Of all the strange silhouettes that make up Madagascar’s spiny forest, none is more instantly recognisable than Didierea. These two species of densely spiny, semi-succulent trees and shrubs — universally nicknamed “octopus trees” for their writhing, spine-covered branches — are the type genus of the family Didiereaceae and, for many visitors, the single most photographed plant of the southern Madagascan landscape. This page introduces the genus as a whole and indexes its two species.
Taxonomy
Didierea was established in 1880 by the French botanist Henri Ernest Baillon, based on a plant collected near Tuléar in southwestern Madagascar by the naturalist and explorer Alfred Grandidier — after whom the genus is named. The species Baillon described, Didierea madagascariensis, remains the type species of the genus.
The genus belongs to the family Didiereaceae, within the subfamily Didiereoideae that groups the four spiny, dioecious, tree-forming genera endemic to Madagascar (Didierea, Alluaudia, Alluaudiopsis and Decarya). Only two species are currently accepted: Didierea madagascariensis, described by Baillon in 1880, and Didierea trollii, described some eighty years later, in 1961, by Roger Capuron and Werner Rauh. The specific epithet trollii honours the German botanist Wilhelm Troll, known for his work on plant morphology. Both species are listed on CITES Appendix II, meaning their international trade is regulated to prevent overexploitation of wild populations.
Morphology and biology
Like the rest of subfamily Didiereoideae, Didierea species are dioecious, semi-succulent, woody shrubs or small trees whose stems bear short lateral shoots (brachyblasts) in whorls, each producing both a cluster of stout spines and, during the growing season, a rosette of small, narrow, deciduous leaves that photosynthesise before dropping in the dry season, leaving the spiny stem to carry on assimilation. Flowers are small, greenish, and borne in spikes on separate male and female plants; fruit is a small, dry, winged structure typical of the family.
Within this shared plan the two species diverge sharply in habit. Didierea madagascariensis grows as a single, thick, largely unbranched columnar trunk when young, only beginning to branch well above head height, eventually reaching 6–10 m; its spines are arranged in regular whorls of four. Didierea trollii, by contrast, has a distinctive dimorphic growth pattern: young stems first creep and sprawl flat along the ground for some time before an upright, branching woody trunk eventually develops from them, giving mature plants a lower, more tangled, genuinely octopus-like silhouette rarely exceeding 4–5 m; its spines are typically borne in clusters of three per areole, alongside a similar number of fleshy leaves.
Distribution and ecology
Both species are endemic to the spiny thickets of southwestern Madagascar, part of the wider Madagascar spiny forest ecoregion — one of the most botanically distinctive and endangered dry-forest habitats on Earth, recognised as a WWF Global 200 priority region. Didierea madagascariensis is the more widespread of the two, occurring abundantly on red sandy soils from the Toliara area north to the Morondava river, where it grows alongside baobabs (Adansonia species), Alluaudia procera and spiny Euphorbia species in the classic silhouette of the spiny forest. Didierea trollii occupies a comparatively narrower range within the same broad region.
Species index
- Didierea madagascariensis — the classic octopus tree, and the type species of the genus; known locally as sohongy, sony or soribarika (Tanosy dialect names referring to its upward-sprawling branches). Widespread and the best known of the two species, forming a distinctive single, thick trunk that branches only well above ground level.
- Didierea trollii — the smaller, more sprawling octopus plant, distinguished by its dimorphic growth habit of an initially creeping stem base giving rise later to an upright, freely branching woody structure.
Cultivation notes
Both species need maximum sun, a strictly free-draining mineral substrate, and a pronounced dry, cool winter rest during which the deciduous leaves drop and watering should be withheld almost entirely; overwatering during dormancy is the most common cause of losses in cultivation. They are frost-tender and, outside genuinely subtropical or arid climates, are best grown under glass or as container specimens brought under cover for winter. Growth is slow, and plants are propagated almost exclusively from seed, since dioecy rules out self-pollination and cuttings are difficult to root; seed-grown, nursery-propagated stock should always be sought given the CITES-regulated status of wild populations.
Conservation
Conservation status differs between the two species: Didierea madagascariensis is currently assessed as Least Concern by the IUCN, reflecting its comparatively wide range, while Didierea trollii is assessed as Vulnerable. Both nonetheless face the same underlying pressure affecting the whole spiny forest ecoregion — ongoing clearance for charcoal production and subsistence agriculture across a naturally narrow southern Madagascan range — and both are protected from unregulated international trade under CITES Appendix II.
Reference sites
Plants of the World Online (Kew) — https://powo.science.kew.org — the nomenclatural authority used for the accepted species, authorship and synonymy cited on this page.
IUCN Red List of Threatened Species — https://www.iucnredlist.org — source for the differing conservation assessments of Didierea madagascariensis (Least Concern) and Didierea trollii (Vulnerable).
Bibliography
Baillon, H.E. (1880). “Sur le Didierea.” Bulletin Mensuel de la Société Linnéenne de Paris 1: 258. [The original description of the genus Didierea and its type species, Didierea madagascariensis, based on material collected near Tuléar by Alfred Grandidier.]
Rauh, W. (1963). “Didierea trollii Capuron et Rauh, eine neue Didieraceae aus Süd-Madagaskar.” Kakteen und andere Sukkulenten 14. [The formal description and horticultural introduction of Didierea trollii, including its distinctive dimorphic growth habit as distinguished from Didierea madagascariensis.]
Applequist, W.L. & Wallace, R.S. (2000). “Phylogeny of the Madagascan endemic family Didiereaceae.” Plant Systematics and Evolution 221(3–4): 157–166. [Molecular phylogenetic study confirming the placement of Didierea within subfamily Didiereoideae alongside Alluaudia, Alluaudiopsis and Decarya.]