Cycas siamensis: bulb-based cycad of Thailand and Indochina

Cycas siamensis is a cycad of mainland Indochina, found from Myanmar through Thailand, Laos and Cambodia to Vietnam, where it grows in dry deciduous forest on poor soils. This member of the genus Cycas was described in 1863 by the Dutch botanist Friedrich Anton Wilhelm Miquel from a plant collected in Thailand, the former kingdom of Siam that gives it its name. It is recognised by the swollen, bulb-like base of its trunk, often partly buried, and by its short leaves with small leaflets. In Thailand, where it is called ปรงนา (prong na), it is one of only two cycads the country allows to be exported. It was placed for a time in the genus Epicycas, as Epicycas siamensis, a name now treated as a synonym. The IUCN lists it as Vulnerable.

Key facts about Cycas siamensis

Accepted name: Cycas siamensis Miq. (1863), accepted by The World List of Cycads (version 2026.08.15); the World Checklist of Vascular Plants (WCVP), on which POWO is based, also accepts it (consulted through GBIF on 31 October 2026)
Former names and synonyms: Cycas immersa Craib (1912); Epicycas siamensis (Miq.) de Laub. (1998); Cycas aurea according to Species+; the species once included, as subspecies, the plants now called Cycas balansae and Cycas inermis
Family: Cycadaceae, order Cycadales; type species of section Indosinenses (Hill 2008)
Common names: no traditional English name; “Thai sago” is a garden label; in Thai ปรงนา (prong na); other local names in Cambodia, Laos, Thailand and Vietnam (see Common names)
Natural range: Myanmar, Thailand (lowlands of almost every region except the south), Laos (Champasak, Saravan), Cambodia, Vietnam (including Dak Lak, Gia Lai, Kon Tum and Nghe An)
IUCN status: Vulnerable (VU A2cd+4cd), assessment by H. T. Nguyen (2010); category kept by the reassessment of J. D. Bösenberg (2020, published in 2023)
CITES and other protection: CITES Appendix II (seeds exempt under annotation #4); Annex B in the European Union; protected plant in Thailand
Documented hardiness: no verifiable data (see Cold hardiness)
Light and soil: in the wild, dry deciduous forest on poor soils, up to 1,200 m
Habit and adult size: bulb up to about 1 m across, trunk up to 2.5 m; leaves 45 to 110 cm long
In the UK, US and Australia: Kew, Glasgow and Treborth (BGCI) in the UK; Montgomery Botanical Center (17 accessions, 1994 to 2000) and some twelve other gardens in BGCI in the US; Royal Botanic Garden Sydney (three plants recorded as of wild Chinese origin)
Toxicity: no analysis specific to the species; treat it as poisonous, like all Cycas (see Toxicity and precautions)

How to recognise Cycas siamensis

Look first at the base of the trunk, swollen like a bulb, then at the leaves: short, with brief, flat leaflets that are often rounded at the tip, a short and usually spiny petiole, and cylindrical male cones. These are the characters de Laubenfels and Adema (1998), who called the plant Epicycas siamensis, used to separate it from Cycas pectinata, and the description below is theirs:

Bulb and trunk: bulb underground or protruding, about 1 m across; trunk, when there is one, up to 2.5 m tall and 8 to 20 cm across.
Leaves: 45 to 110 cm long, with 50 to 103 leaflets on each side; petiole 8 to 18 cm, usually spiny along its whole length, sometimes unarmed; rachis downy when young.
Leaflets: 6 to 12 cm by 4 to 7 mm, with flat margins, a midrib raised on both faces and a tip ranging from sharp to rounded; green, paler underneath.
Male cones: cylindrical or oblong, 20 to 30 cm by 6 to 8 cm, on a stalk of 1 to 2 cm; microsporophylls 17 to 20 mm wide, ending in a sterile point of 7 to 12 mm and an abruptly upturned hook of 5 to 10 mm.
Megasporophylls (the fertile leaves of female plants): 2 to 4 ovules; sterile part triangular, 22 to 35 mm by 17 to 30 mm, with about ten teeth of 6 to 10 mm on each side and a spine-like tip of 14 to 25 mm.
Seeds: yellow, 29 to 35 mm by 30 to 35 mm.

Like the other members of section Indosinenses, it has hard male cones, hairless ovules, a comb-like sterile blade on the megasporophylls and a fibrous layer in the fleshy coat of the seed (Hill 2008). On the French forum Fous de palmiers, a collector who grows it under glass noted in 2012 that its leaves rarely droop, being usually held flat or upright, with fairly short leaflets, often shorter than those of Cycas revoluta; this is a growing observation, not a published description.

According to de Laubenfels and Adema (1998), the seeds are yellow.

Hybrids

No hybrid, natural or horticultural, is documented. The trade name Cycas siamensis “Silver”, given to plants with silvery leaves that are also sold from Thailand as leafless bulbs, does not correspond to any taxon recorded by The World List of Cycads, and no published source explains its origin. On the Fous de palmiers forum, a collector noted in 2010 that this “Silver” had not been firmly linked to the species and that its seeds, judging by the plants he had seen in Thailand, were almost half the size.

Possible confusion

The most important historical confusion is with Cycas pectinata. De Laubenfels and Adema (1998) point out that Smitinand (1971, 1972) illustrated fertile organs of Cycas pectinata as Cycas siamensis, while the figures of Cycas pectinata published by Leandri (1931) mostly show Cycas siamensis. Cycas pectinata has longer leaflets, male cones that are not cylindrical, shorter microsporophylls, much longer teeth on the megasporophylls and no bulb-like base.

Several other South-East Asian species also have a swollen base. Cycas lindstromii, from southern Vietnam, is close, but its often branched bulb never forms an aerial trunk, its megasporophylls are narrower and its leaves shorter (de Laubenfels and Adema 1998). Cycas elephantipes, from Chaiyaphum province, where The World List of Cycads also records Cycas siamensis, and Cycas pachypoda, from southern Vietnam, also have a trunk with a swollen base.

In China, the name was wrongly applied to other species. Cheng and colleagues (1975) used Cycas siamensis for the species now called Cycas collina (Hill 2008), and the Flora of China notes that Chinese plants of the species it calls Cycas miquelii, in other words Cycas sexseminifera, had been misidentified as Cycas siamensis. This matters for anyone reading plant labels: a “Cycas siamensis” of Chinese origin deserves a second look.

The trade adds its own confusions. On the Fous de palmiers forum, a collector reported in 2010 a seller offering Cycas siamensis under the names Cycas pruinosa or Cycas calcicola. Conversely, plants bought as Cycas siamensis belonged, according to the same collector, to another section, Stangerioides: in 2012 a plant grown in the Var, in the south of France, which he likened to Cycas ferruginea, and in 2014 three plants of an unidentified Vietnamese species.

Taxonomy

Cycas siamensis is accepted by The World List of Cycads; the World Checklist of Vascular Plants (WCVP), on which POWO is based, also accepts it (consulted through GBIF on 31 October 2026). Miquel described it in 1863 in the Botanische Zeitung (21: 333–334), in an article entitled “Ueber eine neue Cycas aus Siam”, from a collection made by J. E. Teijsmann in March 1862 in Kanchanaburi province (holotype in the Utrecht herbarium, U; isotype at Leiden, L). This original description has not been read: its details come from The World List of Cycads and from de Laubenfels and Adema (1998).

It is the type species (lectotype, according to Hill and Yang 1998) of section Indosinenses, created by Josef Schuster in 1932. Schuster placed only Cycas siamensis and Cycas micholitzii in it; the latter has since been moved to section Stangerioides. Today the section brings together cycads with hard male cones, hairless ovules, a large and deeply comb-like sterile part on the megasporophylls and a fibrous layer in the fleshy seed coat, from the Himalayan foothills of India and Nepal to Vietnam, southern China and the north of the Malay Peninsula (Hill 2008).

The species was long understood very broadly. Schuster (1932) included as subspecies the plants now called Cycas balansae and Cycas inermis, which are now recognised as separate species; Species+ still keeps “Cycas siamensis subsp. inermis” and “subsp. balansae” among the synonyms of those two species. De Laubenfels and Adema (1998) transferred it to the genus Epicycas, which The World List of Cycads does not recognise. The World List of Cycads retains only one taxonomic synonym, Cycas immersa Craib (1912); Species+ adds Cycas aurea. The names Cycas baguanheensis, Cycas boddamii and Cycas intermedia, given as synonyms by other websites, appear neither in The World List of Cycads nor in Species+.

Etymology and history

The epithet refers to the presence of the species in Thailand, called the kingdom of Siam at the time of the description (Haynes 2022, as taken up by The World List of Cycads). The type plant was collected by Teijsmann in March 1862 in Kanchanaburi province, and the species was described the following year. It then appears in a revision by Regel (1876) and, in 1878, in a column on “new garden plants” in the Gardeners’ Chronicle (Masters 1878), which suggests it was already grown in Europe; the published sources give no details.

Common names

No traditional English name is attested. The Royal Botanic Garden Sydney labels the species “Thai Sago” in its online catalogue, which is a garden label rather than an established name. In Thailand the species is called ปรงนา (prong na), the name used by the Thai Department of Agriculture. The PROSEA database (Plant Resources of South-East Asia) gives other vernacular names: prâng’ in Cambodia, ph’aawz kh’ôôk in Laos, prong-liam (Trat province), prong-pa (centre) and talapat-ruesi (south-west) in Thailand, and thiên tuê xiêm in Vietnam; these names were recorded before the recent revisions of the genus and may also apply to related species. In the trade, the name “Siamensis Silver” (or “Silver”) circulates for a silver-leaved form.

In the wild

Cycas siamensis grows from Myanmar to Thailand, Laos (Champasak and Saravan provinces), Cambodia and Vietnam (including Dak Lak, Gia Lai, Kon Tum and Nghe An), according to The World List of Cycads. In Thailand it is recorded in many provinces, from Tak and Uttaradit to Chachoengsao and Chon Buri; the Thai Department of Agriculture describes it as widespread in the lowlands of almost every region of the country except the south. It grows in dry deciduous forest on poor soils, up to 1,200 m (de Laubenfels and Adema 1998).

Species+ adds China and Malaysia to its range countries, and PROSEA mentions the north of the Malay Peninsula; The World List of Cycads cites neither, and Hill (2008) attributes the old Chinese records to other species.

Conservation status and regulation

The IUCN lists Cycas siamensis as Vulnerable (VU) under criteria A2cd+4cd, that is, for a population reduction inferred from past and continuing habitat decline and exploitation. The assessment is by H. T. Nguyen (2010), and a reassessment by J. D. Bösenberg (dated 2020 and published in version 2023-1 of the Red List) keeps the same category according to GBIF.

In Thailand, the picture looks less serious. A study carried out from 2015 to 2019 by the Department of Agriculture to set export conditions (a CITES non-detriment finding) found traces of illegal digging in the populations of every cycad in the country, but judged Cycas siamensis the least threatened, because it is very widespread and abundant in protected areas, community forests and private land. This is why Thailand allows it to be exported, the only other species being Cycas chamaoensis: artificially propagated plants and wild-origin plants may be exported, but the latter only if they come from private land. In Thailand all native cycads are protected plants under the Plant Varieties Act of 1975.

Like every species in the genus except Cycas beddomei, Cycas siamensis is covered by CITES Appendix II, where the family Cycadaceae has been listed since 4 February 1977. Annotation #4 (version from CoP20, in force since 5 March 2026) exempts in particular seeds, pollen, and seedlings or tissue cultures obtained in vitro in sterile containers, while international trade in live plants still requires permits. In the European Union, the species is listed in Annex B of Regulation (EC) No 338/97, in the version set by Regulation (EU) 2026/1383, applicable since 29 June 2026. Species+, the official CITES database, records the species under its own name and confirms this status (consulted on 13 October 2026). This information does not replace the official texts: check the regulations in force before any purchase, ask the seller for proof of origin, and prefer seed-grown plants, especially for large bulbs, which may have been dug from the wild.

Growing Cycas siamensis

Cycas siamensis is one of the Asian cycads most easily found in Europe, as seed or as bulbs, yet no institution consulted has published growing advice specific to it. The growing reports found come from collectors on the French Fous de palmiers forum who keep it in pots, under glass or indoors.

Outdoor and in-ground cultivation

No result in the ground has been published outside its native range, in Europe, the United States or Australia. Its natural habitat, poor and free-draining soils in a tropical climate with a long dry season, suggests a plant that wants warmth, good drainage and a dry rest, but this is a deduction from the habitat, not a growing result.

Container cultivation

All the documented experience concerns plants in pots:

a collector who grows under glass in the Ain and in the Hérault (France) noted in January 2010 that his seedlings kept producing leaves in winter, and in 2012 described a plant carrying about twenty offsets, each flushing at a different time;
a grower based in Poland reported in January 2010 and again in November 2012 that his Cycas siamensis loses its leaves in winter, like Cycas media, which in his view makes it easy to overwinter under cover;
a collector advised in 2013 against leafless bulbs imported from Thailand, often too dry and hard to bring back into growth.

These observations concern plants in pots and under cover, in conditions that are not always described. A free-draining compost and a dry, frost-free winter are the prudent choices.

In cultivation in the UK, US and Australia

United Kingdom: Kew, Glasgow Botanic Gardens and Treborth Botanic Garden (Bangor) list it in BGCI PlantSearch; this shows that each garden declared it at some point, not that a plant is alive today, and it would be grown under glass.
United States: the online catalogue of Montgomery Botanical Center (Coral Gables, Florida) lists 17 accessions under this name, whose numbers date them to 1994 (2), 1995 (14) and 2000 (1); the catalogue gives the name a “synonym” status without saying of what. BGCI adds some twelve gardens, among them San Diego Botanic Garden, the San Diego Zoo, Naples Botanical Garden and the University of California Botanical Garden at Berkeley.
Australia: the online catalogue of the Royal Botanic Garden Sydney (updated on 1 October 2026) lists three plants of one accession (S1993-0797) in bed 51b, recorded as of wild origin from China; since China lies outside the range accepted by The World List of Cycads and old Chinese records of the name refer to other species (see Possible confusion), the identity of these plants is uncertain.

No English-language growing report with a date, a place and a temperature is documented.

Propagation

The plant is dioecious: a male and a female plant are needed to obtain seed. One sowing is documented, by a grower based in Poland, on the Fous de palmiers forum: sown in early September 2007 in a heated aquarium (30 to 32 °C, or 86 to 89.6 °F, in the upper part) on a sandy, free-draining compost, 8 seeds out of 10 had germinated after two months and 9 out of 10 by January 2008, one immature seed having been discarded. Seeds are exempt from CITES under annotation #4. Only look for seed or plants of legal, documented origin.

Pests and diseases

No pest or disease specific to the species is documented. The cycad scale, Aulacaspis yasumatsui, attacks many Cycas in cultivation, and rot is the usual result of waterlogged compost in cycads.

Cold hardiness

No published, verifiable cold tolerance data exist for Cycas siamensis. It lives in a tropical climate with a long dry season.

European experience

No result in the ground has been published in Europe, and no dated frost episode concerns the species. The reports found on the Fous de palmiers forum all concern plants under glass or indoors, without a minimum temperature. A grower on the Côte Bleue, west of Marseille, mentions in 2012 a Cycas siamensis that “gave up on him”, without saying under what conditions or why. In Italy, neither the published lists of botanic gardens nor the Tropicamente forum mention it. The Jardin zoologique tropical at La Londe-les-Maures, in the Var, has published no observations on this species, so its Mediterranean experience offers no guide here.

United Kingdom, United States and Australia

No verifiable cold value and no dated growing report has been published in these countries. In the UK, treat it as a plant for a heated glasshouse or a bright room; in the United States and Australia, its presence in gardens in southern Florida, coastal southern California and Sydney says nothing about its limits, since none of them records frost damage on it.

Toxicity and precautions

It should be treated as poisonous, even though no analysis specific to Cycas siamensis exists. The risk is well documented in the genus: the tissues of Cycas, and above all their seeds, contain cycasin, which digestion converts into methylazoxymethanol, toxic to the liver and nervous system, along with a neurotoxic amino acid, BMAA. The ASPCA lists the sago palm (Cycas revoluta) among plants toxic to dogs, cats and horses. Keep seeds and leaves out of reach of children and pets, and wear gloves to clean seeds. If any part is swallowed, contact a poison centre, a doctor or a vet straight away.

Traditional uses

Its best-documented use is ornamental: in Thailand the species is sold on the domestic market and may be exported. PROSEA places it among the plants with edible fruits and seeds of South-East Asia, with uses “similar to those of Cycas rumphii”, without giving details. Given the toxicity of the genus, none of this is in any way a recommendation.

Frequently asked questions

What does siamensis mean?

“From Siam”, the former name of Thailand, where the type plant was collected in 1862.

How can I recognise it?

By the bulb-like swollen base of its trunk, its short leaves (45 to 110 cm) with brief leaflets that are often rounded at the tip, and its cylindrical male cones.

Where does it grow in the wild?

In dry deciduous forest on poor soils in Myanmar, Thailand, Laos, Cambodia and Vietnam, up to 1,200 m.

What is Cycas siamensis “Silver”?

A trade name for plants with silvery leaves, also sold as leafless bulbs. It does not match any recorded taxon and its origin is undocumented.

Does it lose its leaves in winter?

It depends on the plant and the conditions. Some European growers see it drop its leaves in pots during the cold season, while others find that their seedlings under glass keep flushing in winter.

Is it endangered?

Yes, for the IUCN, which lists it as Vulnerable because of habitat decline and collecting. In Thailand, however, it is judged the least threatened of the country’s cycads.

Can it be grown outdoors in the UK?

No. No verifiable cold data exist and no trial in the ground has been published anywhere outside the tropics: in the UK, grow it in a pot under heated glass.

Is it poisonous?

It should be assumed so. No analysis specific to the species exists, but every cycad studied contains serious toxins, especially in the seeds.

Sources

Reference sites

The World List of Cycads (version 2026.08.15), Cycas siamensis: https://cycadlist.org/scientific_name/213 — publication, type, range, IUCN status (VU A2cd+4cd), synonyms, etymology, references. Checklist of the IUCN Cycad Specialist Group, reliable; consulted on 13 October 2026.
GBIF, World Checklist of Vascular Plants (Cycas siamensis): https://api.gbif.org/v1/species/206945190 — accepted name in the World Checklist of Vascular Plants (basis of POWO), published in Bot. Zeitung (Berlin) 21: 334 (1863). Kew data distributed by GBIF, reliable; consulted on 31 October 2026.
Species+ (UNEP-WCMC), Cycas siamensis: https://speciesplus.net/species#/taxon_concepts/17514/legal — CITES Appendix II (family listed since 4 February 1977, annotation #4), EU Annex B, synonyms, range countries. Official CITES and EU database; consulted on 13 October 2026.
Commission Regulation (EU) 2026/1383: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=OJ:L_202601383 — annexes to Regulation (EC) No 338/97 in force since 29 June 2026. Official text.
Thai Department of Agriculture (2019), “Non-Detriment Findings for Cycas spp.” (in Thai): https://info.doa.go.th/research/pdf/2562/499_2562.pdf — Thai name, range in Thailand, threats, trade and export rules. Official report, reliable.
GBIF (API), IUCN Red List data, Cycas siamensis: https://api.gbif.org/v1/species/2683286/iucnRedListCategory — current category (VU). Copy of the official Red List data; consulted on 21 October 2026.
Flora of China (eFloras), Cycas miquelii: http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=210000344 — note on Chinese plants misidentified as Cycas siamensis. Reference flora; consulted on 13 October 2026.
PROSEA (Plant Resources of South-East Asia), via Pl@ntUse, Cycas siamensis: https://plantuse.plantnet.org/en/Cycas_siamensis_(PROSEA) — vernacular names in Cambodia, Laos, Thailand and Vietnam, range, uses compared with those of Cycas rumphii. Regional reference work, earlier than the recent revisions of the genus; consulted on 18 October 2026.
Royal Botanic Garden Sydney, Garden Explorer, Cycas siamensis: https://rbgsydney.gardenexplorer.org/taxon-3135.aspx — three plants (S1993-0797) recorded as of wild origin from China, bed 51b, label “Thai Sago”. Garden’s own catalogue, reliable for presence; consulted on 21 October 2026.
Montgomery Botanical Center, living collection catalogue (BG-BASE “Montgomery – Cycads”): https://www.montgomerybotanical.org/research/living-collection/ — Cycas siamensis, 17 accessions. Garden’s own catalogue, reliable for presence; consulted on 19 October 2026.
BGCI PlantSearch: https://plantsearch.bgci.org/ — gardens in the UK and the US declaring the species. Worldwide database of garden collections; a listing shows a declaration, not a living plant; consulted on 19 October 2026.
Fous de palmiers forum (France), threads “cycas siamensis” (2012), “Encephalartos cycadifolius” (2010), “Cycadales pour étés frais” (2012), “Le flusher fou” (2012), “Aquarium – germoir” (2007–2008), “Liste des vendeurs de Cycadales” (2013), “Cycas Elephantipes ?” (2010) and “Cycas petraea” (2012): https://www.fousdepalmiers.fr/html/forum/viewtopic.php?t=12839 — growing observations in pots and under glass, sowing, trade confusions. Growers’ forum, individual reports; cited anonymously.
ASPCA Animal Poison Control, Sago Palm: https://www.aspca.org/pet-care/aspca-poison-control/toxic-and-non-toxic-plants/sago-palm — toxicity of cycads to dogs, cats and horses. Veterinary poison control centre, reliable; genus-level information.

Bibliography

Miquel, F.A.W. (1863). Ueber eine neue Cycas aus Siam. Botanische Zeitung 21: 333–334. [Original description; cited after The World List of Cycads.]
Tang, W. & Yang, S.L. (1995). Focus on Cycas siamensis. Encephalartos 43: 4–11. [Review article; reference taken from The World List of Cycads.]
Laubenfels, D.J. de & Adema, F. (1998). A taxonomic revision of the genera Cycas and Epicycas gen. nov. (Cycadaceae). Blumea 43(2): 351–400. [Description of Epicycas siamensis (pp. 393–394), range, habitat, comparison with Cycas pectinata and Cycas lindstromii.]
Hill, K.D. & Yang, S.L. (1999). The genus Cycas (Cycadaceae) in Thailand. Brittonia 51(1): 48–73. [Description (pp. 53–55); reference taken from The World List of Cycads.]
Hill, K.D., Nguyen, T.H. & Loc, P.K. (2004). The genus Cycas (Cycadaceae) in Vietnam. The Botanical Review 70(2): 134–193. [Vietnamese populations (pp. 180–182); reference taken from The World List of Cycads.]
Hill, K.D. (2008). The genus Cycas (Cycadaceae) in China. Telopea 12(1): 71–118. doi:10.7751/telopea20085804 [Section Indosinenses and its lectotype, names misapplied in China.]
Nguyen, H.T. (2010). Cycas siamensis. The IUCN Red List of Threatened Species: e.T42066A10641954. [VU assessment; reference taken from The World List of Cycads and Crossref.]
Bösenberg, J.D. (2023). Cycas siamensis. The IUCN Red List of Threatened Species 2023: e.T42066A69157502. doi:10.2305/IUCN.UK.2023-1.RLTS.T42066A69157502.en [Reassessment (dated 2020); Crossref metadata consulted, VU category from GBIF.]
Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. doi:10.11646/phytotaxa.550.1.1 [Etymology taken up 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.]