Introduction & Discovery

Dicksonia externa is the second of two Dicksonia species endemic to the Juan Fernandez Islands, sharing its remote oceanic homeland with the closely related D. berteroana and together constituting one of the most notable examples of tree fern endemism on young oceanic islands anywhere in the world. The species name externa, meaning outer or external, has been interpreted as referencing the species' occurrence on the outer or more exposed slopes of the islands, or possibly its distribution on the more westerly Alejandro Selkirk Island, the outermost member of the archipelago, though the precise reasoning of the original author is not entirely clear from the protologue. The Juan Fernandez Islands are famous in world literature as the setting of the real-life castaway experience of the Scottish mariner Alexander Selkirk, who was marooned on Mas a Tierra (now Robinson Crusoe Island) from 1704 to 1709 after a dispute with his captain, surviving alone for over four years by hunting feral goats, fishing, and foraging wild plants in an episode that inspired Daniel Defoe to write Robinson Crusoe in 1719, one of the foundational novels of English literature. The islands are equally famous among botanists for their astonishingly endemic-rich flora, which despite occupying fewer than 100 square kilometres of total land area includes species found nowhere else on the planet representing lineages that span the evolutionary history of flowering plants and ferns. Dicksonia externa belongs to the family Dicksoniaceae, characterised by soft multicellular hairs at the stipe bases rather than the scales of the Cyatheaceae, and by marginal sori protected by bivalved indusia, characters that have been confirmed by molecular phylogenetic analysis as diagnostic for a genuine evolutionary lineage. The species shares the acute conservation challenges that threaten the entire Juan Fernandez endemic flora: invasive plant species, particularly the European blackberry Rubus ulmifolius and the Chilean native maqui Aristotelia chilensis, have aggressively colonised the native forest habitat, forming dense monospecific thickets that exclude native species. Feral animals, including goats introduced in the sixteenth century and rabbits introduced later, have degraded the forest understory and prevented natural regeneration. The Chilean government, CONAF (the National Forest Corporation), and international conservation partners including the Royal Botanic Garden Edinburgh, the Juan Fernandez Islands Conservancy, and Island Conservation have invested significantly in invasive species control and native habitat restoration, offering realistic hope for the long-term survival of D. externa and its ecosystem.

Dicksonia is a genus of plants. Dicksonia is a genus of tree ferns in the order Cyatheales. It is regarded as related to Cyathea, but is considered to retain more primitive traits, dating back at least to the Jurassic and Cretaceous periods. The fossil record includes stems, pinnules, and spores.

Discovery & Naming

Dicksonia externa was described from material collected on the Juan Fernandez Islands during one of the botanical expeditions that documented the archipelago's extraordinary endemic flora over the course of nearly two centuries of scientific exploration. The botanical investigation of the Juan Fernandez Islands began in earnest with the visits of European naturalists in the early nineteenth century, including the ill-fated Carlo Bertero who collected on the islands in the late 1820s before his death at sea in 1831. Other important collectors included Rodulfo Amando Philippi, the German-born Chilean naturalist who directed the Museo Nacional de Historia Natural in Santiago for over four decades and described many Chilean plant species, and Federico Johow, who published the first comprehensive botanical account of the islands in 1896. The definitive treatment of the Juan Fernandez flora was provided by the Swedish botanist Carl Skottsberg (1880 to 1963), whose monumental multi-volume Natural History of Juan Fernandez and Easter Island (1920 to 1956) documented the flora, fauna, and physical geography of the archipelago with a thoroughness and scientific rigour that has rarely been equalled in island biology. Skottsberg visited the islands on multiple occasions, making extensive herbarium collections and field observations that clarified the identity and relationships of the endemic species, including the Dicksonia taxa. His work established the Juan Fernandez Islands as a key case study in island biogeography and plant evolution. The genus Dicksonia was established by Charles Louis L'Heritier de Brutelle in 1789, honouring the English nurseryman and bryologist James Dickson (1738 to 1822), and the family Dicksoniaceae is recognised as a natural evolutionary group distinct from the Cyatheaceae in all modern classification systems including the PPG I system of 2016, supported by molecular phylogenetic analyses using multiple DNA markers.

Native Range & Distribution Map

Distribution map showing the native range of Dicksonia externa.

Biology & Frond Morphology

Dicksonia externa belongs to the genus Dicksonia in the family Dicksoniaceae, producing bipinnate to tripinnatifid fronds produced in a terminal crown, reaching 1.5 to 2.5 metres in length. blades are broadly lanceolate, dark green, and of moderately leathery texture. pinnae are deeply pinnatifid to pinnate. stipes are stout, dark at the base, and densely clothed with soft rufous to brown multicellular hairs characteristic of the dicksoniaceae, distinguishing the species from any scale-bearing cyatheaceae. young croziers bear dense woolly indument. sori are marginal, at the tips of pinnule veins, with bivalved indusia consisting of an outer reflexed marginal valve and an inner true indusium. fronds that unfurl from coiled fiddleheads (croziers). Like all ferns, it reproduces via spores borne on the underside of fertile fronds rather than flowers and seeds, and its life cycle alternates between a dominant sporophyte (the visible plant) and a small, short-lived gametophyte stage.

Reproduction & Propagation

Propagation of Dicksonia externa can be achieved through several methods:

• Spores: Collect ripe spores from the underside of fertile fronds, sow on sterilised peat or peat/perlite mix. Do not cover. Keep humid and in bright indirect light. Prothalli (gametophytes) typically develop in 4–12 weeks, and young sporophytes appear after a further 2–6 months.
• Division: Mature clumps with multiple crowns or creeping rhizomes can be divided in spring as new fronds emerge.
• Rhizome cuttings / offsets: Epiphytic genera (Davallia, Polypodium, Phlebodium) can be propagated from 5–10 cm rhizome segments with at least one frond and visible roots.

Cultivation & Substrate

Successful cultivation of Dicksonia externa depends on matching three conditions to its natural habitat: consistent moisture without waterlogging, a humus-rich yet well-drained substrate, and the correct light level for its frond type — whether dappled woodland shade, bright filtered light, or, for a handful of rock ferns, direct sun.

Common Mistakes to Avoid

Confusing D. externa with D. berteroana is easy given their geographic co-occurrence and morphological similarity; precise determination requires examination of diagnostic characters. Treating the species as cold-hardy like D. antarctica will result in frost damage or death. Allowing humidity to drop below 60 percent damages fronds. Neglecting trunk-watering is the universal Dicksonia cultivation failure. Sourcing material from wild populations without authorisation is illegal and threatens the critically small remaining habitat.

Seasonal Considerations

The Juan Fernandez Islands have a Mediterranean-influenced oceanic climate with mild wet winters and warmer somewhat drier summers, though cloud immersion maintains moisture at higher elevations year-round. In cultivation, maintain consistent moisture and humidity throughout the year. Increase watering slightly during warmer months and protect from frost during winter. No true dormancy occurs. New frond production may occur at any time of year in equable conditions.

Diseases & Pests

Common issues affecting Dicksonia externa in cultivation:

• Root/rhizome rot: Caused by waterlogged substrate, compacted soil, or overwatering in cool weather. Ensure the growing medium is well-aerated and never let pots sit in standing water for prolonged periods.
• Fungal leaf spot & Botrytis: Brown or grey blotches appear in stagnant, overly humid conditions. Improve air circulation, remove affected fronds, and avoid wetting foliage late in the day.
• Scale insects & mealybugs: The most common fern pests, hiding on stipes and frond undersides. Wipe off with a cotton swab dipped in diluted isopropyl alcohol, or treat with horticultural soap. Many chemical pesticides scorch fern fronds — always test on one frond first.
• Spider mites: Fine webbing and stippled fronds, common in dry indoor air. Raise humidity and rinse fronds regularly with tepid water.
• Frond browning (tip burn): Caused by dry air, direct hot sun, fluoridated or chlorinated tap water (especially in Nephrolepis, Calathea-loving filmy ferns), or soluble-salt build-up from fertiliser. Flush the pot with rainwater and reduce feeding.

Indoor Growing & Terrariums

Dicksonia externa can be grown indoors as a houseplant or terrarium subject when its humidity and light requirements are met.

Indoor Setup

• Light: Bright indirect light — an east- or north-facing window, or 30–60 cm under an LED grow light (10–12 hours/day). Most ferns scorch in direct midday sun.
• Humidity: 50–80%. Group plants, stand pots on a pebble-and-water tray, or run a humidifier; misting alone rarely raises ambient humidity enough.
• Temperature: 16–24°C (60–75°F) for most indoor species; avoid cold drafts and hot radiators.
• Substrate: Peat-free potting mix with added perlite and orchid bark for drainage; epiphytic genera (Platycerium, Davallia) grow best mounted on bark or in a bark-heavy orchid mix.
• Water: Keep consistently moist but never waterlogged. Let the top 1–2 cm of substrate dry slightly between waterings in winter.
• Air circulation: A gentle fan discourages fungal leaf spot without drying out the fronds.

Landscape & Garden Use

Dicksonia externa can be used in the garden wherever its hardiness and habitat preferences are matched. Ferns are classic choices for shaded borders, woodland gardens, stumperies, rockeries, stream-side plantings, and ground-cover under trees.

Landscape Tips

• Companions: Hostas, Astilbe, Heuchera, Tiarella, Epimedium, hellebores, snowdrops and other shade-tolerant perennials are classic partners.
• Soil preparation: Incorporate leaf mould or composted bark to improve moisture retention and mimic a forest floor.
• Mulching: A 3–5 cm mulch of chipped bark or leaf litter protects the rhizomes, conserves moisture, and slowly releases nutrients.
• Watering: Establish new plantings with regular deep watering during the first growing season; most hardy ferns need only occasional supplemental irrigation once established.

Conservation & Collector Notes

Dicksonia externa is endemic to the Juan Fernandez Islands and shares the acute conservation predicament of the entire archipelago's endemic flora, which is one of the most threatened island floras in the world. The islands are a Chilean National Park (designated 1935) and UNESCO Biosphere Reserve (designated 1977), and significant conservation investment has been directed toward invasive species control and native habitat restoration over recent decades. The species is not known to be subject to CITES trade regulation, as commercial exploitation is not a threat; the danger is entirely from habitat degradation by invasive organisms. The species' IUCN status may be Vulnerable or has not been formally assessed, but the extremely small total habitat area (the remaining native forest fragments total only a few hundred hectares across the entire archipelago), the ongoing pressure from invasive species despite control efforts, and the vulnerability of the island ecosystem to stochastic events such as wildfire or severe storms mean that the species is at significant and ongoing conservation risk. The co-occurrence of two endemic Dicksonia species on the archipelago (D. externa and D. berteroana) heightens the conservation importance of the island's remaining forest, as the loss of this habitat would result in the simultaneous extinction of both species and represent an irreversible loss of evolutionary history for the genus Dicksonia. The Chilean government, CONAF, and international partners have made substantial progress in invasive species removal, particularly the systematic clearing of blackberry and maqui from critical forest areas, and the replanting of nursery-raised native species into restored sites. The long-term success of these efforts depends on sustained funding, continued community engagement by the small resident population of approximately 900 people on Robinson Crusoe Island, and effective biosecurity measures to prevent the introduction of new invasive organisms.

Collector Notes

Dicksonia externa, together with D. berteroana, represents one of the most compelling tree fern endemism stories in the world: two sister species confined to a tiny oceanic archipelago in the south-eastern Pacific. Herbarium material is held at the Museo Nacional de Historia Natural in Santiago, the Royal Botanic Gardens Kew, and institutions with Pacific and Chilean collections. Living material outside Chile is virtually unknown. The species pair is a high priority for ex situ conservation through spore banking and propagation in botanical gardens, and any international collaboration with Chilean conservation authorities to establish living collections would be of great scientific and conservation value.

Ethnobotany & Cultural Significance

The Juan Fernandez Islands had no indigenous human population before European discovery in 1574, so no traditional ethnobotanical knowledge of the species exists. The islands served as a haven for sailors, pirates, and whalers from the sixteenth century onward, and tree ferns may have been used for temporary shelter and fuel by these transient residents, though no specific records exist for D. externa. The archipelago's modern significance is overwhelmingly ecological and conservational rather than utilitarian. The literary association with Robinson Crusoe gives the islands and their flora a unique cultural resonance that supports conservation advocacy.

Frequently Asked Questions