Introduction & Discovery

Dicksonia arborescens is the tree fern of Saint Helena, one of the most remote and historically evocative islands in the world and the place of Napoleon Bonaparte's exile and death. This species is far more than a botanical curiosity; it is the living emblem of an entire island ecosystem that has been devastated by five centuries of human impact and is now the focus of one of the most ambitious conservation restoration projects in the South Atlantic. Before the arrival of Portuguese sailors in 1502, Saint Helena's highland plateau was clothed in a lush endemic cloud forest dominated by this tree fern and a suite of unique flowering plants, ferns, mosses, and invertebrates found nowhere else on Earth. Deforestation for timber and fuel, browsing by introduced goats, and invasion by aggressive exotic plants reduced this forest to tiny fragments clinging to the steepest and most inaccessible parts of the Diana's Peak ridge. Dicksonia arborescens survived in these remnants, its trunks rising above a carpet of endemic mosses and filmy ferns in a landscape that evokes the primeval forests of a world before human dominion. Classified within the family Dicksoniaceae and distinguished from the Cyatheaceae by its hairy rather than scaly stipe bases and its marginal bivalved sori, the species represents a Gondwanan lineage that colonised this isolated volcanic speck in the South Atlantic by long-distance spore dispersal, an event of extraordinary improbability that underscores the notable capacity of fern spores to cross oceanic barriers. Today, Dicksonia arborescens is classified as Vulnerable by the IUCN and is the subject of active propagation and replanting efforts coordinated by the Saint Helena National Trust and international conservation partners, offering hope that this ancient survivor may yet reclaim its place in a restored highland forest.

Dicksonia arborescens, the Saint Helena tree fern, is a characteristic plant of the "tree fern thicket" vegetation of the highest parts of the central ridge of the island of Saint Helena. It is the type species of the genus Dicksonia.

Discovery & Naming

Dicksonia arborescens was described by Carl Linnaeus in 1753 in Species Plantarum, making it one of the earliest tree fern species to enter the scientific literature. The material on which Linnaeus based his description was collected during one of the early botanical visits to Saint Helena, and the species has been consistently recognised under its original binomial since that time. The island's flora attracted botanical attention from the seventeenth century onward, with Joseph Dalton Hooker publishing a comprehensive account of Saint Helena's plants in 1867 that documented the alarming decline of the endemic vegetation. Subsequent botanical expeditions throughout the nineteenth and twentieth centuries recorded the progressive shrinkage of the cloud forest and the species' declining population. The genus Dicksonia was named by L'Heritier de Brutelle in 1789, honouring the English nurseryman and botanist James Dickson. Molecular phylogenetic studies have confirmed the placement of D. arborescens within Dicksoniaceae and its generic affinity with other Dicksonia species, while also highlighting the deep evolutionary divergence between this isolated South Atlantic lineage and its distant relatives in Australasia and the Americas.

Native Range & Distribution Map

Distribution map showing the native range of Dicksonia arborescens.

Biology & Frond Morphology

Dicksonia arborescens belongs to the genus Dicksonia in the family Dicksoniaceae, producing evergreen, bipinnate to tripinnatifid fronds forming a crown of 10 to 20 fronds at the trunk apex, typically 1.5 to 2.5 metres long. blades are broadly lanceolate, dark green, and moderately leathery in texture. pinnae are deeply pinnatifid to pinnate, with oblong pinnules bearing entire to crenate margins. stipes are stout, dark at the base, and densely clothed with soft rufous-brown multicellular hairs characteristic of the dicksoniaceae family, clearly distinguishing the species from any scale-bearing cyatheaceae. young croziers are enveloped in dense tawny-brown woolly indument. sori are marginal, positioned at the tips of pinnule veins, protected by bivalved indusia with an outer valve of reflexed marginal tissue and an inner true indusial valve. the overall habit is of a small to medium tree fern with a relatively compact crown adapted to the windy, exposed conditions of the saint helena highland ridge. 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 arborescens 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 arborescens 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

Treating Dicksonia arborescens as equivalent to D. antarctica in terms of cold-hardiness is the most consequential error; this species cannot tolerate frost and should not be planted outdoors in any climate where freezing temperatures occur. Underestimating humidity requirements leads to rapid frond desiccation and crown death. Allowing invasive species to overwhelm planted specimens in conservation settings has been a historical problem on Saint Helena. In glasshouse cultivation, poor ventilation combined with high humidity promotes fungal crown rot. The species requires consistent equable conditions; large temperature fluctuations between day and night or between seasons stress the plant and reduce vigour. Given the species' conservation significance, any cultivated specimens should be managed with the highest standard of care and their provenance documented for potential future conservation use.

Seasonal Considerations

Saint Helena's oceanic climate provides minimal seasonal variation, with temperatures ranging from about 14 to 22 degrees Celsius year-round and rainfall distributed relatively evenly. In cultivation in the Northern Hemisphere, spring and summer require monitoring to ensure temperatures do not exceed 25 degrees Celsius, with shading and ventilation as needed. Maintain constant humidity and trunk moisture throughout the year. In autumn and winter, supplemental heating may be necessary in glasshouse cultivation to maintain temperatures above 8 degrees Celsius. There is no dormant season, and the species may produce new fronds at any time of year. Year-round vigilance against fungal pathogens is necessary in the high-humidity growing environment. In conservation replanting on Saint Helena, maintenance focuses on keeping planted sites free of invasive species and monitoring survival and growth of transplanted specimens.

Diseases & Pests

Common issues affecting Dicksonia arborescens 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 arborescens 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 arborescens 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 arborescens is classified as Vulnerable on the IUCN Red List, though many conservation biologists argue that this assessment may understate the true level of threat given the extremely small remaining habitat area and ongoing pressures. The total population is estimated at several thousand mature individuals, confined to a few hectares of remnant cloud forest on the Diana's Peak ridge. 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 loss and degradation. The Saint Helena government, the Saint Helena National Trust, and international conservation partners including the Royal Botanic Gardens Kew and the Falklands Conservation have invested significant resources in cloud forest restoration, including propagation and replanting of D. arborescens and associated endemic species. Invasive species removal, particularly of New Zealand flax, has created space for replanting, and fencing excludes remaining feral animals. Climate change poses a long-term threat through potential alteration of the trade wind patterns and orographic cloud formation that sustain the cloud forest. The species' survival depends entirely on the success of the ongoing restoration programme and the continued commitment of the island's government and international partners to maintaining the highland cloud forest ecosystem.

Collector Notes

Dicksonia arborescens is one of the most desirable species for institutional fern collections, combining extreme rarity, extraordinary biogeographic interest, and urgent conservation significance. Living specimens in ex situ collections are exceedingly few, with documented plants at the Royal Botanic Gardens Kew, a small number of other European botanical gardens, and the conservation nursery on Saint Helena itself. Any institution acquiring this species should treat it as a conservation asset and participate in coordinated conservation efforts. Herbarium material is held at Kew, the Natural History Museum London, and in collections made during various scientific expeditions to Saint Helena. The species is a priority for ex situ conservation under the Global Strategy for Plant Conservation, and any opportunity to establish additional living collections from legally obtained spore material should be pursued. The story of Dicksonia arborescens and the Saint Helena cloud forest is one of the most compelling narratives in plant conservation and deserves wider recognition.

Ethnobotany & Cultural Significance

The early human history of Saint Helena's cloud forest is one of destruction rather than sustainable use. When Portuguese mariners discovered the uninhabited island in 1502, the highland forest was intact. Timber was cut for ship repair and fuel, goats were released to provide fresh meat for passing ships, and systematic deforestation for agriculture and settlement followed during the East India Company's administration from 1659. Dicksonia arborescens trunks were used as building material and the fronds as animal bedding, but no sophisticated ethnobotanical tradition developed around the species, as Saint Helena had no indigenous human population. During Napoleon's exile on the island from 1815 to 1821, the deforestation was already advanced, and contemporary accounts describe the denuded state of the highlands. The most significant modern human interaction with D. arborescens is the conservation programme to save it and its ecosystem from extinction. The species has become a symbol of Saint Helena's endemic natural heritage and a focal point for environmental education on the island, appearing in local conservation literature and interpretive materials.

Frequently Asked Questions