Encephalartos horridus

Encephalartos horridus

Encephalartos horridus is a small South African cycad endemic to the area around Port Elizabeth (now Gqeberha) and Uitenhage (Kariega), in the Eastern Cape. This species of the genus Encephalartos, described in 1801 by Nikolaus Joseph von Jacquin as Zamia horrida, is known in English as the Eastern Cape blue cycad. It is recognised by its stiff, intensely blue leaves curving outwards, whose leaflets carry one to three spiny lobes twisted out of their plane. Its trunk usually stays underground, and the plant readily forms clumps of several heads. Much sought after by collectors, it has disappeared from part of its range through urban development and poaching, and is listed as Endangered by the IUCN. Its habitat is almost frost-free, and outdoor reports from cooler climates are few.

Key facts about Encephalartos horridus

  • Accepted name: Encephalartos horridus (Jacq.) Lehm. (1834), accepted by The World List of Cycads (version 2026.08.15), GBIF and Species+
  • Former names and synonyms: Zamia horrida Jacq. (1801, basionym); Encephalartos nanus Lehm. (1838), Encephalartos van-hallii de Vriese (1838), Encephalartos altensteinii var. eriocephalus de Vriese (1846)
  • Family: Zamiaceae, order Cycadales
  • Common names: Eastern Cape blue cycad, blue cycad (English), according to PlantZAfrica and Species+
  • Natural range: South Africa, Port Elizabeth and Uitenhage districts (Eastern Cape); extent of occurrence about 431 km² (SANBI, 2023)
  • IUCN status: Endangered (EN A2acd+4acd), assessment by J. D. Bösenberg dated June 2020 and published in 2022
  • CITES and other protection: CITES Appendix I (whole genus, since 1977); Annex A in the European Union, with a negative EU opinion on artificially propagated plants from South Africa since 2013; wild collection banned in South Africa
  • Documented hardiness: almost frost-free habitat; tolerates light to moderate frosts (PlantZAfrica); in Roussillon (southern France), a patio plant without leaf damage at an absolute minimum of −4 °C (24.8 °F) in 2011; in Ravenna (Italy), in a cold glasshouse, leaf tips scorched at −7 °C (19.4 °F) in winter 2009–2010 (see Cold hardiness)
  • Light and soil: full sun, excellent drainage, little water, slightly acidic soil (PlantZAfrica)
  • Habit and adult size: stems usually underground or rising about 30 cm, exceptionally 1 m; clumps of four to eight heads; leaves 0.5 to 1 m long
  • In the UK, US and Australia: Kew, Oxford, the Eden Project, RHS Garden Wisley and Sheffield (BGCI); Montgomery (six accessions), Fairchild, the Huntington, Lotusland, the San Diego Botanic Garden, the Missouri Botanical Garden (plant donated in 1918) and some twenty other US gardens; Royal Botanic Garden Sydney, Royal Botanic Gardens Victoria (outdoors in Melbourne), Adelaide and the University of Melbourne
  • Toxicity: toxicity demonstrated as early as 1944 (Van der Walt); seeds and tissues poisonous (see Toxicity and precautions)

How to recognise Encephalartos horridus

It is one of the easiest cycads to recognise: a low clump of stiff, intensely silvery-blue leaves, strongly curved outwards in their upper third, with broad leaflets bearing twisted, spiny lobes. R. A. Dyer (1965) describes it as follows, with additions from SANBI’s PlantZAfrica:

  • Stems: single or more often branched from the base, underground or rising about 30 cm, exceptionally up to 1 m; crown only slightly woolly. PlantZAfrica describes dense clumps of four to eight stems with entangled foliage, the plant reaching up to 80 cm long and 30 cm across
  • Leaves: stiff, 0.5 to 1 m long, curved back in their upper third, covered with an intense bluish or silvery bloom that contrasts with the older leaves; they do not droop during coning (PlantZAfrica)
  • Leaflets: median leaflets up to about 10 cm long and 2.5 cm wide, with a sharp tip and one to three spiny lobes, up to 4 cm long, on the lower margin, twisted out of the plane of the leaflet; towards the base of the leaf they become smaller and entire
  • Cones: one per stem, sometimes two in males (PlantZAfrica), dark green with a fine blackish or reddish-brown down; male cones subcylindrical, 20 to 40 cm long and 6 to 12 cm across; female cones more or less ovoid, 20 to 40 cm long and 15 to 20 cm across, their scales ribbed towards the tip. PlantZAfrica describes them as bluish green when mature; M. J. Wells observed in Grahamstown that they turned dull orange-yellow beneath their grey-green surface (cited by Dyer, 1965)
  • Seeds: pale red to carmine, 3 to 3.5 cm long and about 2.5 cm across

Two forms occur in the wild: a typical, more robust form and a dwarf form, with smaller stems, leaves and cones (PlantZAfrica). According to SANBI’s 2023 non-detriment findings document, the common form has stems 0.5 to 1 m tall and 20 to 30 cm across, entirely above ground among rocks and mostly underground in deep soil, while the less frequent dwarf form has stems about 20 cm tall and no more than 15 cm in diameter (after Kemp, 1986). In cultivation the dwarf form may lose its dwarf character (PlantZAfrica). The San Diego County nursery Jungle Music adds that the colour varies from blue-green to dark blue or almost silvery white, that some plants curl their leaf tips almost into a full circle, and that its cones are yellow-green and rusty brown.

Encephalartos horridus
Encephalartos horridus in the Estufa Fria, Lisbon, Portugal.

Hybrids

Natural hybrids are reported with Encephalartos longifolius: Guy Wrinkle, in a portrait of the species published in 1983 by The Cycad Society (United States), wrote that he had seen “very beautiful hybrids” between the two species in the Eastern Cape, where they grow together. The cover note of issue 142 of Encephalartos, the journal of the Cycad Society of South Africa (September 2026), states that recent DNA analysis shows Encephalartos horridus to be closely related to Encephalartos lehmannii, with high gene flow between the two species; the article itself is reserved for members. In cultivation, a grower warned on the French Fous de palmiers forum in 2011 that many plants sold under this name come from cross-pollination with Encephalartos lehmannii: a plant raised from collection seed is not necessarily pure. Montgomery Botanical Center also lists an accession of an Encephalartos horridus hybrid in its catalogue.

Possible confusion

The most frequent confusion is with Encephalartos trispinosus, first treated as a variety of Encephalartos horridus (Hooker, 1863). According to Dyer (1965), Encephalartos trispinosus more often has aerial stems, up to 1 m, generally narrower leaflets (1.5 to 2.5 cm) with less regularly twisted lobes, and above all different cones: the female cone scales are strongly wrinkled and warty, bluish green or yellowish, whereas those of Encephalartos horridus are fairly smooth, ribbed towards the tip and dark reddish brown before maturity. Without cones, origin helps: Encephalartos trispinosus grows further east, in the Bushmans and Great Fish river valleys, and Dyer notes that no related species grows within the range of Encephalartos horridus.

Encephalartos lehmannii shares the same blue foliage, but its leaflets are almost always entire, its leaves rather straight and the faces of its female cone scales almost smooth; it is also more robust (Dyer, 1965). The two species meet in the Uitenhage district, Encephalartos horridus mainly in the central, hilly part of the Zwartkops valley and Encephalartos lehmannii in the Klein Winterberg mountains and the catchments of the Groot and Sundays rivers; according to the Cycad Society of South Africa (2026), Encephalartos horridus grows closer to the coast and Encephalartos lehmannii in the more arid interior.

Dyer was surprised that some authors had confused it with Encephalartos latifrons, a species with a massive trunk over 3 m tall and broad dark green leaves, which shares only the lobed leaflets. Encephalartos arenarius has light to mid-green, barely glaucous leaves and leaflets more than 2.5 cm wide with three or four lobes.

Taxonomy

Encephalartos horridus is an accepted name (The World List of Cycads, GBIF, Species+). Jacquin first described the species in 1801 as Zamia horrida, in his Fragmenta botanica (pp. 27–28, plates 27 and 28), from plants grown in the imperial garden at Schönbrunn, Vienna. Lehmann transferred it to Encephalartos, the genus he had just created, on 15 March 1834, in the Allgemeine Gartenzeitung (vol. 2, no. 11, p. 86), and then in the sixth part of his Novarum et minus cognitarum stirpium pugillus. His diagnosis already stresses the hairless trunk, the four-angled rachis, the bloomed, glaucous leaflets and the upper leaflets “almost three-cleft”.

Several names were later created for cultivated plants and are now treated as synonyms by The World List of Cycads: Encephalartos nanus Lehm. (1838), which became Encephalartos horridus var. nanus (Lehm.) J.Schust. (1932), Encephalartos van-hallii de Vriese (1838), which became Encephalartos horridus var. hallianus (de Vriese) Miq. (1842), and Encephalartos altensteinii var. eriocephalus de Vriese (1846). Species+ also lists the horticultural names Zamia aurea and Zamia tricuspidata. Dyer (1965) thought that the dwarf form from Port Elizabeth might deserve varietal rank but kept it within the species, as he could not compare the two forms in cultivation; no variety is recognised today.

Etymology and history

The epithet comes from the Latin horridus, “bristly, dreadful”, for the stiff, prickly leaflets (Haynes, 2022, cited by The World List of Cycads; PlantZAfrica). The genus name chosen by Lehmann means “bread in the head”, because bread was made from the pith of these plants in their native country (Lehmann, 1834).

The species was among the first South African cycads grown in Europe. According to Dyer (1965), the Viennese gardener and collector Scholl brought back plants of it, together with Encephalartos longifolius and Encephalartos cycadifolius, for the imperial garden at Schönbrunn, where Jacquin described and illustrated them. Once locally abundant, to judge by the number of plants in cultivation, it was later collected on a large scale; in Dyer’s time a few could still be seen here and there along the road from Uitenhage to Steytlerville. It was also among the first cycads whose toxicity was demonstrated, in 1944 (see Toxicity and precautions).

Common names

PlantZAfrica gives the English names Eastern Cape blue cycad and blue cycad, and Species+ Eastern Cape blue cycad, after the colour of the foliage. No Afrikaans or isiXhosa name specific to the species is attested in a verifiable source; names such as “prickly cycad” or “bloubroodboom” found on some websites are not supported by these sources.

In the wild

Encephalartos horridus is endemic to South Africa, confined to the Port Elizabeth and Uitenhage districts of the Eastern Cape (Dyer, 1965; PlantZAfrica). SANBI’s non-detriment findings document (approved in 2023, updated in June 2025) estimates its extent of occurrence at about 431 km². The typical form grows near Uitenhage in Karoo-type thicket rich in succulents; the dwarf form grows a few kilometres west of Port Elizabeth, on a sparsely vegetated quartzite hill (Dyer, 1965). Dyer also illustrates a form with an aerial trunk near Uitenhage, on shale.

The species belongs to the Albany Thicket biome, where it grows in mesic or dry thicket, often on poor quartzite outcrops and ridges but also in deep, fertile soil (SANBI, 2023); the 2017 Biodiversity Management Plan describes its habitat as xeric thicket between Port Elizabeth and Uitenhage. PlantZAfrica cites Karoo scrub, sourveld (acid grassland), deep soils and rocky ridges, always in full sun. Annual rainfall ranges from 250 to 650 mm; summers are hot and winters cool to mild, and the area is almost frost-free, frosts being rare (PlantZAfrica).

Populations consist mainly of very old adults (generation length estimated at more than 200 years) that produce many suckers; young plants are scarce. Each stem bears a single cone, rarely two, on average every six years, and the seeds, dispersed by animals, usually fall within 20 m of the parent plant (SANBI, 2023). Pollination is carried out by insects (PlantZAfrica).

Conservation status and regulation

Encephalartos horridus is listed as Endangered (EN) on the IUCN Red List, under criteria A2acd+4acd, which concern a population reduction of more than 50 % over three generations, past and continuing. The assessment, by J. D. Bösenberg, is dated 1 June 2020 and was published in 2022 (The World List of Cycads, GBIF, Crossref). PlantZAfrica gives the same category from the Red List of South African plants.

The IUCN cycad action plan (Donaldson, 2003) already listed it as Endangered, with 3,000 to 7,000 plants, a high rate of decline and over-collection and habitat destruction as threats, citing urban expansion and coastal development among the causes of lost populations. SANBI’s 2023 document estimates the wild population at more than 2,000 plants but very probably fewer than 5,000 adults. The species has disappeared from several areas, notably with urban development and the roads and railways between Uitenhage and Despatch. According to the same document, nearly 300 wild plants were illegally removed in the Eastern Cape in 2008–2009, and 36 were recovered near Uitenhage in 2016; one subpopulation lost half its plants and has fallen to fewer than 20 since 2014, and another, of about 150 plants, has disappeared entirely. Plants have also been stolen from the national botanical gardens, and international demand remains strong: the document lists it among the species most sought after by collectors.

The 2017 Biodiversity Management Plan (Government Gazette No. 40793) notes a decline of more than 50 % through habitat destruction and collection, local extinctions, and the fact that no complete population survey had ever been carried out, so that the exact extent of the range and the size of the subpopulations remained unknown. It plans such a survey and biodiversity management agreements with the landowners concerned, and reports that recent thefts had significantly reduced the number of plants in the national botanical gardens’ collections, which therefore need to be secured.

In South Africa, the collection of wild cycads (plants and seeds) is banned (Government Notice No. 371 of 2012), and the species has been covered by a national management plan since 2017, which provides for marking wild plants with microdots. Like the whole genus, it has been listed in CITES Appendix I since 4 February 1977 and, in the European Union, in Annex A of the wildlife trade regulations (current list: Regulation (EU) 2026/1383), according to Species+. For artificially propagated plants from South Africa, the European Union has kept a negative opinion since 7 February 2013, whatever their size; a review covering the whole genus was under consultation in September 2026. Databases consulted on 25 October 2026: buy only nursery-propagated plants with the required CITES documents, and check the current rules before any purchase or import.

Growing Encephalartos horridus

PlantZAfrica describes Encephalartos horridus as a very hardy, adaptable, fairly slow-growing cycad suited to temperate and subtropical regions. It needs full sun, excellent drainage, little water and a slightly acidic soil, and is best kept away from paths because of its prickly leaflets; in cultivation, male plants may produce several cones in succession in a year. Jungle Music stresses a soil through which water passes quickly, and recommends siting the plant close enough to a viewpoint to be seen but not beside a path.

Outdoor and in-ground cultivation

In southern California, with its winter rainfall, the species grows outdoors without difficulty although it comes from a region with summer rain (Wrinkle, 1983); Jungle Music considers it a good grower in the ground that soon makes a sizeable plant. In Roussillon, in southern France, a grower reports on the Fous de palmiers forum (January 2011) that his plant, bought young and planted about twelve years earlier in a patio, receives only the rain of the cool season and makes about three leaves per flush, against six for his Encephalartos lehmannii; in June 2012 he showed five new leaves and a sucker forming, and explained that he now waters it, like his Encephalartos lehmannii, once the heat has set in, until the leaves are fully developed. It produced a male cone in January 2014. On the Côte d’Azur, the Villa Thuret garden at Antibes grows the species alongside Encephalartos lehmannii and Encephalartos longifolius (photographs of 2009 to 2014 on the same forum). Another member of the forum reported in November 2008 having seen Encephalartos horridus planted in the ground at the Jardin d’oiseaux tropicaux, now the Jardin zoologique tropical, at La Londe-les-Maures in the Var; no cold data have been published for those plants. These Mediterranean experiences are useful for readers in comparable climates, such as coastal California or southern Australia, but they concern sheltered positions and mild winters.

Encephalartos horridus
Encephalartos horridus in the garden of the Villa Thuret, Antibes, France.

Container cultivation

PlantZAfrica recommends it as a container plant. A pot makes it easy to keep the plant dry and frost-free in winter: at the Villa Carlotta, on Lake Como in Italy, a plant spends the summer outdoors in a pot sunk into the ground and the winter under cover, and in Switzerland, north of the Alps, a grower showed in 2012 a potted plant overwintered in a cellar that had produced a new flush. A very free-draining mix is essential.

Encephalartos horridus
Encephalartos horridus in a pot sunk into the ground outdoors for the summer and kept under cover in winter, at the Villa Carlotta, Lake Como, Italy.

In cultivation in the UK, US and Australia

The species has been grown in Europe since Jacquin’s time, and in Britain it is today declared to BGCI PlantSearch by the Royal Botanic Gardens, Kew, Oxford Botanic Garden, the Eden Project in Cornwall, RHS Garden Wisley and Sheffield Botanical Gardens; none of these sources says whether the plants are under glass, as is usual for cycads in Britain.

In the United States, Montgomery Botanical Center in Coral Gables lists six accessions in its online catalogue, and Fairchild Tropical Botanic Garden lists the species in its Garden Explorer. At Ganna Walska Lotusland (Montecito, California), it is among the species of the cycad garden. In St. Louis, the Missouri Botanical Garden still grows an Encephalartos horridus donated in 1918 by a local collector, D. S. Brown (accession 1980-1156): its stem measures 43.2 cm in height and 25.4 cm in width, its sex is unknown, it has never been recorded coning, and it grows in the Climatron conservatory, with plans to move it to the Shoenberg Arid House (Deloso and colleagues, 2025). In all, twenty-eight US institutions declare the species to BGCI, among them the Huntington, the San Diego Botanic Garden, the San Diego Zoo, the Los Angeles County Arboretum, the University of California Botanical Garden at Berkeley, Naples Botanical Garden, Longwood Gardens, the New York Botanical Garden and the United States Botanic Garden.

In Australia, the Royal Botanic Garden Sydney lists the species in its Garden Explorer, and the current census of the Royal Botanic Gardens Victoria records it in outdoor beds at Melbourne Gardens; the Botanic Garden of South Australia in Adelaide and the University of Melbourne System Garden also declare it to BGCI. A BGCI listing shows that a garden has declared the species at some point, not that a plant is alive today.

In Italy, the Botanic Garden of Naples grew six South African plants in 1993–1994 (Gaudio and colleagues, 1997) and offered pollen from two of them in its 2012–2013 Index Seminum; at the Hanbury gardens of La Mortola, near Ventimiglia, Zamia horrida already appears in the 1897 catalogue of plants grown in the open air, and Encephalartos horridus in the Hortus Mortolensis of 1912.

Encephalartos horridus
Encephalartos horridus under glass at the Palmengarten, Frankfurt, Germany.

Propagation

Encephalartos horridus is propagated from seed or from suckers, which it produces freely (PlantZAfrica; SANBI, 2023; Jungle Music). Suckers transplant readily, but removing them takes experience; for a first cycad, PlantZAfrica advises buying a nursery seedling, which grows more easily and faster. In the Var, a grower described on the Fous de palmiers forum (April 2022) an orange-sized sucker removed with the beginning of a root, whose wound he left to dry for three days before potting it in a deep pot filled with 80 % pumice and 20 % well-rotted compost, kept dry in a glasshouse. Suckers normally have the same sex as the parent plant (SANBI, 2023).

To obtain seed, a female cone must be pollinated at the right stage with pollen from a male of the same species. The red seeds are poisonous: wear gloves to remove their fleshy coat. Seeds, like plants, fall under CITES Appendix I.

Pests and diseases

For cycads in southern Africa, PlantZAfrica lists moth caterpillars and aphids on the leaves, weevils on the cones, beetle larvae that bore into the stems and cause rot, and ants and termites that attack the roots and the underground part of the trunk; porcupines and baboons can damage stems and cones. In cultivation in Europe, the main risk remains root and trunk rot in a cold, wet soil.

Cold hardiness

The habitat of Encephalartos horridus is almost frost-free, but PlantZAfrica states that the species tolerates light to moderate frosts, and SANBI’s 2023 document notes that the “blue” cycads are more frost-tolerant than green species, which makes them popular on the Highveld, the inland plateau where winter frosts are common. Dated reports remain few and concern mostly plants kept dry or sheltered. As in all Encephalartos, survival in the cold often concerns young plants whose caudex is still underground, protected by the warmth stored in the soil; in this species the trunk often stays underground, which may help, but adult plants with exposed stems do not enjoy that protection.

Mediterranean and European experience

In Roussillon, the grower mentioned above reports that his plant, in a patio where the temperature falls to −3 to −4 °C (26.6 to 24.8 °F) when the garden reaches −7 °C (19.4 °F), has never had leaf damage, with an absolute minimum of −4 °C (24.8 °F) (January 2011); it was intact after four nights of frost and a minimum of −5 °C (23 °F) in December 2009. In August 2026 he summed it up: it withstands “intact around −3 °C, at most −4 °C” in dry conditions. In Ravenna (Emilia-Romagna, Italy), a grower reports on the Tropicamente forum that in a cold glasshouse, after one night at −7 °C (19.4 °F) and another at −5 °C (23 °F) with daytime temperatures below freezing (winter 2009–2010), all the leaves of his Encephalartos horridus had scorched tips but stayed upright, whereas his Encephalartos lehmannii was completely defoliated; the plant then recovered and even produced a sucker (2012). Another Italian grower, without giving his location, reported that a single night at −4 °C (24.8 °F) in winter 2010–2011 damaged his plants. No report exists for the Atlantic coast of France or for inland areas.

United Kingdom, United States and Australia

In southern California the species grows outdoors without problems (Wrinkle, 1983), and at Melbourne the Royal Botanic Gardens Victoria grows it in outdoor beds; neither source publishes minimum temperatures. No other dated cold report has been published in a verifiable English-language source. No USDA zone is published for the species by a botanical source; the zone ratings of some websites, such as “zone 9b”, cite no measurement. In Britain, it is a plant for an unheated or cool glasshouse rather than the open garden: the European reports above show damage between −4 and −7 °C (24.8 and 19.4 °F), depending on shelter and dryness.

Toxicity and precautions

Encephalartos horridus is one of the first cycads whose toxicity was demonstrated: in 1944 S. J. Van der Walt listed it among four plants, out of sixteen studied, whose toxicity was proved for the first time according to the literature available to him. Like other Encephalartos, it contains toxic glycosides (cycasin, macrozamin), from which methylazoxymethanol is derived; these substances affect the liver, the digestive tract and the nervous system, and the seeds are the most dangerous part. The ASPCA reports vomiting, diarrhoea, liver damage and deaths in dogs, cats and horses that have eaten cycads.

Wear gloves to handle the seeds and the prickly leaflets, and keep plants and seeds out of reach of children and pets. If any part is swallowed, contact a poison centre, a doctor or a vet without delay.

Traditional uses

SANBI’s 2023 document recalls that cycads have cultural importance in South Africa and that all species are exposed to collection for traditional medicine and spiritual uses, since collectors do not distinguish between species; for Encephalartos horridus, such collection, possibly still continuing, is thought to be limited but destructive where plants are accessible. Lehmann (1834) reports, for the southern African Encephalartos as a whole, that bread was made from their pith; no published source describes this use for this species in particular.

Frequently asked questions

Is Encephalartos horridus frost-hardy?

Only to light frost. Its habitat is almost frost-free; in southern France, a sheltered patio plant has stayed intact at −3 to −4 °C (26.6 to 24.8 °F) in dry conditions, while in Italy a cold glasshouse plant had scorched leaf tips at −7 °C (19.4 °F). In cool climates, grow it in a container under cover in winter.

How can I tell it from Encephalartos trispinosus?

Mainly by the cones. The female cone scales of Encephalartos trispinosus are strongly wrinkled and warty, those of Encephalartos horridus fairly smooth; its leaflets are also broader with more regularly twisted lobes. Origin helps too: Encephalartos trispinosus comes from further east.

And from Encephalartos lehmannii?

Encephalartos lehmannii has leaflets that are almost always entire, without spiny lobes, straighter leaves and a more robust habit. The two species are nevertheless genetically very close, and hybrids are common in cultivation.

Why is it so sought after?

For its intense blue colour and armed leaflets, unique in the genus. This demand fuels the plundering of wild populations: buy only nursery-propagated plants with their documents.

Does it grow fast?

No, it is fairly slow. In southern France, a patio plant makes about three leaves per flush, half as many as an Encephalartos lehmannii in the same garden.

Can it be grown in a pot?

Yes. PlantZAfrica recommends it as a container plant. A pot makes it easy to keep it dry and frost-free in winter; a very free-draining mix is essential.

Is it dangerous?

Yes, in two ways. Its leaflets are very prickly, and the plant is poisonous: its toxicity was demonstrated as early as 1944.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • Jacquin, N.J. (1801). Zamia horrida. Fragmenta botanica: 27–28, plates 27–28. Vienna. [Basionym; cited by The World List of Cycads and Dyer (1965).]
  • Lehmann, J.G.C. (1834). Encephalartos horridus. Allgemeine Gartenzeitung 2(11): 86; and Novarum et minus cognitarum stirpium pugillus 6. Hamburg. [New combination.]
  • Hooker, W.J. (1863). Encephalartos horridus var. trispinosus. Curtis’s Botanical Magazine, plate 5371. [Cited by Dyer (1965).]
  • Dyer, R.A. (1965). The cycads of southern Africa. Bothalia 8: 405–515. [Description, distribution, forms, distinctions from Encephalartos trispinosus, Encephalartos lehmannii and Encephalartos latifrons, history.]
  • Wrinkle, G. (1983). Portrait of a species: Encephalartos horridus; Encephalartos altensteinii. The Cycad Newsletter, July 1983.
  • Kemp, M. (1986). Focus on Encephalartos horridus. Encephalartos 7: 8–13. [Journal of the Cycad Society of South Africa, members only; cited by SANBI.]
  • Gaudio, L., Aceto, S., Caputo, P., Cozzolino, S., Siniscalco Gigliano, G., Moretti, A. & De Luca, P. (1997). Enzyme polymorphism in Encephalartos Lehmann (Zamiaceae). Delpinoa n.s. 35–36: 55–64. [Six plants grown in Naples in 1993–1994; table 1.]
  • Donaldson, J.S. (ed.) (2003). Cycads. Status Survey and Conservation Action Plan. IUCN/SSC Cycad Specialist Group, Gland and Cambridge. [Status table, threats.]
  • Department of Environmental Affairs (2017). Biodiversity Management Plan for Critically Endangered and Endangered Encephalartos species. Government Gazette No. 40793, 21 April 2017. [Section 4.10 on Encephalartos horridus.]
  • Van der Walt, S.J. (1944). Recent investigations into the toxicity of plants, etc., in the Union of South Africa. Onderstepoort Journal of Veterinary Science and Animal Industry 20(1). [Toxicity of Encephalartos horridus demonstrated.]
  • Bösenberg, J.D. (2022). Encephalartos horridus. The IUCN Red List of Threatened Species 2022: e.T41905A51056703. [Assessment cited by The World List of Cycads, SANBI and Crossref.]
  • Deloso, B.E., Colligan, A., Sucher, R. & Ramirez, V. (2025). Missouri Botanical Garden’s cycad collection: a journey through time. Plant Species Biology. https://doi.org/10.1111/1442-1984.12509 [The Encephalartos horridus donated in 1918.]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. https://doi.org/10.11646/phytotaxa.550.1.1 [Etymology, cited 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. https://doi.org/10.5281/zenodo.21958524 [Checklist.]