Cycas bougainvilleana

Cycas bougainvilleana is a tall cycad of the islands north-east of New Guinea: Bougainville, at the heart of its range, New Britain and the Solomon Islands, where it grows close to the shore. Described by Ken Hill in 1994, this species of the genus Cycas belongs to the Cycas rumphii group, whose seeds float and can be carried by the sea. The IUCN lists it as Near Threatened (NT). It is one of the less documented species of the genus: its original description could not be consulted, and this page relies mainly on the type specimen and other herbarium records.

Key facts about Cycas bougainvilleana

  • Accepted name: Cycas bougainvilleana K.D.Hill (1994), accepted by The World List of Cycads (version 2026.08.15); POWO could not be consulted directly
  • Former names and synonyms: no synonym of its own; included in Cycas rumphii by de Laubenfels and Adema (1998)
  • Family: Cycadaceae, order Cycadales
  • Common names: none in English; the label of the type specimen records the local name “Diduna” (see Common names)
  • Natural range: Papua New Guinea (Bougainville, New Britain) and the Solomon Islands
  • IUCN status: Near Threatened (NT); assessed by Ken Hill (2010, criterion A4c), reassessed by John Donald Bösenberg in Red List version 2023-1, NT according to GBIF
  • 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: sea shores and the edge of coastal rainforest, according to herbarium labels; soil not documented at the source
  • Habit and adult size: trunk sometimes forked, 4.5 to 7.6 m tall on the herbarium labels; leaves 1.8 to 2.7 m long in the wild
  • In the UK, US and Australia: listed by Kew, Montgomery Botanical Center and Cairns Botanic Gardens; herbarium vouchers from plants in the Royal Botanic Garden Sydney (2016)
  • Toxicity: no data specific to the species; treat it as poisonous, like all Cycas (see Toxicity and precautions)

How to recognise Cycas bougainvilleana

The most reliable character is shared by the whole subsection Rumphiae: beneath the hard shell, the seed contains a layer of spongy tissue that makes it float (Lindström et al. 2009). The original description by Ken Hill (Australian Systematic Botany, 1994) could not be consulted, as only its abstract is freely available; the details below come from the labels of herbarium specimens, starting with the type, as published through GBIF:

  • Trunk: up to 7.6 m (25 ft) on the type, forked at 4.5 m with two crowns, about 20 cm across at breast height; a plant on Bougainville was recorded at about 4.5 m with a trunk 46 cm (18 in) thick, branched and covered with the remains of old leaf bases.
  • Leaves: up to 2.7 m (9 ft) long on the type, 1.8 to 2.4 m on another collection; dark green and glossy above, paler beneath; petiole unarmed at the base, then with a few spines.
  • Male cones: one collection from New Britain records a cone about 46 cm long and 20 cm across (18 by 8 in).
  • Megasporophylls (the fertile leaves of female plants): 6 to 9 ovules, of which only one to three, rarely four, are fertilised; blade triangular at the tip, with a fringed margin and a long point.
  • Seeds: green when young, red-brown to olive-brown with a leathery skin when ripe; a spongy inner layer, as in the whole subsection Rumphiae; they take two years to ripen, a feature confirmed in this species (Lindström et al. 2008, 2009).

The figures given on some websites (trunk to 5 m, 130 to 260 leaflets 25 to 34 cm long) are attributed to Ken Hill but could not be checked against his text, so they are not used here.

Hybrids

No garden hybrid is documented. In the wild, Lindström and colleagues (2009) suggest that genetic input from Cycas bougainvilleana may explain why, in the east of its range, the upper tissue of the leaflets of Cycas rumphii is sometimes interrupted by the midrib, a feature normally continuous across it. More generally, they see dispersal by floating seeds, with successive colonisations of the same islands, as the source of the strong local variability of the whole group.

Possible confusion

  • Cycas rumphii, from the Moluccas and northern New Guinea, in which de Laubenfels and Adema included it: the same floating seed, but in Cycas rumphii the megasporophylls lack distinct lateral teeth and the seed carries an apical crest (Lindström et al. 2009);
  • Cycas seemannii, from the south-west Pacific (Fiji, Tonga, Vanuatu, New Caledonia), another member of the subsection whose seeds also take two years to ripen;
  • Cycas micronesica, from western Micronesia, described in the same 1994 paper, and the other species of the group, which are told apart chiefly by their geographical origin and the details of their sporophylls. Hill’s key could not be consulted.

Taxonomy

Cycas bougainvilleana is accepted by The World List of Cycads; POWO could not be consulted directly. Ken Hill described it in 1994 in Australian Systematic Botany (volume 7, pages 557 to 560), in his account of the Cycas rumphii complex in New Guinea and the western Pacific. According to its abstract, that paper created subsection Rumphiae, recognised six species in the region, three of them new (Cycas bougainvilleana, Cycas apoa and Cycas micronesica), and discussed the misapplication of the name Cycas circinalis in the western Pacific.

Subsection Rumphiae is defined by the spongy layer of the seed, which keeps it afloat and favours dispersal by sea. A two-year seed maturation period may be a second shared character; it has been confirmed only in Cycas seemannii, Cycas thouarsii and Cycas bougainvilleana (Lindström et al. 2008, 2009). De Laubenfels and Adema (Blumea, 1998), who took a very broad view of species, placed it under Cycas rumphii; The World List of Cycads does not follow them.

Etymology and history

The epithet refers to the island of Bougainville, in Papua New Guinea, the centre of the species’ range (Haynes 2022, taken up by The World List of Cycads).

The type was collected on 26 June 1964 by C. D. Sayers (NGF 19739) at Tonolei Harbour, in the Buin sub-district of southern Bougainville; the holotype is at the National Herbarium of New South Wales in Sydney (NSW), with isotypes at Brisbane, Canberra, Lae, Kew, Leiden and Honolulu (Bishop Museum). The label describes a scattered tree at the edge of rainforest on the sea shore. Older herbarium collections, now identified under this name, go back to 1930 at Kieta, on Bougainville, and to 1932 on Makira (San Cristobal), in the Solomon Islands.

Common names

No English common name is attested. The label of the type specimen, collected in southern Bougainville in 1964, records the local name “Diduna”, without naming the language. No other vernacular name was found in the sources consulted.

In the wild

The World List of Cycads places Cycas bougainvilleana in Papua New Guinea, on Bougainville and New Britain, and in the Solomon Islands. Herbarium labels describe coastal plants: at the edge of rainforest on the sea shore (the type), in beach strand-line vegetation near Rabaul, on New Britain (1964 and 1968), and in beach forest on Ambitle Island, in the Feni group (2003). A 1968 collector noted that it was frequent on headlands along the coast of Bougainville, while adding that the plant he sampled was probably planted. Other labels mention rainforest, on Bougainville in 1930 and on Makira in 1932.

GBIF also carries the name for specimens from other islands of the Bismarck Archipelago and from the Milne Bay islands; these identifications have not been checked and lie outside the range given by The World List of Cycads, so they are not used here. The habitat on coral-sand dunes described on some websites could not be checked against Ken Hill’s text.

Its floating seeds take two years to ripen, whereas the other Cycas studied ripen their seeds in less than a year (Lindström et al. 2008, 2009). The pollinator is not documented.

Conservation status and regulation

Cycas bougainvilleana is listed as Near Threatened (NT) by the IUCN. The World List of Cycads gives this category with criterion A4c (a decline in numbers linked to the loss of habitat, over a period spanning past and future), citing the assessment by Ken Hill (2010). A reassessment by John Donald Bösenberg was published in Red List version 2023-1 (reference checked on Crossref), and the IUCN data relayed by GBIF give the NT category (consulted on 21 October 2026). The Red List itself could not be consulted directly.

Like every species in the genus except Cycas beddomei, Cycas bougainvilleana is listed in CITES Appendix II, where the family Cycadaceae has been included since 4 February 1977. The listing carries annotation #4 (CoP20 version, in force since 5 March 2026): seeds, pollen, seedlings or tissue cultures obtained in vitro and transported in sterile containers, and cut flowers of artificially propagated plants, are not subject to CITES, whereas international trade in the plants themselves requires an export permit. Australia, the United Kingdom and the United States apply CITES through their own national legislation; in the European Union the species is in Annex B of Regulation (EC) No 338/97, whose annexes in force are set by Commission Regulation (EU) 2026/1383, applicable since 29 June 2026. Species+, the official CITES database, names the species and confirms this status. This general information does not replace the official texts: check the rules in force before buying or exchanging plants, ask the seller for proof of provenance and prefer plants raised from seed in cultivation.

Growing Cycas bougainvilleana

In short: a large tropical cycad of sea shores, grown in a few botanic gardens in tropical and warm climates. No growing guide from an institution has been found, and no trial has been published in a Mediterranean or temperate climate.

Outdoor and in-ground cultivation

The species is grown at the George Brown Darwin Botanic Gardens, in tropical northern Australia, where a member of the French forum Fous de palmiers photographed it in September 2016. In Sydney, herbarium vouchers made in 2016 come from plants in the beds of the Royal Botanic Garden, raised from material collected by Ken Hill on Guadalcanal, in the Solomon Islands, in May 1993; their labels describe positions under trees or at the top of a slope, in full sun or part shade, which points to outdoor planting. After some twenty-three years, these plants had trunks of only 25 to 60 cm, with leaves of 0.8 to 1.6 m, far smaller than plants in the wild. By simple deduction, its seashore habitat suggests a plant for frost-free, humid tropical or subtropical gardens, in a well-drained soil; claims on amateur websites that it tolerates salt spray could not be confirmed.

Container cultivation

In the United Kingdom and other temperate regions, it would be a plant for a heated glasshouse, given its equatorial origin and adult size. No account of pot culture has been found. The Jardin zoologique tropical at La Londe-les-Maures, in the south of France, has published no observation on Cycas bougainvilleana.

In cultivation in the UK, US and Australia

In short: a few botanic garden records, none with a growing report.

  • United Kingdom: the Royal Botanic Gardens, Kew, declares the species in BGCI PlantSearch (consulted in October 2026); where it is grown at Kew is not stated.
  • United States: the online catalogue of Montgomery Botanical Center (Coral Gables, Florida) lists three accessions of Cycas bougainvilleana, whose short numbers do not allow the year of entry to be read with certainty; the garden also declares it in BGCI.
  • Australia: Cairns Botanic Gardens, in tropical Queensland, declares it in BGCI; at the Royal Botanic Garden Sydney, herbarium vouchers were taken in 2016 from plants of wild origin (Solomon Islands, 1993), but the species does not appear in the garden’s online catalogue as consulted in October 2026.

Outside these three countries, herbarium sheets made in 2011 document plants grown at Nong Nooch Tropical Botanical Garden, in Thailand. No first-hand report from a private grower in the UK, US or Australia could be read; PalmTalk and Dave’s Garden could not be consulted.

Propagation

Like all Cycas, Cycas bougainvilleana is dioecious: a male and a female plant are needed for fertile seed, which takes two years to ripen on the plant. No germination data specific to the species have been published. 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 is documented. Scale insects, including Aulacaspis yasumatsui, threaten all cultivated Cycas, but no attack has been reported on this species in particular.

Cold hardiness

No cold-tolerance data exist for Cycas bougainvilleana. Its range is entirely equatorial; nothing suggests that it can take frost.

European experience

No cultivation result has been published in Europe, in the open ground or in a pot, and no dated frost episode concerns the species. A search of the French forum Fous de palmiers and the Italian forum Tropicamente found no cold report for this species.

United Kingdom, United States and Australia

No cold reading and no growing report were found, either on forums or from botanic gardens. The plants recorded in Sydney show only that the species survives in the beds of a warm-temperate garden; the labels say nothing about winter temperatures or damage. Treat it as frost-tender; no USDA zone can be given without a documented trial.

Toxicity and precautions

No toxicity analysis specific to Cycas bougainvilleana has been found. The risk is, however, 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. Until proven otherwise, treat the plant as poisonous: keep seeds and leaves out of reach of children and pets, and wear gloves to clean the seeds. If any part is swallowed, contact a poison centre, a doctor or a vet straight away.

Traditional uses

Very little is documented. A herbarium label from 1954, collected near Cape Hoskins on New Britain, notes that scrapings from the seed (“nut”) were applied to sores. This is a single collector’s note, not a documented remedy, and no other use was found in the sources consulted.

Frequently asked questions

Where does Cycas bougainvilleana grow?

On the islands north-east of New Guinea: Bougainville, New Britain and the Solomon Islands, close to the coast.

Why do its seeds float?

Because they contain a layer of spongy tissue beneath the hard shell, a feature of the Cycas rumphii group; this allows them to be carried by the sea.

Is it a form of Cycas rumphii?

No, according to current checklists. A 1998 revision placed it under Cycas rumphii, but The World List of Cycads keeps it as a distinct species.

How long do its seeds take to ripen?

Two years, a period confirmed in this species; most other Cycas ripen their seeds in less than a year.

How tall does it grow?

Herbarium labels record plants 4.5 to 7.6 m tall in the wild, sometimes forked with two crowns. Cultivated plants in Sydney remained much smaller after more than twenty years.

Is it endangered?

It is Near Threatened (NT) according to the IUCN, because of the loss of its habitat.

Can it be grown outdoors in the UK?

No, almost certainly not. No cold data exist and the species is equatorial; it needs a heated glasshouse.

Is it poisonous?

It must be assumed so. No toxicity analysis specific to the species exists, but the Cycas studied contain dangerous toxins, especially in the seeds.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • Hill, K.D. (1994). The Cycas rumphii complex (Cycadaceae) in New Guinea and the western Pacific. Australian Systematic Botany 7(6): 543–567. doi:10.1071/SB9940543 [Original description (pp. 557–560); only the abstract could be consulted (paywalled).]
  • Lindström, A.J., Hill, K.D. & Stanberg, L.C. (2009). The genus Cycas (Cycadaceae) in Indonesia. Telopea 12(3): 385–418. doi:10.7751/telopea20095826 [Subsection Rumphiae, floating seeds, possible genetic input into Cycas rumphii; full text consulted.]
  • Lindström, A.J., Hill, K.D. & Stanberg, L.C. (2008). The genus Cycas (Cycadaceae) in The Philippines. Telopea 12(1): 119–145. doi:10.7751/telopea20085805 [Two-year seed maturation confirmed in this species; full text consulted.]
  • de Laubenfels, D.J. & Adema, F. (1998). A taxonomic revision of the genera Cycas and Epicycas gen. nov. (Cycadaceae). Blumea 43(2): 351–400. [Very broad species concept; cited after The World List of Cycads.]
  • Hill, K.D. (2010). Cycas bougainvilleana. The IUCN Red List of Threatened Species: e.T42070A10643325. doi:10.2305/IUCN.UK.2010-3.RLTS.T42070A10643325.en [Previous assessment; reference checked on Crossref, Red List page not consulted (access blocked).]
  • Bösenberg, J.D. (2023). Cycas bougainvilleana. The IUCN Red List of Threatened Species 2023: e.T42070A67337344. doi:10.2305/IUCN.UK.2023-1.RLTS.T42070A67337344.en [Current assessment; reference checked on Crossref, NT category relayed by GBIF; 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 etymologies taken up 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.]