Cycas tropophylla

Cycas tropophylla is a cycad of north-eastern Vietnam, endemic to the limestone karst of Ha Long Bay and Cát Bà Island, where it grows on bare limestone walls, sometimes on cliffs rising straight out of the sea. This species of the genus Cycas, described in 2004 by Ken Hill and Phan Kế Lộc, takes its name from its distinctly keeled leaves, a feature unique among the cycads of northern Vietnam. The IUCN lists it as Vulnerable.

Key facts about Cycas tropophylla

  • Accepted name: Cycas tropophylla K.D.Hill & P.K.Lôc (2004), accepted by The World List of Cycads (version 2026.08.15) and by GBIF; POWO could not be consulted
  • Former names and synonyms: none
  • Family: Cycadaceae, order Cycadales; section Stangerioides according to Hill (2004), a source not consulted
  • Common names: none attested in the sources consulted
  • Natural range: north-eastern Vietnam, Quảng Ninh (Ha Long Bay) and Hải Phòng (Cát Bà Island) provinces
  • IUCN status: Vulnerable (VU A2c+4c; B1ab(iii,v)), assessment by J.D. Bösenberg and T.H. Nguyen dated 2021 and published in Red List version 2023-1
  • CITES and other protection: CITES Appendix II (seeds exempt under annotation #4); Annex B in the European Union
  • Documented hardiness: no data (see Cold hardiness)
  • Light and soil: bare, steep limestone walls with no visible soil around the roots (Hill 2008)
  • Habit and adult size: a fairly compact crown according to a field account (2010); the measurements in the original description could not be consulted
  • In the UK, US and Australia: Montgomery Botanical Center (Florida), four accessions from 1994 and 2010; Royal Botanic Garden Sydney (online catalogue); the Huntington, the New York Botanical Garden and Cairns Botanic Gardens declared to BGCI
  • Toxicity: no data specific to the species; treat it as poisonous, like all Cycas (see Toxicity and precautions)

How to recognise Cycas tropophylla

By its keeled leaves. The name says it: according to The World List of Cycads, which follows the etymological compendium of Haynes (2022), its distinctly keeled leaves, raised like the keel of a boat on either side of the rachis, are unique among the cycads of northern Vietnam. To this is added a very particular habitat: the bare limestone walls of the karst of Ha Long and Cát Bà.

The full description, with the measurements of the trunk, leaves, cones and seeds, is in the original description (Hill, Nguyen and Loc 2004, pp. 168–170, fig. 10), which could not be consulted: as a precaution, no measurement is given here. A French collector who visited Ha Long Bay in the summer of 2010 describes on the Fous de palmiers forum plants with a fairly compact crown, and female plants bearing cones, all out of reach at least 5 to 6 m above the water.

Hybrids

No natural hybrid has been documented. In the early 2010s, however, seed of an artificial hybrid sold as (Cycas revoluta × Cycas micholitzii) × Cycas tropophylla was circulating among European collectors. A grower living between Belley (Ain) and Montpellier (Hérault), in France, reports on the Fous de palmiers forum that it germinated in January 2010; by 2012 his plants had grown very little and one had rotted before recovering, and in 2013 he partly blamed this “very, very slow” growth on his first compost, based on perlite and very calcareous sand, results being much better in another mix. The identity of this hybrid rests on the seller’s word: these observations do not concern the pure species.

Possible confusion

  • Cycas sexseminifera, from southern China and northern Vietnam, and Cycas ferruginea, from Guangxi and Vietnam, also grow on bare limestone walls with no visible soil around the roots (Hill 2008), but lack the keeled leaves of Cycas tropophylla.
  • The other cycads of northern Vietnam, such as Cycas dolichophylla, do not have such clearly keeled leaves either: according to the etymology given by The World List of Cycads, this character is unique in the region.

Outside Ha Long Bay and Cát Bà, the origin of a plant sold under this name should always be checked. In December 2012, a participant on the Fous de palmiers forum was already asking whether a plant offered under this name by a German online seller really was Cycas tropophylla.

Taxonomy

Cycas tropophylla is accepted by The World List of Cycads; POWO could not be consulted. The species was described in 2004 by Ken Hill and Phan Kế Lộc in the account of the genus in Vietnam published by Hill, Nguyen and Loc (The Botanical Review 70(2): 168–170, fig. 10). The World List of Cycads abbreviates the authors as “K.D.Hill & P.K.Lôc”, IPNI as “K.D.Hill & L.K.Phan”: both refer to the same person, Phan Kế Lộc, whose Vietnamese name is abbreviated in two ways. Species+, the CITES database, gives “K.D.Hill & S.L.Yang” in error.

Hill (2008), in his account of the genus in China, cites the species alongside Cycas sexseminifera and Cycas ferruginea for their shared preference for limestone walls. Its placement in section Stangerioides, the section of those two species, is attributed to an analysis by Hill (2004) that could not be consulted.

Etymology and history

The epithet comes from the Greek trŏpis (“boat keel”) and phyllon (“leaf”), in reference to the keeled leaves (Haynes 2022, quoted by The World List of Cycads). It therefore has nothing to do with the botanical term “tropophyte”, used for plants with seasonal foliage.

The type was collected on 10 March 1999 by Ken Hill, Phan Kế Lộc and N.V. Dzu on an island in Ha Long Bay (Hill, Loc & Dzu 5020; holotype in Hanoi, isotypes in Sydney, at Kew and in New York). In 2005 the journal of the Cycad Society of South Africa, Encephalartos, ran a feature on the species (Osborne 2005), which could not be consulted.

Common names

No common name is attested for this species in the sources consulted, in English, Vietnamese or any other language. “Ha Long Bay cycad” is a description, not a recorded name.

In the wild

Cycas tropophylla lives in north-eastern Vietnam, in Quảng Ninh and Hải Phòng provinces, that is in Ha Long Bay and on Cát Bà Island (The World List of Cycads). It grows on bare limestone cliffs with no visible soil around the roots: Hill (2008) describes this habitat for Cycas ferruginea, which shares the preference with Cycas sexseminifera and Cycas tropophylla. The French collector who visited the bay in 2010 confirms plants clinging to vertical cliffs, where other plants cannot establish, and very hard to approach.

Pollination, phenology and the exact climate of its range are not documented in the sources consulted.

Conservation status and regulation

The IUCN lists Cycas tropophylla as Vulnerable (VU) under criteria A2c+4c and B1ab(iii,v): a past and projected population decline linked to habitat loss, and a restricted range in continuing decline. The World List of Cycads cites the 2010 assessment by H.T. Nguyen; according to Crossref, the most recent assessment is by J.D. Bösenberg and T.H. Nguyen, dated 2021 and published in Red List version 2023-1, and the Red List data relayed by GBIF confirm the VU category (consulted on 24 October 2026). The Red List itself could not be consulted, and the category of the 2010 assessment could not be checked. In 2010 the French collector mentioned above thought the species little threatened, given how inaccessible the plants are and the inscription of Ha Long Bay on UNESCO’s World Heritage List: a personal impression that the IUCN assessment does not support.

Like every species in the genus except Cycas beddomei, Cycas tropophylla is listed in CITES Appendix II, where the family Cycadaceae has appeared since 4 February 1977. The listing carries annotation #4 (the version in force since 5 March 2026): seeds, pollen, seedlings and tissue cultures obtained in vitro in sterile containers are exempt, while international trade in plants requires an export permit. In the European Union the species falls under Annex B of Regulation (EC) No 338/97, in the version set by Regulation (EU) 2026/1383, applicable since 29 June 2026. Species+ records the species in Vietnam and confirms this status (consulted on 24 October 2026).

This summary does not replace the official texts: check the regulations in force before any purchase or exchange, ask the seller for proof of origin and choose nursery-raised plants grown from seed.

Growing Cycas tropophylla

No cultivation guidance specific to Cycas tropophylla has been published, and the forums consulted report no result for the pure species. Its natural setting, bare limestone walls where water never lies, suggests a very free-draining mineral compost, moderate watering and no standing water: these are precautions inferred from the habitat, not tested results. The only report available, on the hybrid described above, notes very slow growth in a mix of perlite and calcareous sand.

Outdoor and in-ground cultivation

Only warm, frost-free or nearly frost-free climates can be considered for planting out, ideally on a slope or among rocks where water drains away at once. No planted specimen with a documented history has been found; Montgomery Botanical Center, in southern Florida, holds the species, but its catalogue does not say whether its plants are in the ground.

Container cultivation

In cooler climates, grow it in a pot, wintered in a bright, heated shelter, in a gritty mineral compost and with sparing water in cool weather. In the absence of data, keep it under cover as soon as frost threatens.

In cultivation in the UK, US and Australia

In short: present in a handful of American and Australian collections; no British record.

  • United States: Montgomery Botanical Center, in Coral Gables (Florida), lists four accessions in its online catalogue, numbered 941417, 941419 and 941420, dated to 1994 by their numbers, and 20100508 (2010) (consulted on 24 October 2026). The Huntington (San Marino, California) and the New York Botanical Garden, where it would be grown under glass, also declare it to BGCI PlantSearch.
  • Australia: the Royal Botanic Garden Sydney lists it in its online catalogue (inventory of 19 October 2026) and declares it to BGCI; the Cairns Botanic Gardens also declare it.
  • United Kingdom: no record in BGCI or in the British catalogues checked in October 2026. The British nursery Trebrown lists the species with a USDA zone 9b rating, its own commercial classification, not a documented trial.

A BGCI declaration shows that a garden held the plant at some point, not that it is alive today. Elsewhere, BGCI lists the Marimurtra botanic garden at Blanes, in Spain. No report from a private grower in these three countries was found.

Propagation

The plant is dioecious: a male and a female are needed to obtain seed. No germination data specific to the species has been published; Cycas seed is usually sown fresh, cleaned of its sarcotesta, in a free-draining mix and in warmth. Seeds are exempt from CITES under annotation #4: ask for seed from cultivated plants.

Pests and diseases

No pest or disease specific to the species has been documented. In cultivation, Cycas are mainly exposed to the Asian cycad scale, Aulacaspis yasumatsui, and to root rot when overwatered: these are risks documented for the genus, not for this species in particular.

Cold hardiness

There is no data on the cold tolerance of Cycas tropophylla: no source consulted describes how it behaves below freezing, and nothing supports the idea that it can withstand frost.

Mediterranean and European experience

No cultivation result has been published from southern France, outdoors or in a pot, and no dated cold event concerns the species. In December 2012, a grower on the Côte Bleue (Bouches-du-Rhône, France) asked on the Fous de palmiers forum whether the species could withstand frost; no data was offered in reply. The Jardin zoologique tropical at La Londe-les-Maures, in the Var, has published no observation of this species, and it does not appear in the published inventories of the Italian botanic gardens consulted.

United Kingdom, United States and Australia

No cold value and no dated cultivation report was found, from forums or from botanic gardens. Trebrown’s zone 9b rating is a commercial classification, not a measured result.

Toxicity and precautions

No toxicity analysis specific to Cycas tropophylla has been found, but the risk is well documented in the genus: Cycas tissues, and the seeds above all, contain cycasin, which is converted during digestion into methylazoxymethanol, toxic to the liver and nervous system, along with a neurotoxic amino acid, BMAA. The ASPCA lists the sago palm (Cycas revoluta) as poisonous to dogs, cats and horses, with vomiting, diarrhoea and liver damage that can be fatal. In the absence of evidence to the contrary, treat the plant as poisonous: keep seeds and leaves out of reach of children and animals and wear gloves to clean seeds. If any part is swallowed, contact a poison centre, a doctor or a vet straight away.

Traditional uses

No use of this species is documented in the sources consulted.

Frequently asked questions

Where does Cycas tropophylla grow?

Only in north-eastern Vietnam, on the limestone walls of Ha Long Bay and Cát Bà Island.

What does tropophylla mean?

“Keel leaf”, from Greek: the leaves are keeled. The name has nothing to do with the seasonal foliage of so-called tropophytes.

How can I recognise it?

By its keeled leaves, unique among the cycads of northern Vietnam, and by its habitat on bare limestone walls.

Which species are closest to it?

Cycas sexseminifera and Cycas ferruginea, which share the same rocky limestone habitat.

Is it endangered?

Yes. The IUCN lists it as Vulnerable, because of habitat decline and its restricted range.

Is it hardy in the UK?

No one knows, and it is unlikely: there is no cold data at all. Grow it in a pot and keep it under glass in winter.

Is there a hybrid with Cycas revoluta?

Yes, at least in the trade: a hybrid sold as (Cycas revoluta × Cycas micholitzii) × Cycas tropophylla was raised by French collectors around 2010, with very slow growth.

Is it poisonous?

It should be assumed so. No analysis exists for this species, but all the Cycas studied contain dangerous toxins, especially in their seeds.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • Hill, K.D., Nguyen, H.T. & Loc, P.K. (2004). The genus Cycas (Cycadaceae) in Vietnam. The Botanical Review 70(2): 134–193. [Original description of Cycas tropophylla, pp. 168–170, fig. 10; reference checked on The World List of Cycads, text not consulted (paywalled).]
  • Hill, K.D. (2008). The genus Cycas (Cycadaceae) in China. Telopea 12(1): 71–118. doi:10.7751/telopea20085804 [Habitat shared with Cycas sexseminifera and Cycas ferruginea; full text consulted.]
  • Hill, K.D. (2004). Character evolution, species recognition and classification concepts in the Cycadaceae. In: Walters, T. & Osborne, R. (eds), Cycad Classification: Concepts and Recommendations: 23–44. CABI Publishing, Wallingford. [Classification into sections; not consulted.]
  • Osborne, R. (2005). Focus on Cycas tropophylla. Encephalartos 82: 8–13. [Feature article; not consulted, cited from The World List of Cycads.]
  • Nguyen, H.T. (2010). Cycas tropophylla. The IUCN Red List of Threatened Species 2010: e.T42087A10626392. doi:10.2305/IUCN.UK.2010-3.RLTS.T42087A10626392.en [Assessment cited by The World List of Cycads; page not consulted, category not checked.]
  • Bösenberg, J.D. & Nguyen, T.H. (2023). Cycas tropophylla. The IUCN Red List of Threatened Species 2023-1: e.T42087A69827956. doi:10.2305/IUCN.UK.2023-1.RLTS.T42087A69827956.en [Assessment dated 11 November 2021; reference checked on Crossref, VU category relayed by GBIF; Red List page not consulted (access blocked).]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. doi:10.11646/phytotaxa.550.1.1 [Source of the etymology given by The World List of Cycads; not consulted directly.]
  • 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; consulted directly.]