There are cycads, and then there is Dioon spinulosum. At up to 16 metres tall, this is the undisputed giant of the genus Dioon — and one of the tallest cycads on Earth, rivalling only the largest Lepidozamia hopei of Australia for the title. Where most Dioon species are compact, drought-adapted plants of dry mountain slopes and canyon walls, Dioon spinulosum takes a radically different ecological path: it grows in lowland tropical evergreen rainforest on limestone hills and cliffs in Veracruz and Oaxaca, rising through the humid understory on a slender, columnar trunk that may lean and arch gracefully under its own weight over the centuries. Its female cones — approaching 80 cm long and weighing up to 15 kg or more — are among the largest reproductive structures in the cycad order. Researchers at Montgomery Botanical Center have documented the single largest and heaviest female cone of any Dioon: nearly 1 metre in length and 65 kg in weight, of which 34 kg were seeds alone. This is a species of superlatives — and yet in cultivation it is surprisingly tractable, growing faster than most other Dioon species and adapting well to warm temperate and subtropical climates far outside its tropical native range.
Quick Facts
| Scientific name | Dioon spinulosum Dyer ex Eichler |
| Family | Zamiaceae |
| Origin | Lowland southeastern Mexico — Veracruz and northern Oaxaca, at the foot of the Sierra de Oaxaca |
| Adult size | Trunk to 16 m tall (typically 3–6 m in cultivation), 30–40 cm diameter; crown spread 2–4 m |
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 |
| IUCN | Endangered (EN) — A2acd+4acd; B1ab(iii,v) |
| CITES | Appendix II (all Dioon species) |
| Cultivation difficulty | 2/5 |
Taxonomy and Nomenclature
Dioon spinulosum was described by August Wilhelm Eichler, crediting William Turner Thiselton-Dyer, in 1883 in the Gartenzeitung (Berlin) (2: 411–413, 438–439, fig. 80). The type specimen was collected by C.J. Hoge from cultivated plants at Progreso, Yucatán — not from the wild habitat. Chamberlain (1912) later noted that Dioon spinulosum “occurs only in cultivation in Progreso,” confirming that the type material was garden-grown. The holotype is at K (Kew).
The specific epithet spinulosum derives from the Latin spinulosa, meaning “spiny,” referring to the conspicuously spinose margins of the leaflets — a character that immediately distinguishes this species from its smooth-margined relative Dioon edule.
POWO lists no synonyms. Dioon spinulosum belongs to the “massive” clade of the genus, alongside Dioon mejiae (from Honduras) and Dioon rzedowskii (from the Río Santo Domingo canyon, Oaxaca). These three species are characterised by large fronds, well-developed trunks, and massive cones — in contrast to the smaller “edule clade” containing the remaining species. Phylogenetic analysis (Moretti et al., 1993) confirms that these three form a well-defined monophyletic group.
Common names: Giant Dioon, Spiny Dioon, Gum Palm (English); Palma de chicle, Palma de chicalite, Coyolillo, Chamal, Changó, Jangó, Coyolito de cerro (Spanish).
Morphological Description
Dioon spinulosum is the largest species in the genus Dioon and one of the tallest cycads in the world. It is generally single-stemmed — though older specimens may occasionally branch — with a slender, columnar trunk of irregular breadth that can lean with age. The trunk reaches up to 16 m in height and 30–40 cm in diameter, clothed in persistent petiole scars and becoming smooth and grey-brown with age. Like Dioon edule, growth rings are present in the trunk. In a garden setting, heights of 3–5 m are realistic over several decades; specimens approaching 16 m are centuries old.
Leaves: numerous, forming a large, open, spreading crown at the apex of the trunk. Individual fronds are 1.5–2 m long, arching gracefully — a character quite different from the stiff, erect fronds of Dioon edule. Leaves are flat (not keeled) in cross-section. Colour is a bright, glossy green — distinctly lighter and more vivid than the grey-green or blue-green of most other Dioon species. New leaves emerge with a bronze tint and transient tomentum. Leaflets number 140–240 per frond, are broader than those of Dioon edule (10–20 mm wide, lanceolate), and bear the eponymous character: small spines (spinules) along the margins, particularly near the tips. The petiole is smooth and leafless before the leaflets begin — another character distinguishing Dioon spinulosum from Dioon mejiae, which has spines/leaves along the entire leaf length.
Cones: the cones of Dioon spinulosum are among the largest in the cycad world. Female seed cones are ovoid, pale brown to grey, and can reach up to 80 cm long and 30 cm in diameter, weighing up to 15 kg — with exceptional specimens documented at nearly 1 m and 65 kg (Montgomery Botanical Center records). A single cone can contain up to 300 seeds. Male pollen cones are elongated, large, and woolly. The extreme size of the cones reflects the lowland tropical ecology of this species — ample resources in a high-rainfall environment allow the production of reproductive structures on a scale impossible for the drought-stressed mountain species.
Seeds: large, ovoid, with a cream or white fleshy sarcotesta. Seeds are spherical to ellipsoidal, reaching 2–4 cm in diameter. Germination is cryptocotylar.
Similar Species and Common Confusions
| Character | Dioon spinulosum | Dioon mejiae | Dioon rzedowskii | Dioon edule |
|---|---|---|---|---|
| Adult trunk height | Up to 16 m | Up to 12 m | Up to 10 m | 1–3 m (max 4 m) |
| Leaf length | 1.5–2 m | 1.5–2 m | 1–1.5 m | 0.7–1.6 m |
| Leaflet width | 10–20 mm | 10–18 mm (no prickles at maturity) | 10–15 mm | 6–10 mm |
| Leaflet prickles | Present (spinose margins) | Usually absent at maturity | Present | Absent on mature leaves |
| Petiole | Smooth, leafless | Spines/leaves along entire length | Smooth | Smooth, short |
| Leaf colour | Bright glossy green | Dull green to lime green | Green | Grey-green to blue-green |
| Cone size (female) | Up to 80 cm (max ~100 cm) | Very large, pendant | Large, pendant | 20–35 cm |
| Habitat | Lowland tropical rainforest, 30–300 m | Lowland Honduras | Río Santo Domingo canyon, Oaxaca | Dry slopes, 0–1,500 m |
| Cold hardiness | −2 to −4 °C | ~0 °C (essentially frost-free) | −2 to −4 °C | −7 to −12 °C |
| IUCN | Endangered | Endangered | Vulnerable | Near Threatened |
At the juvenile stage, Dioon spinulosum can be confused with Dioon purpusii (both have broad, prickly leaflets), but the adult plants are dramatically different: Dioon spinulosum is a lowland tropical giant with glossy bright green leaves, while Dioon purpusii is a mid-altitude dry-forest species with grey-green, lusterless foliage. From Dioon edule, the separation is straightforward: Dioon spinulosum has broader leaflets with spinose margins, larger cones, brighter green colour, and much greater ultimate size. Dioon mejiae (Honduras) is the closest relative; the key difference is the petiole — smooth and leafless in Dioon spinulosum, armed with spines along its entire length in Dioon mejiae.
Distribution and Natural Habitat
Dioon spinulosum is endemic to lowland Veracruz and northern Oaxaca, Mexico, at the foot of the Sierra de Oaxaca. It occurs in at least three main locations near the towns of Ciudad Alemán, San Juan Bautista Tuxtepec, and San José Ojitlán. The species is found at low elevations, typically 30–300 m above sea level — a strikingly different ecological niche from the mid-altitude or montane habitats of all other Dioon species treated in this series.
The habitat is lowland tropical evergreen rainforest on limestone hills and cliffs. Dioon spinulosum is typically a part of the forest understory, with only a few of the tallest individuals emerging above the canopy. The limestone substrate provides excellent drainage despite the high rainfall, and the steep slopes and cliff faces where the species predominates are often inaccessible to agriculture — which has been its principal conservation asset.
Climate in the native range:
| Parameter | Typical range |
|---|---|
| Altitude | 30–300 m (rarely to 460 m) |
| Mean annual temperature | 24–26 °C |
| Average winter minimum (coolest month) | 16–20 °C |
| Historical minimum | Estimated 5–8 °C during exceptional cold fronts; frost essentially absent |
| Summer maximum average | 32–36 °C |
| Historical maximum | ~40 °C |
| Annual rainfall | 2,000–3,500 mm (among the highest in any Dioon habitat) |
This climate profile is radically different from that of every other Dioon species covered in this silo. Dioon spinulosum is a genuine tropical rainforest cycad — warm year-round, receiving very high rainfall, and virtually never exposed to frost in the wild. This has direct implications for cultivation: it is the least cold-hardy species commonly grown in the genus, and it appreciates more moisture and humidity than any other Dioon. Yet its natural occurrence on freely draining limestone means that even in its wet habitat, root drainage remains excellent — a requirement that persists in cultivation.
Conservation
Dioon spinulosum is classified as Endangered (EN) on the IUCN Red List under criteria A2acd+4acd; B1ab(iii,v). This is one of the more threatened species in the genus, with significant historical habitat loss. The primary threats are severe habitat destruction through farming, cattle ranching, and the creation of reservoirs, plus historical over-collection for the horticultural trade. The lowland tropical forest habitat of Dioon spinulosum has been far more heavily impacted by agricultural conversion than the inaccessible canyon habitats of Dioon purpusii or Dioon califanoi.
All Dioon species are listed on CITES Appendix II. Despite the Endangered status, Dioon spinulosum is one of the more common Dioon species in cultivation — possibly the most common in tropical regions worldwide, where it is widely planted as an ornamental and landscape specimen.
For buyers: nursery-propagated plants are widely available from specialist and general nurseries. This is one of the most accessible Endangered cycads for the average gardener. Ensure any international purchase includes CITES documentation.
Cultivation
| Hardiness | −2 to −4 °C (28 to 25 °F) / USDA zones 9b–11 |
| Light | Partial shade to full sun (partial shade preferred, especially in hot climates) |
| Soil | Well-drained, rich — tolerates limestone; benefits from organic matter |
| Watering | Moderate to regular — more moisture than other Dioon; less water is still better than waterlogging |
| Adult size | Trunk to 3–5 m in cultivation (16 m in wild over centuries) × crown 2–4 m |
| Growth rate | Moderate to fast for a cycad — one of the faster Dioon species |
| Difficulty | 2/5 |
Dioon spinulosum is surprisingly easy to grow despite its tropical origins. It is widely regarded as one of the most adaptable large cycads for warm climates, performing well across southern Florida, southern California, coastal Australia, and comparable subtropical to warm-temperate regions worldwide. In southern Florida, it is rated for USDA zones 9b–11 and is described by the University of Florida IFAS Extension as “a suitable alternative for the queen sago.”
Light: unlike the sun-demanding mountain Dioon species, Dioon spinulosum grows naturally in the forest understory and prefers partial shade, especially in hot, dry climates. Full sun is tolerated — and can produce more compact growth — but in arid inland locations, afternoon shade prevents leaf scorch. This shade tolerance makes Dioon spinulosum particularly valuable as an understory planting beneath large palms or trees, a niche where Dioon edule also performs but where Dioon spinulosum provides superior scale and a more tropical aesthetic.
Soil and drainage: well-drained soil is still essential — despite the high rainfall in the native habitat, the limestone substrate ensures perfect drainage. In cultivation, a rich, well-drained mix is ideal: garden soil amended with coarse grit and quality compost. Dioon spinulosum tolerates richer soil than the mountain species and benefits from organic matter, but waterlogging remains the universal cycad killer.
Watering: Dioon spinulosum appreciates more regular watering than other Dioon species — consistent with its high-rainfall native habitat. During the growing season, keep the soil evenly moist (not waterlogged). Reduce watering in winter, especially in cooler climates where root rot risk increases. The mantra “less water is better” still applies relative to most tropical plants, but this species needs more moisture than Dioon edule, Dioon califanoi, or Dioon purpusii.
Cold hardiness: this is the principal limitation of Dioon spinulosum in cultivation. As a lowland tropical species, it is the least frost-tolerant of the commonly cultivated Dioon species. Published sources generally cite USDA zones 9b–11, meaning it tolerates only brief frosts to approximately −2 to −4 °C. Home growers on forums (Houzz, Agaveville) report conflicting experiences, with some claiming survival down to 20 °F (−7 °C) for brief periods in dry soil, but this is at the absolute edge and involves significant leaf damage. In USDA zone 9a, outdoor cultivation is risky without winter protection. For gardeners in zones 8b–9a, Dioon edule or Dioon angustifolium are far safer choices.
Growth rate: one of the faster-growing Dioon species — significantly faster than Dioon edule in warm, moist conditions. Llifle notes that “a plant with 30 cm of stem can be quite old (10–20 or more years)” for Dioon spinulosum, compared to 20–40+ years for Dioon edule with the same trunk height. In the tropics, growth is appreciably faster still.
Container culture: Dioon spinulosum makes an impressive container specimen, especially for gardeners who need to move it indoors during winter. It tolerates pot culture well, though its ultimately large size means that very large containers are needed for mature plants. Use a well-drained but moisture-retentive mix. Overwinter in a bright, warm location (minimum 10 °C is ideal).
Fertilization: responds well to regular feeding — more so than the mountain Dioon species. A balanced slow-release fertilizer with trace elements during the growing season produces faster growth and darker, more vigorous foliage.
Buying Advice
Availability: Dioon spinulosum is the second most widely available Dioon in the trade after Dioon edule, and is probably the most common in tropical regions. Seeds and seedlings are readily available from specialist and general nurseries at moderate prices. Larger specimens are increasingly available from landscape nurseries in Florida, California, and Australia.
What to expect: young plants have stiff, upright leaves that become progressively more arching as the plant matures. The glossy bright green colour and spinose leaflet margins are visible even on seedlings, making identification straightforward.
Do not confuse with Dioon edule: Dioon spinulosum has broader leaflets, spinose margins, brighter green (not grey-green or blue-green) foliage, and is significantly less cold-hardy. These are not interchangeable species for garden planning.
Propagation
Seed: Dioon spinulosum is among the easiest cycad species to germinate from seed. Remove the sarcotesta (use gloves), soak cleaned seeds 24–48 hours, then sow horizontally, half-buried in a well-draining mix. Maintain 25–30 °C. Keep medium consistently moist (slightly more moisture than for mountain species). Germination typically occurs in 2–6 months. Growth from seed is faster than for Dioon edule.
Offsets: mature plants produce basal offsets, which can be separated and potted independently once they have developed their own root system.
Pests and Diseases
Cycad aulacaspis scale (Aulacaspis yasumatsui): the most significant pest. Dioon spinulosum is moderately susceptible. Infestations have been documented both in cultivation and in the wild. Regular inspection and early treatment with horticultural oil are essential.
Root rot: caused by waterlogging, especially in cooler temperatures. Ensure excellent drainage despite the species’ preference for more moisture than other Dioon species.
Mealybugs and scale: occasional in cultivation. Standard treatments apply.
Landscape Use
Dioon spinulosum is the Dioon of choice for creating a truly tropical landscape effect. Its large, gracefully arching fronds, bright green colour, and impressive eventual stature make it a superb specimen for subtropical and tropical gardens. It works beautifully as a focal point, as an understory planting beneath large palms or Ficus trees, in poolside plantings, and in large public garden displays. In southern Florida, it is used as a landscape alternative to the queen sago (Cycas rumphii). Its spiny leaflets should be considered when siting near pathways. For xeric or Mediterranean gardens in cooler climates, Dioon edule is a more appropriate choice; Dioon spinulosum shines where warmth and moisture are abundant.
Frequently Asked Questions
How tall does Dioon spinulosum get?
In the wild, up to 16 m — one of the tallest cycads on Earth. In cultivation, 3–5 m is realistic over several decades. The tallest specimens in the wild represent many centuries of growth.
Is Dioon spinulosum cold-hardy?
Less so than most other Dioon species. It tolerates brief frosts to approximately −2 to −4 °C, but it is a lowland tropical species and should be considered frost-sensitive compared to Dioon edule (−7 to −12 °C) or Dioon angustifolium (−10 to −14 °C). USDA zones 9b–11 for year-round outdoor cultivation.
Is Dioon spinulosum faster-growing than Dioon edule?
Yes, significantly. In warm, moist conditions, Dioon spinulosum produces more leaves per year and develops trunk height faster than Dioon edule. It is one of the faster-growing species in the genus.
Can Dioon spinulosum grow in shade?
Yes — and it often prefers it. Unlike the sun-demanding mountain Dioon species, Dioon spinulosum grows naturally in the forest understory and tolerates partial shade well. Full sun is acceptable in cooler climates but can cause leaf scorch in hot, dry conditions.
Is Dioon spinulosum toxic?
Yes. Like all cycads, all parts contain cycasin and other toxic compounds. 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:80656-2
The accepted nomenclatural record: native range confirmed as Mexico (Oaxaca, Veracruz), seasonally dry tropical biome. No synonyms listed.
World List of Cycads — cycadlist.org
https://cycadlist.org/scientific_name/262
Detailed nomenclatural record: type specimen (C.J. Hoge s.n., Progreso, Yucatán — from cultivation), etymology (“spiny”), and IUCN status Endangered A2acd+4acd; B1ab(iii,v).
IUCN Red List — Dioon spinulosum
https://www.iucnredlist.org/species/42135/10663756
The full Endangered assessment with population data, habitat description, and threat analysis. Notes severe habitat destruction from farming, ranching, and reservoir creation.
iNaturalist
https://www.inaturalist.org/taxa/135615-Dioon-spinulosum
Citizen science observations with photographs from the Veracruz/Oaxaca habitat and cultivated specimens worldwide.
Tropicos — Missouri Botanical Garden
https://www.tropicos.org/name/32600027
Nomenclatural data and specimen records.
Chicago Botanic Garden — Dioon spinulosum
https://www.chicagobotanic.org/plant-collections/plant-finder/dioon-spinulosum-giant-dioon
Concise profile from a major US public garden: notes on height, leaf morphology, and cultivation.
Gardenia.net — Dioon spinulosum
https://www.gardenia.net/plant/dioon-spinulosum
Detailed cultivation factsheet including RHS Award of Garden Merit information, USDA zones, and landscape use recommendations.
Dave’s Garden — Dioons: The Hardy Mexican Cycads
https://davesgarden.com/guides/articles/view/1981/
Grower’s guide by Geoff Stein. Compares Dioon spinulosum with Dioon mejiae, noting the diagnostic petiole character and the fact that Dioon spinulosum “could be more common [than Dioon edule] in the tropics.”
University of Florida IFAS — Dioon
https://gardeningsolutions.ifas.ufl.edu/plants/trees-and-shrubs/palms-and-cycads/dioon/
Extension factsheet for Florida gardeners. Lists Dioon spinulosum as suitable for USDA zones 9b–11, best in partial to full shade, and “a suitable alternative for the queen sago.”
Llifle — Encyclopedia of Living Forms
https://www.llifle.com/Encyclopedia/PALMS_AND_CYCADS/Family/Zamiaceae/24411/Dioon_spinulosum
Comprehensive species page with morphological data, habitat notes, and cultivation guide. States trunk can reach “up to 16 m in height” in habitat and “rarely exceeds 3.5 m” in garden conditions.
Bibliography
Chamberlain, C. J. (1912). Morphology of Ceratozamia. Botanical Gazette, 53(1), 1–19.
Eichler, A. W. (1883). Dioon spinulosum Dyer. Gartenzeitung (Berlin), 2, 411–413, 438–439.
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.
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.