08.15), GBIF, Species+ and Kew’s World Checklist of Vascular Plants, the database behind POWO (June 2026 version, published on GBIF)
  • Former names and synonyms: no published synonym; the unpublished names “Dioon dohenyi” and “Dioon pinoi” circulated before 1981; “Dioon edule var. merolae” is not a valid name according to the 2020 revision
  • Family: Zamiaceae, order Cycadales
  • Common names: no English name is attested; espadaña and nimalari in Chiapas, also yerba sagrada, maíz viejo, morrito, palma espinuda and palmilla
  • Natural range: Mexico, Chiapas (Central Depression and Sierra Madre), and Oaxaca according to The World List of Cycads; about 500 to 1,200 m (1,600 to 3,900 ft)
  • IUCN status: Vulnerable (VU A2cd; C2a(i)), assessment by J. Chemnick and T. Gregory dated July 2020 and published in 2022
  • CITES and other protection: CITES Appendix II (annotation #4); Annex B in the European Union; “in danger of extinction” (category P) in Mexico under the standard NOM-059-SEMARNAT-2010
  • Documented hardiness: growing point killed at about −7 °C (19 °F) in Florida (1989); a plant cut to the ground at −10.6 °C (13 °F) in Texas (1983) resprouted from the base (see Cold hardiness)
  • Light and soil: full sun on the coast, morning sun inland according to a Californian nursery; thin, very free-draining soils in the wild
  • Habit and adult size: trunk 3 m or more, up to 5 to 6 m, 25 to 40 cm (10 to 16 in) across, often reclining and branched on old plants; leaves 1 to 1.4 m long
  • In the UK, US and Australia: Kew and the National Botanic Garden of Wales (BGCI); Montgomery Botanical Center (two accessions), Fairchild, Naples Botanical Garden and some 16 other American gardens; Royal Botanic Gardens Victoria (BGCI)
  • Toxicity: to be treated as toxic, especially the seeds (see Toxicity and precautions)

How to recognise Dioon merolae

Dioon merolae is recognised by its leaves with very crowded leaflets, which leave the axis at an acute angle, are inserted obliquely and overlap along almost the whole length of the leaf. According to Chemnick and Pérez-Farrera (2007), the leaflets curve up and then down on either side of the axis, so that in cross-section the leaf recalls a gull in flight, wings lowered. The original description by De Luca, Sabato and Vázquez Torres (1981) gives the following measurements:

  • Trunk: cylindrical, upright or reclining, 3 m or more and 25 to 40 cm across; the largest plants observed are 5 to 6 m tall and the oldest have reclining trunks.
  • Leaves: numerous, stiff and leathery, woolly when young and later hairless, 1 m long or more, on a petiole of 7 to 10 cm.
  • Leaflets: more than 120 on each side, linear-lanceolate, sharp, with rolled margins; the middle ones 7 to 9 cm by 10 to 12 mm, with entire margins or one or two small spiny teeth 1 mm long.
  • Male cone: elongated and cylindrical, 30 to 40 cm long and 8 to 10 cm across.
  • Female cone: ovoid, about 45 cm by 25 cm, with densely woolly scales.
  • Seeds: ovoid, about 3 cm across, with a yellow fleshy coat (2020 revision).
Dioon merolae
Dioon merolae grown under glass at the Palmengarten in Frankfurt, Germany.

Young plants always have strongly spiny leaflets on both margins and very woolly leaves. In Chiapas, Lázaro-Zermeño and co-authors (2012), who followed more than 800 plants, add leaves up to 1.4 m long, female cones weighing 3.6 kg on average (more than 6.4 kg for the heaviest) and carrying 107 to 149 seeds, and male cones 20 to 60 cm long. Old plants can carry up to 18 secondary stems around the main trunk. The Californian nursery Jungle Music describes the foliage as green or distinctly grey-green, the leaves gently keeled, with the leaflets reflexed towards the ground.

Hybrids

No natural or garden hybrid of Dioon merolae has been documented.

Possible confusion

With Dioon oaxacensis and Dioon salas-moralesiae

These two species from Oaxaca were separated from Dioon merolae in 2020 and 2021. According to the key published in 2021 by Gutiérrez-Ortega and co-authors, Dioon merolae has strongly overlapping leaflets clearly bent downwards at the tip; those of Dioon oaxacensis are flat, stiff and straight, those of Dioon salas-moralesiae broad, less overlapping and only slightly bent. A plant from Oaxaca sold as Dioon merolae before these descriptions may therefore belong to one of these species. The trade names “Oaxaca Gold” or “Golden Oaxacan”, discussed on the French Fous de palmiers forum between 2010 and 2015, referred to plants that growers could not tell from an ordinary Dioon merolae.

With Dioon califanoi

Dioon califanoi, from northern Oaxaca and Puebla, is its closest relative according to the 2020 revision. Its leaves are strongly keeled, V-shaped; the revision gives finer characters: Dioon merolae has cataphylls with a long, tail-like point, keeled and recurved leaflets and trowel-shaped female cone scales.

With Dioon edule

The original description compares Dioon merolae with the form of Dioon edule once named Dioon imbricatum: in that form the leaflets leave the axis at right angles, are not inserted obliquely and overlap only at the tip of the leaf.

With Dioon holmgrenii

Dioon holmgrenii, from the Pacific slope of Oaxaca, is genetically very close (a single difference in the chloroplast DNA analysed by Moretti and co-authors in 1993, according to Chemnick, Gregory and Salas-Morales, 1997), but its leaflets are flat, spaced and set almost at right angles, with three or four spines.

Taxonomy

Paolo De Luca, Sergio Sabato and Mario Vázquez Torres described Dioon merolae in 1981 in the journal Brittonia (33(2): 179–185). The type is the collection Vázquez Torres 2301, made on 27 May 1979 on the Cerro de la Placa, about 4 km west of Tres Picos, in the district of Tonalá (Chiapas); the holotype is kept in the Naples herbarium (NAP), with duplicates in Florence, Mexico City, New York and Xalapa (The World List of Cycads). The 2020 revision dates the collection to 29 May 1979 and places it at 1,200 m.

The plant was known before it was described. In 1909 the Californian collector Edward Howard found a population near Tres Picos and called it “Dioon dohenyi”, after the sponsor of his expedition, E.L. Doheny; he had up to 50 plants exported to California, and the herbarium specimens were later labelled Dioon tomentosum or Dioon purpusii. In 1925 Carl Purpus collected it near Montserrate under a name attributed to Joseph Rose, “Dioon pinoi”. None of these names was published (De Luca and co-authors, 1981; Chemnick and Pérez-Farrera, 2007). The 2020 revision states that “Dioon edule var. merolae”, occasionally seen, is not a valid name.

The species has recently been redefined. In 2020 a genetic, morphological and climatic study by Gutiérrez-Ortega and co-authors (New Phytologist) showed that Dioon merolae as then understood included distinct lineages. The populations of the arid valleys of Oaxaca were described the same year as Dioon oaxacensis, those of south-eastern Oaxaca in 2021 as Dioon salas-moralesiae. According to the abstract of the 2021 paper, the name Dioon merolae now applies to the lineage east of the Isthmus of Tehuantepec, in Chiapas; The World List of Cycads still includes Oaxaca in its range. Gutiérrez-Ortega and co-authors (2021) place it in the Purpusii clade, together with Dioon califanoi, Dioon purpusii and Dioon holmgrenii among others.

Etymology and history

The epithet honours Aldo Merola (1924–1980), professor of botany and director of the botanic garden of the University of Naples, for his contribution to the knowledge and conservation of cycads (De Luca and co-authors, 1981; The World List of Cycads). The authors brought back living plants from Tonalá and Montserrate, which were growing in the Naples botanic garden when the species was described.

In the mid-1990s Silvia Salas-Morales and Leo Schibli discovered populations in Oaxaca, about 160 km west of the Chiapas localities, showing that the Isthmus of Tehuantepec was not an impassable barrier (Chemnick and Pérez-Farrera, 2007). Some of these populations now belong to the species described in 2020 and 2021.

Common names

No English common name is attested. In Chiapas the species is called espadaña, a name used in the original description and in the Mexican standard NOM-059, and nimalari in Chiapanec, an extinct Amerindian language, from a word meaning “feather” (Bonta and co-authors, 2019). Chemnick and Pérez-Farrera (2007), after Bonta and Osborne, also give yerba sagrada, maíz viejo, morrito, palma espinuda and palmilla; Species+ lists nimalari and morrito. The English name “Merola’s dioon”, sometimes put forward, is not documented in any reliable source.

In the wild

In Chiapas, Dioon merolae forms at least 12 populations in two regions: the Central Depression (Cintalapa, Jiquipilas, La Concordia and Villaflores) and the Sierra Madre (Arriaga and Tonalá) (Lázaro-Zermeño and co-authors, 2012). The original description records it in the understorey of pine-oak forest at Montserrate and on the Cerro Grande, and in tropical semi-deciduous forest on the Cerro de la Placa. The 2020 revision also cites xerophytic scrub and oak forest, at 500 to 800 m; Lázaro-Zermeño and co-authors studied it at 530 to 600 m in the Central Depression and 1,100 to 1,200 m in the Sierra Madre, and a letter of 1926 quoted by the original description places a station near Tres Picos at about 1,500 m.

In the Central Depression the adult plants grow on rocky sandstone knolls overlooking the plain, mainly on north-facing slopes, in very thin soils under a very open canopy; the climate is warm, with 700 to 1,000 mm (28 to 39 in) of rain from May to October. The stems bend towards the light and end up lying on the ground (Lázaro-Zermeño and co-authors, 2012; González-Espinosa and co-authors, 2012).

Growth is very slow: less than 1 cm of trunk a year during the first ten years, then 2 to 3 cm a year. Adults produce 30 to 40 leaves in a single flush, in March and April, before the rainy season. According to Lázaro-Zermeño and co-authors, a plant begins to reproduce at 80 to 100 years, and a 4 m trunk may be at least 700 years old.

The caterpillars of the butterfly Eumaeus debora eat 40 to 100% of the young leaves in some years. The male cones host the weevil Parallocorynus jonesi (O’Brien and Tang, 2015) and the small beetle Pharaxonotha occidentalis, described in 2022 from a cone of Dioon merolae from El Campanario (Chiapas); their exact role in pollination has not been established.

Conservation status and regulation

Dioon merolae is listed as Vulnerable (VU) on the IUCN Red List, in the assessment by J. Chemnick and T. Gregory dated 3 July 2020 and published in 2022, under criteria A2cd and C2a(i): a population decline linked to habitat loss and collecting, and a small, declining population (The World List of Cycads, GBIF and Crossref). The IUCN cycad action plan of 2003 estimated it at 3,000 to 5,000 adult plants.

The threats are many. According to that action plan and to Chemnick and Pérez-Farrera (2007), collectors have decimated populations: hundreds of wild trunks were still being smuggled into the United States in the 1990s. Clearing for crops and pasture, fires, goats browsing the seedlings and seed collecting add to the pressure. The ritual harvest of the leaves is debated: Chemnick and Pérez-Farrera considered it without notable effect, since only old leaves are taken, whereas Lázaro-Zermeño and co-authors (2012) observed plants defoliated up to 100% every year, whose females reproduce less, in populations of fewer than 200 adults. Community nurseries have been set up in the buffer zone of La Sepultura Biosphere Reserve, and about 2,000 seedlings have been reintroduced near Villaflores.

The species is listed in CITES Appendix II, like the whole genus since 4 February 1977, with annotation #4, which exempts seeds and pollen in particular. In the European Union it is listed in Annex B of Regulation (EC) No 338/97, whose current annexes are set by Regulation (EU) 2026/1383, in force since 29 June 2026, according to Species+. In Mexico the standard NOM-059-SEMARNAT-2010 lists it as “in danger of extinction” (category P), as the text published in the Diario Oficial de la Federación confirms. Databases checked on 26 October 2026: check the regulations in force before buying, and prefer seed-grown plants from cultivation.

Growing Dioon merolae

Dioon merolae is a much admired collectors’ Dioon, but a slow one. Growers who have published their experience give these benchmarks:

  • Exposure: full sun on the coast and morning sun inland, according to the Californian nursery Jungle Music. In the wild the plant is a sun-lover: in full light the trunks stay upright.
  • Soil: thin, stony, fast-draining soils in the wild. On the French Fous de palmiers forum, a grower in Barcelona lost half of his eighteen-month-old seedlings after an over-watered summer: the roots had rotted (2010).
  • Growth: slow. In central Florida, Tom Broome sees a single flush of leaves a year, sometimes none, followed the next year by an abundant flush, and his plants do not respond to fertiliser (The Cycad Newsletter, 2007). According to Jungle Music, a plant with a 90 cm (3 ft) trunk may be a hundred years old.

In cultivation the flush can shift: at the Montgomery Botanical Center in Florida, plants produce their leaves mainly in July (Calonje, 2007), and a grower in Poland sees his pot plants flush in winter, near a heated window (2014).

Outdoor and in-ground cultivation

Dioon merolae is grown outdoors in subtropical gardens: in Florida at the Montgomery Botanical Center and at Naples Botanical Garden, which displays plants grown from material legally collected in Mexico in the early 1980s by a Florida horticulturist and later given to the garden, and in coastal southern California, where Jungle Music grows old specimens. Give it full sun or bright light, a very free-draining, gritty soil and moderate summer watering. In Mediterranean-type climates no in-ground trial has been documented; a sheltered, sunny raised bed would follow the logic of its habitat, but frost remains the limiting factor (see Cold hardiness). No observation of the species has been published for the Jardin zoologique tropical at La Londe-les-Maures, in the Var.

Container cultivation

Its slow growth makes Dioon merolae a good subject for a large pot, in a mineral mix that dries quickly; the Barcelona experience shows that over-watering in summer is the main danger for seedlings. In climates with frost, move the pot in winter into a bright, frost-free place.

In cultivation in the UK, US and Australia

  • United Kingdom: the Royal Botanic Gardens, Kew, and the National Botanic Garden of Wales declare the species to BGCI PlantSearch; both are glasshouse collections, and no outdoor cultivation is documented in Britain.
  • United States: the Montgomery Botanical Center (Coral Gables, Florida) lists two accessions in its online catalogue, one from 2001; the Fairchild Tropical Botanic Garden (Miami) lists the species in its online catalogue, and Naples Botanical Garden (Florida) displays plants grown from material legally collected in Mexico in the early 1980s and donated by a private collector. BGCI PlantSearch also lists the Huntington, Lotusland, the Los Angeles County Arboretum, San Diego Botanic Garden, the San Diego Zoo, the University of California Botanical Garden at Berkeley, the Missouri Botanical Garden, the Chicago Botanic Garden, Marie Selby Botanical Gardens and the Edison and Ford Winter Estates, among others. A BGCI listing is a declaration by the garden, not proof that the plant is alive today, and plants of Oaxacan origin acquired before 2020 may belong to Dioon oaxacensis or Dioon salas-moralesiae.
  • Australia: Royal Botanic Gardens Victoria (Melbourne) declares the species to BGCI; no other Australian record has been found.

Propagation

Dioon merolae is propagated mainly from seed. The seeds have no dormancy: as soon as the embryo is mature they germinate, at over 80% in the trial by Lázaro-Zermeño and co-authors, and the first leaf appears three months after germination. They do not keep long. On the French Fous de palmiers forum, two growers, one of them in Nîmes, saw the first germinations after about two months (2010). Seedlings make only one leaf a year for years.

To obtain seed, a male and a female plant are needed. Tom Broome advises, in Florida, opening the top of the receptive female cone and pouring dry pollen into it; the seeds stay 18 to 24 months in the cone (16 to 28 months at the Montgomery Botanical Center). The plant produces offsets at the base, especially male plants, but it is not known whether these root once detached (Lázaro-Zermeño and co-authors, 2012).

Pests and diseases

No disease specific to the species is documented. In the wild, the caterpillars of Eumaeus debora can destroy most of the young leaves, and beetles and rodents attack the seeds. In cultivation the main risk is rot of the roots and trunk in a cold or over-wet mix, including in seedlings in summer.

Cold hardiness

Dioon merolae survives light frosts, but its foliage tolerates them poorly, and the growing point can die at around −7 °C (19 °F). Two accounts published in 2007 in The Cycad Newsletter, both from the United States, document this.

Mediterranean and European experience

No in-ground cultivation is documented in France. On the French Fous de palmiers forum, a grower in Agen, who had never left it outdoors in winter, thought it could take light frosts (2010–2011). Another grower considers that leaf damage appears at about −4 to −5 °C (25 to 23 °F) in this group of Dioon (2021): these are opinions, not dated observations. For readers in comparable Mediterranean-type climates, the American reports below are the best available guide.

United Kingdom, United States and Australia

  • Central Florida: during the freeze of 1989, plants had their growing point killed at about −7.2 °C (19 °F), and some produced new offsets; according to the author, the species tolerates about −6 to −5 °C (the low 20s °F) under a tree canopy, but its foliage dislikes frost (Broome, 2007).
  • South Texas (Goliad County): during the 1983 cold wave, after heavy rain, a large female plant in the ground went through −3.3 °C (26 °F), −10.6 °C (13 °F), −8.9 °C (16 °F) and then −7.2 °C (19 °F) on four nights, with about 65 hours below freezing. It was killed to the ground and its trunk fell over a few months later, but the base resprouted in early summer and the plant recovered (Frasier, 2007).
  • Coastal southern California: Jungle Music claims hardiness to about −6.7 to −5.6 °C (20 to 22 °F) and judges it less hardy than Dioon edule.
  • United Kingdom and Australia: no dated report has been found; in Britain the species needs a heated glasshouse.

These accounts suggest leaf damage from the first hard frosts, death of the growing point at around −7 °C (19 °F) and possible regrowth from the base; this is a synthesis of isolated reports, not a measurement.

Toxicity and precautions

Dioon merolae should be treated as toxic, especially the seeds. Cycads contain toxic glycosides (cycasin, macrozamin), from which methylazoxymethanol is derived, a toxin affecting the liver and the nervous system. No analysis specific to the species is documented. According to Chemnick and Pérez-Farrera (2007), some local people eat the fried seed flesh, and the kernel has been used as famine food: these traditional uses should not be copied. Wear gloves to handle the seeds, and keep plants and seeds out of reach of children and animals. If any part has been eaten, contact a poison centre, a doctor or a vet without delay.

Traditional uses

Dioon merolae lies at the heart of a religious tradition in Chiapas, described by Pérez-Farrera and Vovides (2006):

  • Pilgrimage: at the end of April, about a hundred men from Suchiapa, the espadañeros or hojeros, walk 60 to 70 km to the Cerro Nambiyugua, near Villaflores, pick the old leaves by hand, without cutting tools, and carry them back in bundles called tercios. According to Bonta and co-authors (2019), a load can reach 80 kg.
  • Feast: the leaves decorate the altars for the feast of the Holy Cross, on 3 May, at Suchiapa, Terán and Nuevo Tenochtitlán (Villaflores); at Suchiapa a herald’s chant dedicated to the nimalari is still sung in Chiapanec (Bonta and co-authors, 2019). Lázaro-Zermeño and co-authors (2012) also report harvests on the cerros El Estoraque and El Sauz (Jiquipilas).
  • Other uses: leaves for Holy Week, weddings and wreaths, seed shells for children’s games, bracelets and necklaces (Chemnick and Pérez-Farrera, 2007).

This tradition, probably pre-Hispanic in origin and blended with Catholicism in colonial times, is also practised by Zoque and mestizo people. Deliberate fires, attributed to religious groups hostile to the feast, have affected the Cerro Nambiyugua (Pérez-Farrera and Vovides, 2006).

Frequently asked questions

Is Dioon merolae hardy?

Only to light frosts. In Florida the growing point was killed at about −7 °C (19 °F); in Texas a plant destroyed at −10.6 °C (13 °F) regrew from the base. Plant it out only where frost is rare and light; elsewhere, grow it in a pot moved to a frost-free place in winter.

How can it be told from Dioon edule?

By its leaflets, inserted obliquely and crowded to the point of overlapping along almost the whole leaf, then bent downwards. In Dioon edule they leave the axis at right angles and, in some forms, overlap only at the tip of the leaf.

Is a Dioon merolae from Oaxaca really Dioon merolae?

Not necessarily. Since 2020 and 2021, part of the Oaxacan populations belong to Dioon oaxacensis or Dioon salas-moralesiae. Without a precise origin, a plant labelled “merolae from Oaxaca” cannot be identified with certainty.

Does it grow fast?

No. It often makes a single flush of leaves a year, sometimes none. In the wild the trunk gains 2 to 3 cm a year in adults, and a plant produces its first cones only at 80 to 100 years.

What is espadaña?

It is the name of Dioon merolae in Chiapas. Every spring, pilgrims from Suchiapa gather its leaves on the Cerro Nambiyugua to decorate the altars for the feast of the Holy Cross on 3 May.

Is it toxic?

Yes, it should be treated as toxic, especially the seeds, like all cycads. Wear gloves to handle them and call a poison centre if any part is eaten.

Is it endangered?

It is threatened. The IUCN lists it as Vulnerable (assessment published in 2022), and Mexico as in danger of extinction, because of collecting, clearing and fires. It is protected by CITES (Appendix II).

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • De Luca, P., Sabato, S. & Vázquez Torres, M. (1981). Dioon merolae (Zamiaceae), a new species from Mexico. Brittonia 33(2): 179–185. doi:10.2307/2806317 [Original description: type, history, habitat, measurements, comparison with Dioon imbricatum; copy of The Cycad Society.]
  • Chemnick, J. & Pérez-Farrera, M.A. (2007). Dioon merolae De Luca, Sabato & Vázquez-Torres: its history, ethnobotany and conservation. The Cycad Newsletter 30(2): 8–10. [History, names, uses, threats.]
  • Broome, T. (2007). Growing Dioon merolae in Central Florida; Calonje, M. (2007). Phenology of Dioon merolae; Frasier, R.L. (2007). My personal experience with Dioon merolae in Texas. The Cycad Newsletter 30(1): 10–11. [Cultivation, phenology, cold.]
  • 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 Botánica de México 78: 107–113. doi:10.17129/botsci.1724 [Pilgrimage and feast of the Holy Cross.]
  • Lázaro-Zermeño, J.M., González-Espinosa, M., Mendoza, A. & Martínez-Ramos, M. (2012). Historia natural de Dioon merolae (Zamiaceae) en Chiapas, México. Botanical Sciences 90(1): 73–87. doi:10.17129/botsci.387 [Habitat, growth, offsets, cones, seeds, fauna, age, harvest.]
  • González-Espinosa, M., Pérez-Espinosa, B.L. & Ramírez-Marcial, N. (2012). Distribución espacial y respuesta fototrópica de Dioon merolae (Zamiaceae) en poblaciones relictuales del centro de Chiapas, México. Botanical Sciences 90(1): 63–72. doi:10.17129/botsci.386 [Soils, exposure, leaning trunks; abstract.]
  • Hernández-Tapia, J.E., Jiménez-Ramírez, J. & Vovides, A.P. (2020). Taxonomic revision of the genus Dioon (Zamiaceae). Phytotaxa 442(4): 267–290. doi:10.11646/phytotaxa.442.4.2 [Description, habitats, names, invalid names; treatments on Plazi.]
  • Gutiérrez-Ortega, J.S., Salinas-Rodríguez, M.M., Ito, T. et al. (2020). Niche conservatism promotes speciation in cycads: the case of Dioon merolae (Zamiaceae) in Mexico. New Phytologist 227(6): 1872–1884. doi:10.1111/nph.16647 [Lineages of the former Dioon merolae; abstract.]
  • Gutiérrez-Ortega, J.S., Pérez-Farrera, M.A., Chemnick, J. & Gregory, T.J. (2021). A reassessment of Dioon merolae (Zamiaceae) leads to the description of Dioon salas-moralesae, a new cycad species from Southeastern Oaxaca, Mexico. Phytotaxa 528(2): 93–110. doi:10.11646/phytotaxa.528.2.3 [Chiapas lineage, key and Purpusii clade; abstract.]
  • Chemnick, J., Gregory, T.J. & Salas-Morales, S. (1997, issued 1998). A revision of Dioon tomasellii (Zamiaceae) from western México, a range extension of D. merolae, and clarification of D. purpusii. Phytologia 83(1): 1–6. [Genetic closeness to Dioon holmgrenii.]
  • Bonta, M., Pulido-Silva, M.T., Diego-Vargas, T., Vite-Reyes, A., Vovides, A.P. & Cibrián-Jaramillo, A. (2019). Ethnobotany of Mexican and northern Central American cycads (Zamiaceae). Journal of Ethnobiology and Ethnomedicine 15: 4. doi:10.1186/s13002-018-0282-z [Espadaña pilgrimages, the name nimalari.]
  • Skelley, P.E., Tang, W. & Pérez-Farrera, M.A. (2022). Review of Pharaxonotha Reitter (Coleoptera: Erotylidae: Pharaxonothinae) inhabiting the cycad genus Dioon Lindl. (Cycadales), with descriptions of nine new species and comments on P. kirschii Reitter. Insecta Mundi 917. doi:10.5281/zenodo.6391973 [Pharaxonotha occidentalis on Dioon merolae; treatments on Plazi.]
  • O’Brien, C.W. & Tang, W. (2015). Revision of the New World cycad weevils of the subtribe Allocorynina, with description of two new genera and three new subgenera (Coleoptera: Belidae: Oxycoryninae). Zootaxa 3970(1). doi:10.11646/zootaxa.3970.1.1 [Parallocorynus jonesi in the male cones.]
  • Donaldson, J.S. (ed.) (2003). Cycads. Status Survey and Conservation Action Plan. IUCN/SSC Cycad Specialist Group, Gland and Cambridge. [2003 status, numbers, illegal collecting, nurseries of La Sepultura.]
  • Chemnick, J. & Gregory, T. (2022). Dioon merolae. The IUCN Red List of Threatened Species 2022: e.T42146A69830866. doi:10.2305/IUCN.UK.2022-1.RLTS.T42146A69830866.en [Category VU; reference from The World List of Cycads, checked on Crossref.]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. doi:10.11646/phytotaxa.550.1.1 [Dedicatee, as given by The World List of Cycads.]
  • Calonje, M., Stevenson, D.W. & Osborne, R. (2026). The World List of Cycads, version 2026.08.15. Montgomery Botanical Center, Coral Gables. doi:10.5281/zenodo.21958524 [Reference checklist.]