Introduction & Discovery

Sphaeropteris medullaris, known in New Zealand as mamaku, is the tallest tree fern in Aotearoa and one of the tallest in the world, with mature specimens reaching 15 to 20 metres in sheltered bush — towering columns of jet-black trunk crowned with enormous spreading fronds that define the character of the New Zealand bush as powerfully as the pohutukawa or the rimu. The name mamaku comes from te reo Maori, and the species holds deep cultural significance for Maori as a food source, a navigational aid, and a symbol of the forest. The species name medullaris refers to the medulla, the soft pithy core of the trunk, which was historically an important carbohydrate source for Maori. Under modern phylogenetic classification, this species sits at the heart of the Sphaeropteris nomenclatural debate. It has been widely known as Cyathea medullaris under the broad Cyathea concept adopted by the PPG I classification of 2016, but molecular phylogenetic analyses by Korall and colleagues have provided robust support for recognising Sphaeropteris as a monophyletic genus distinct from Cyathea sensu stricto. Sphaeropteris species are characterised by large, pale, conforming scales on the stipe, typically absent or reduced indusia, softer trunk tissue, and often faster growth rates — all characters that distinguish S. medullaris from the stiffer, darker-scaled, typically slower Cyathea species sensu stricto. The jet-black stipe and rachis are the single most dramatic identification feature, immediately separating S. medullaris from all other tree ferns in New Zealand and worldwide. All Cyatheaceae, including Sphaeropteris, are listed on CITES Appendix II, and international trade in any part of the plant requires appropriate export documentation. For gardeners in mild maritime climates, S. medullaris offers one of the most spectacular tree fern experiences available: a species that combines massive scale, fast growth, jet-black architecture, and deep cultural significance into a single extraordinary plant.

Sphaeropteris medullaris, synonym Cyathea medullaris, commonly known as mamaku or black tree fern, is a large tree fern up to 20 m (66 ft) tall with a trunk up to 20 cm diameter at breast height. It is distributed across the south-west Pacific from Fiji to Pitcairn Island and is a common plant found in forests of New Zealand.

Discovery & Naming

Sphaeropteris medullaris was among the first New Zealand tree ferns described to European science, collected during the voyages of exploration that brought European naturalists to the Pacific in the late eighteenth and early nineteenth centuries. The species was described by Georg Forster from material collected during Captain James Cook's second voyage of 1772 to 1775, and the soft pithy trunk that inspired the epithet medullaris was noted by early observers. The species has been known under various generic placements: Polypodium medullare in the earliest literature, subsequently transferred to Cyathea and Alsophila by different authors. The placement in Sphaeropteris follows the molecular phylogenetic evidence from Korall and colleagues demonstrating the monophyly of the pale-scaled clade. The PPG I classification of 2016 retained the species in Cyathea sensu lato, but many pteridologists favour Sphaeropteris based on the strength of the molecular and morphological evidence. Living plants reached European cultivation in the nineteenth century, primarily through introductions from New Zealand by collectors associated with Kew and other major botanic gardens. The species has been cultivated in the mildest parts of Britain since the late Victorian era and is now grown in sheltered gardens across the Atlantic seaboard of Europe. Its Maori name mamaku and its cultural significance have become increasingly recognised in international horticultural and botanical circles. The early European naturalists who encountered the species were impressed by both its stature and the curious black colouration of the stipes, which one early commentator described as resembling polished ebony. The combination of dramatic appearance and cultural significance has made mamaku one of the most recognised New Zealand plants in the global botanical community, alongside the silver fern (Cyathea dealbata), which serves as a national emblem. The gradual resolution of the generic placement debate — Cyathea versus Sphaeropteris — continues to generate discussion in both scientific literature and horticultural reference works, with the practical result that the species appears under both names in different sources.

Native Range & Distribution Map

Distribution map showing the native range of Sphaeropteris medullaris.

Biology & Frond Morphology

Sphaeropteris medullaris belongs to the genus Sphaeropteris in the family Cyatheaceae, producing bipinnate to tripinnate, forming a massive arching crown of 6 to 12 fronds spanning 4 to 7 metres at maturity, among the largest of any tree fern. individual fronds reach 4 to 6 metres in length in favourable conditions, with stout stipes up to 1.5 metres long and 4 to 6 centimetres in diameter at the base. the stipe and rachis are the primary identification character: they are jet-black to very dark brown, smooth and glossy, with large conforming paleae (scales) that are dark brown at the base but golden-brown along the rachis, readily detaching when handled. the black stipe colouration is diagnostic and immediately distinguishes sphaeropteris medullaris from all other new zealand tree ferns. the blade is bright green above, paler beneath, and of relatively thin texture. pinnae are alternate, 40 to 70 centimetres long, with pinnules 4 to 8 centimetres long that are deeply pinnatifid. the overall crown has a somewhat more open, less densely layered appearance than sphaeropteris cooperi, reflecting the larger individual frond size and lower simultaneous frond count. croziers unfurl with spectacular speed in spring and early summer, and the unfolding fiddleheads with their dark stipes and golden scale coverings are among the most photogenic moments in the fern world. 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 Sphaeropteris medullaris 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 Sphaeropteris medullaris 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

Underestimating the eventual size is the most common error; a tree fern that can reach 15 metres and spread 5 to 7 metres in crown diameter needs significant space. Planting in an exposed windy site shreds the large thin-textured fronds and stresses the plant. Allowing the trunk to dry out, even briefly, risks crown death — the soft pithy trunk has less moisture reserve than Dicksonia. Cold exposure below minus 4 degrees Celsius without crown protection is fatal. Attempting to transplant by cutting the trunk, as works with Dicksonia antarctica, kills the plant — S. medullaris does not regenerate from cut trunks. Full sun in hot dry conditions causes rapid frond scorch. Poor drainage promotes Phytophthora crown rot. Planting too deeply buries and rots the crown meristem. Ignoring the CITES status when sourcing or transporting plants internationally leads to legal complications.

Seasonal Considerations

Spring (September to November in New Zealand, March to May in the Northern Hemisphere): the dramatic growth flush begins. New croziers unfurl with spectacular speed, revealing jet-black stipes and golden scales. Remove winter-damaged fronds. Apply slow-release fertiliser into the crown. Resume regular watering and trunk misting. Watch for slug damage on unfurling fiddleheads. Summer (December to February in NZ, June to August in NH): peak growth period with rapid trunk elongation. Water deeply 2 to 3 times weekly. Overhead trunk misting is valuable in dry weather. Monthly liquid feed. Monitor for scale insects. Provide afternoon shade in hot climates. Autumn (March to May in NZ, September to November in NH): growth slows. Apply second fertiliser dose. Reduce overhead watering. Prepare frost protection in marginal zones. Remove damaged fronds. Winter (June to August in NZ, December to February in NH): in frost-free zones, minimal growth continues. Water monthly to prevent trunk desiccation. In frost-risk zones, wrap crown and upper trunk with fleece when temperatures approach minus 2 degrees Celsius.

Diseases & Pests

Common issues affecting Sphaeropteris medullaris 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

Sphaeropteris medullaris 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

Sphaeropteris medullaris 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

Sphaeropteris medullaris is not considered threatened in New Zealand, where it is common and widespread in suitable forest habitat and actively colonises regenerating bush. It is classified as Not Threatened under the New Zealand Threat Classification System. The species' pioneer ecology means that it benefits from the forest regeneration occurring on retired farmland across much of New Zealand, and populations are likely increasing in many areas. In the Pacific Islands (Fiji, Samoa, Tonga), local populations may be under greater pressure from deforestation and habitat degradation, but no formal conservation assessments have been published for these island populations. All Cyatheaceae are listed on CITES Appendix II, meaning that international trade requires export permits. In New Zealand, tree fern harvesting for the horticultural trade is regulated under the Forests Act and Resource Management Act; sustainable harvest of fronds and limited trunk collection is permitted under appropriate consent. The species is not at risk of over-exploitation in New Zealand given its abundance and fast growth, but the CITES listing ensures that international trade is documented and monitored. The broader conservation context for New Zealand tree ferns includes the impact of introduced mammalian predators (particularly possums, which browse croziers) and competitors on native forest ecosystems. Mamaku benefits from predator control programmes that reduce possum damage to native bush, and the ongoing expansion of conservation-managed public and private land across New Zealand provides increasing habitat for tree fern populations. Climate change projections for New Zealand suggest that the species' range may expand southward and to higher elevations as temperatures increase, though the loss of reliable rainfall in some eastern regions could locally reduce suitable habitat.

Collector Notes

Sphaeropteris medullaris is a must-have for the serious tree fern collector, offering the unique combination of jet-black stipes, massive scale, rapid growth, and deep Maori cultural significance that no other species replicates. In New Zealand, the species is widely available from native plant nurseries at affordable prices. Outside New Zealand, sourcing requires specialist channels: CITES-compliant nurseries, pteridological society networks, or botanic garden seed banks. Spore is occasionally available through the New Zealand Fern Society and international exchanges. Wild-provenance material from different parts of the New Zealand range and from Pacific Island populations offers opportunities for comparative cultivation and genetic diversity preservation. Named cultivars do not exist. The species is best displayed alongside other New Zealand natives — Cyathea dealbata, Dicksonia squarrosa, Rhopalostylis sapida — for a biogeographically coherent southern hemisphere temperate rainforest collection. The jet-black stipes retain their colour indefinitely when dry and have decorative value in dried floral arrangements, though harvesting from wild trees is regulated in New Zealand. For living collections, mature mamaku with visible black trunk scars and a full crown of dark-stiped fronds is one of the most visually striking tree ferns in cultivation, and its presence elevates any collection from merely interesting to genuinely dramatic. Specimens in accessible public collections include plantings at the Wellington Botanic Garden, the Auckland Domain, and in Britain at Tresco Abbey Garden and the Lost Gardens of Heligan.

Ethnobotany & Cultural Significance

Sphaeropteris medullaris, mamaku, holds a central place in Maori culture and traditional knowledge. The soft, starchy pith of the trunk was an important traditional food source, prepared by felling the tree fern, splitting the trunk, extracting the white pithy core, and cooking it in earth ovens or boiling it in water. The cooked pith has a bland, starchy flavour and served as a carbohydrate staple in areas where kumara (sweet potato) cultivation was marginal. Mamaku pith was also used medicinally: poultices of the pith were applied to wounds and skin irritations, and decoctions were used internally for digestive complaints. The dark frond stipes were used as building material and as markers in bush navigation — the black stipes are conspicuous against the green forest and served as trail indicators. In Maori cosmology, tree ferns feature in traditions relating to the creation of the forest and the relationship between the sky father Ranginui and the earth mother Papatuanuku. The contemporary Maori renaissance has renewed interest in traditional uses of mamaku, and the species features in cultural education programmes, native bush restoration projects, and indigenous food sovereignty initiatives. European settlers in New Zealand used tree fern trunks extensively for fencing, garden borders, and rough construction, and the fibrous outer mantle of the trunk was used for wall panels in bush huts — a practice documented throughout the colonial period. In modern New Zealand, mamaku is valued as a cultural taonga (treasure) and a symbol of the native forest heritage.

Frequently Asked Questions