Cycas petraea

Cycas petraea is a large cycad of the limestone cliffs and boulders of north-eastern Thailand, in the provinces of Loei and Khon Kaen, near the Phu Kradueng massif. This species of the genus Cycas was described in 2003 by Anders Lindström and Ken Hill; its name, “of rocky places”, recalls its habitat. Its seeds, offered by seed merchants in the late 2000s, were sown by many European amateurs, whose reports shed some light on its cultivation. The IUCN now lists it as Endangered.

Key facts about Cycas petraea

  • Accepted name: Cycas petraea A.Lindstr. & K.D.Hill (2003), accepted by The World List of Cycads (version 2026.08.15) and by GBIF; 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: none; spelt “petrae” in one line of the original description, corrected to petraea; often written “petrea” in the trade
  • Family: Cycadaceae, order Cycadales
  • Common names: no English name attested; ปรงเขาภูกระดึง (“prong khao Phu Kradueng”, “mountain cycad of Phu Kradueng”) in Thai
  • Natural range: north-eastern Thailand (Loei and Khon Kaen provinces); The World List of Cycads also cites Laos (Vientiane province)
  • IUCN status: Endangered (EN B1ab(v)), assessment by L. Averyanov and A. Lindström dated 2020 (Red List version 2023-1); Near Threatened (NT) in 2010; EN according to GBIF
  • CITES and other protection: CITES Appendix II (seeds exempt under annotation #4); Annex B in the European Union; protected plant in Thailand (Plant Varieties Act 1975)
  • Documented hardiness: an adult plant in a pot spent the winter of 2012–2013 outdoors, unprotected, with a minimum of −2 °C (28.4 °F) on the Côte Bleue, west of Marseille (France); young potted plants in Ravenna (Italy), sheltered only from rain and wind, withstood −3 °C (26.6 °F), but were lost to fungal attack in a wet winter (see Cold hardiness)
  • Light and soil: in the wild, cliffs and open areas of limestone hills; in cultivation, a very free-draining substrate; full sun tolerated by an adult plant
  • Habit and adult size: a large cycad, with a trunk of about 10 m and 30 cm or more in diameter according to the Thai Department of Agriculture; leaf blade 70 cm to 1.65 m long
  • In the UK, US and Australia: two accessions at Montgomery Botanical Center (Florida), received in 1998 and 2000; declared in BGCI PlantSearch by Glasgow Botanic Gardens and by several gardens in California, Florida and Australia
  • Toxicity: no data specific to the species; treat it as poisonous, like all Cycas (see Toxicity and precautions)

How to recognise Cycas petraea

The original description by Lindström and Hill (2003) could not be consulted. The characters below come from the 2019 report of the Thai Department of Agriculture (DOA) on the country’s Cycas, written in Thai; they describe a large cycad with smooth bark, leaves held upright then spreading, and crowded, regular leaflets.

According to that report, Cycas petraea reaches about 10 m or more, with a straight trunk 30 cm or more in diameter and smooth, yellow to grey bark. The numerous green leaves, upright then spreading, have a petiole 15 to 25 cm long edged with short spines and a blade 70 cm to 1.65 m long bearing 45 to 110 leaflets on each side. The crowded, regular leaflets measure 9 to 20 cm by 5 to 9 mm; those at the base are not reduced to spines, and the midrib is raised on both surfaces. The cataphylls (the scale leaves that protect the growing point) are triangular and soft, 4 to 8 cm long, with light brown or grey hairs.

The male cones are ovoid, thick and yellowish brown, with sharp points. The megasporophylls (the fertile leaves of female plants) are covered in yellow to grey felt and have a comb-like toothed blade. The seeds are round and flattened, fairly large, with a yellow coat and a fibrous layer. Amateurs note that seedlings soon form a small caudex covered with a corky skin, and that the first leaf can be distinctly bluish.

Hybrids

No hybrid is documented for this species.

Possible confusion

In Thailand, Cycas petraea can be confused with other cycads with comb-like megasporophylls from the hills of the centre and north-east. According to the DOA report, Cycas nongnoochiae, from the limestone hills of Nakhon Sawan, has black, deeply fissured bark and basal leaflets reduced to spines, and Cycas pectinata (as understood in the report), a large cycad of the moist forests of the west and north, has brown, almost smooth bark and a swollen trunk base. Cycas elephantipes, from the sandstone cliffs of Chaiyaphum, also has a swollen, “elephant’s foot” trunk base.

Among amateurs, young plants sold under this name are hard to identify without cones: on the French forum Fous de palmiers, a large plant bought unlabelled on the Paris quays was identified as this species only from photographs, and other identification requests remain without a firm answer.

Taxonomy

Cycas petraea is a name accepted by The World List of Cycads and by GBIF; the World Checklist of Vascular Plants (WCVP), on which POWO is based, also accepts it (consulted through GBIF on 31 October 2026). The species was described by Anders Lindström and Ken Hill in “New species and a new record of Cycas (Cycadaceae) from Thailand”, in Brittonia 54(4): 298–304 (issue dated 2002, published in 2003); the same paper describes Cycas elephantipes, from Chaiyaphum. Its placement in a section of the genus is not stated.

The original description once spells the name “petrae” before the words “sp. nov.”, but “petraea” everywhere else. The World List of Cycads treats the first spelling as an orthographic error to be corrected, based on the etymology given by the authors: the correct name is Cycas petraea. The spelling “petrea”, common in the trade and on forums, is wrong.

Etymology and history

The epithet comes from the Latin petraeus, “of rocky places”, referring to the species’ habitat on bare limestone cliffs and boulders (Haynes 2022, after the authors).

The type was collected by Ken Hill and P. Vatcharakorn on 1 February 2000 at Ban Pong Khao, Phu Kradueng, Loei province (Hill & Vatcharakorn 5086; holotype at the Forest Herbarium in Bangkok, BKF, with isotypes at Kew, Sydney and New York). A note on the species by C. Nazor and A. Lindström appeared in 2010 in Cycad Newsletter (33(3): 10–13). French amateurs discovered it soon afterwards: a group order of a hundred seeds placed with a seed merchant in September 2008 was shared among members of the Fous de palmiers forum, and the forum’s seed bank offered some in January 2009.

Common names

No English common name is attested. In Thai the species is called ปรงเขาภูกระดึง (“prong khao Phu Kradueng”), “mountain cycad of Phu Kradueng” (DOA 2019); “prong” refers to cycads in general.

In the wild

Cycas petraea grows on the limestone hills of north-eastern Thailand, in the provinces of Loei and Khon Kaen, on cliffs, boulders and open ground (DOA 2019; the authors’ etymology). In a survey published in 2019, the DOA recorded three sites with about 300 plants in all: in the rock park of Suan Hin Pha Ngam and at Suan Sawan, in Nong Hin district (Loei), and in Phu Pha Man district (Khon Kaen). Signs of illegal collection were found there.

The World List of Cycads adds Vientiane province, in Laos, but the published sources give no details of this occurrence. Pollination and phenology are not documented.

Conservation status and regulation

The IUCN now lists Cycas petraea as Endangered (EN, criterion B1ab(v)), in the assessment by L. Averyanov and A. Lindström dated 2020 and published in version 2023-1 of the Red List: the range is very small, the sites few and the number of mature plants declining. The previous assessment, by J.D. Bösenberg (2010), listed it as Near Threatened (NT), the category still cited by the DOA in 2019. The World List of Cycads shows the EN category but refers to the 2010 assessment. The Red List data relayed by GBIF confirm the EN category (consulted on 24 October 2026).

In Thailand, all Cycas species are “protected plants” under the Plant Varieties Act of 1975 (B.E. 2518) and its amendments. The DOA report counts Cycas petraea among the endemic species with a low capacity for dispersal, whose populations are declining and highly threatened, notably by collection for ornamental use.

Like every species in the genus except Cycas beddomei, Cycas petraea 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 petraea

In short: Cycas petraea is one of the few Thai cycads whose cultivation in Europe is at least partly documented, thanks to amateurs who sowed it from 2008 onwards. Their reports, on the French forum Fous de palmiers and the Italian forum Tropicamente, come down to three points: the plant grows slowly at first, it dislikes moisture around the roots, and an adult plant tolerates full Mediterranean sun.

Outdoor and in-ground cultivation

No in-ground planting has been documented. The only outdoor experience comes from potted plants left outside in winter on the French Mediterranean coast and in northern Italy (see Cold hardiness). An adult plant with a large caudex, grown in a pot on the Côte Bleue, west of Marseille, produced seven leaves in spring 2013 after its first winter outdoors, then two flushes in the year, in full sun; the owner noted that it had never produced a second flush when grown in Paris. Repotted in 2014, it was still in its pot in 2016, when the owner was considering planting it in the ground; what happened next is not known. This Mediterranean experience may be useful to readers in comparable climates, such as coastal California or southern Australia, but it concerns a single plant over a few mild winters.

Container cultivation

The growers who reported on it use a mix of potting compost, sand and pozzolana (volcanic gravel), sometimes perlite, in pots deeper than they are wide; watering is reduced in winter, when their cycads are kept almost dry in a glasshouse or conservatory. All the losses reported come from rot: young plants sown in 2009 rotted in 2011 in a poor-quality compost, in the Lot (south-western France), and in Ravenna, young potted plants suffered a fungal attack during the wet winter of 2008–2009, after which only one survived. A grower whose location is not given reported in 2012 leaflet edges that turned yellow and dried; another member suggested sensitivity to chlorides in tap water, an unverified hypothesis. The Jardin zoologique tropical at La Londe-les-Maures, in the south of France, has published no observation on this species.

In cultivation in the UK, US and Australia

In short: present in a fair number of collections, mainly in the United States, but documented at the level of the individual plant only at Montgomery Botanical Center. Declarations in the BGCI PlantSearch database (consulted on 24 October 2026) show that a garden has listed the species at some point, not that a living plant is there today.

  • United States: Montgomery Botanical Center, in Coral Gables (Florida), holds two accessions, numbered 981855 and 2000140, which indicate that they were received in 1998 and 2000 (online catalogue consulted on 24 October 2026). The Huntington (San Marino), San Diego Botanic Garden (Encinitas), the San Diego Zoo, Descanso Gardens and Sonoma Botanical Garden, in California, and Naples Botanical Garden and Pinecrest Gardens, in Florida, declare the species in BGCI. None of these sources says whether the plants grow outdoors or under cover.
  • Australia: Brisbane Botanic Gardens Mount Coot-tha (Queensland) and the System Garden of the University of Melbourne (Victoria) declare it in BGCI. It does not appear in the Garden Explorer of the Royal Botanic Garden Sydney or of the Australian Botanic Garden Mount Annan.
  • United Kingdom: Glasgow Botanic Gardens declare it in BGCI; in Glasgow it can only be grown under glass.

No first-hand report from a private grower in these countries has been published.

Propagation

Like all Cycas, Cycas petraea is dioecious: a male and a female plant are needed for fertile seed. Sowing is the only method documented, by amateurs. Their observations agree that seeds can take a year or more to germinate after harvest, and that the first leaf can be slow to appear (about eight months after germination for a seedling in the Lot, in 2009). An Italian grower, by contrast, considers that fresh seeds “come up easily”, recalling that the embryo of cycads must mature for a few months after the seed falls.

A seedling in the Lot developed several basal buds in 2009, unusual behaviour in a species reputed to be single-trunked. Seeds are exempt from CITES under annotation #4; collecting them in the wild remains subject to Thai law.

Pests and diseases

No pest specific to the species is documented. The problems reported in cultivation are rots of the roots and collar, linked to a substrate that is too wet or of poor quality, and fungal attacks in wet winters, described above. Scale insects, including Aulacaspis yasumatsui, can attack any cultivated Cycas.

Cold hardiness

The available reports document only light frosts, and they are few: no survival of Cycas petraea below −3 °C (26.6 °F) is documented. Its real limit is unknown; for young plants, winter wet seems more dangerous than cold itself.

Mediterranean and European experience

A grower on the Côte Bleue (Bouches-du-Rhône, France) reports on the Fous de palmiers forum that an adult plant of Cycas petraea, grown in a pot, spent the winter of 2012–2013 outdoors without protection, with a minimum of −2 °C (28.4 °F), and produced a crown of seven leaves in spring 2013. It is the only French report of a winter in the open, on the Mediterranean coast; there are no data for the ground or for colder regions.

A grower in Nîmes (Gard, France) reports on the same forum that a young seedling spent the winter of 2009–2010 in an unheated conservatory where the temperature fell to 3 °C (37.4 °F), with no damage reported, watered every fortnight.

A grower in Ravenna (Emilia-Romagna, Italy; USDA zone 8a/8b according to the grower’s forum signature) reports on the Tropicamente forum, in July 2009, that young plants of Cycas petraea, grown in pots outdoors and sheltered only from rain and wind, withstood −3 °C (26.6 °F). In August 2009 the same grower added that during the previous winter, presumably that of 2008–2009, the young plants suffered a fungal attack caused by high humidity and only one survived. In October 2009 the grower rated the species, from personal experience, as “fairly hardy”, USDA zone 9a (average annual minimum −6.7 to −3.9 °C, or 20 to 25 °F), between the cycads the grower considered hardy, such as Cycas revoluta, and the more delicate Dioon spinulosum, placed in zone 9b–10a. This rating remains a personal opinion: no plant has been observed below −3 °C.

United Kingdom, United States and Australia

No cold reading and no dated growing report are documented from these countries, either on forums or from botanic gardens. In the United Kingdom it is a plant for a heated or at least frost-free glasshouse; in mild, frost-free climates such as southern Florida or coastal southern California, outdoor trials would be possible but none has been published. No USDA zone can be given beyond the Italian grower’s personal opinion quoted above.

Toxicity and precautions

No toxicity analysis specific to Cycas petraea has been published. 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

No traditional use is documented. Its main current use is ornamental: the DOA (2019) recorded collection from the wild linked to the garden-plant trade.

Frequently asked questions

Should it be spelt petraea or petrea?

Petraea. The spelling “petrea”, frequent in the trade, is wrong; the original description once has “petrae”, which The World List of Cycads corrects on the basis of the etymology given by the authors.

Where does Cycas petraea grow?

In north-eastern Thailand, on the limestone cliffs and hills of Loei province (Phu Kradueng area and Nong Hin district) and Khon Kaen province; The World List of Cycads also cites Laos.

Is Cycas petraea hardy?

Only slightly. An adult plant in a pot spent one winter unprotected at −2 °C (28.4 °F) on the French Mediterranean coast, and young potted plants sheltered from rain and wind withstood −3 °C (26.6 °F) in Ravenna, but no lower value is documented; to be safe, grow it in a pot and bring it under cover in winter.

Are its seeds slow to germinate?

Often, yes. Several amateurs report up to a year or more between harvest and germination, followed by a first leaf that is sometimes slow to appear.

Why is my young Cycas petraea rotting?

Usually because of excess moisture. The losses reported came from poor-quality compost or a wet winter; a very free-draining mineral substrate and reduced watering in winter limit the risk.

Is it endangered?

Yes. The IUCN now lists it as Endangered: a 2019 survey found only about 300 plants at three sites, with signs of collection.

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

  • Lindström, A.J. & Hill, K.D. (2003). New species and a new record of Cycas (Cycadaceae) from Thailand. Brittonia 54(4): 298–304 (“2002”). doi:10.1663/0007-196X(2003)54%5B298:NSAANR%5D2.0.CO;2 [Original description (p. 299, fig. 1), together with that of Cycas elephantipes; cited after The World List of Cycads and Crossref.]
  • Nazor, C. & Lindström, A.J. (2010). Cycas petraea. Cycad Newsletter 33(3): 10–13. [Reference taken from The World List of Cycads.]
  • Sonsuphap, Y., Sripotha, D., Thuamsomboon, P. & Thongdon-ae, S. [names transliterated from Thai] (2019). วิจัยสถานภาพพืชอนุรักษ์สกุลปรง (Cycas spp.) — Non-Detriment Findings for Cycas spp. Department of Agriculture, Bangkok. [Report in Thai.]
  • Averyanov, L. & Lindström, A. (2023). Cycas petraea. The IUCN Red List of Threatened Species 2023: e.T178873A69154793. doi:10.2305/IUCN.UK.2023-1.RLTS.T178873A69154793.en [Current assessment (dated 2020, published in 2023); reference checked on Crossref, EN category relayed by GBIF.]
  • Bösenberg, J.D. (2010). Cycas petraea. The IUCN Red List of Threatened Species 2010: e.T178873A7632560. doi:10.2305/IUCN.UK.2010-3.RLTS.T178873A7632560.en [Earlier assessment (NT), cited by the DOA (2019).]
  • 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.]
  • 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.]