Introduction & Discovery

Cyathea medullaris is the mamaku, the black tree fern of New Zealand, the tallest and most magnificent of the country's six native tree fern species and one of the most dramatic plants in the entire fern kingdom. Rising to heights of 15 to 20 metres on a jet-black trunk topped by a massive crown of arching fronds spanning 5 to 6 metres, the mamaku dominates the gully forests and moist lowland valleys of New Zealand with an architectural presence that borders on the prehistoric. The species' most striking character is the glossy black colouration of the stipe and rachis, a pigmentation so dark and lustrous that it appears almost lacquered. This black colour extends from the thick stipe base through the entire rachis and onto the primary pinna axes, creating a dramatic visual contrast with the dark green lamina. No other commonly cultivated tree fern shares this character. In te reo Maori, mamaku is a name rich with cultural resonance. The species was a significant food plant for Maori, who harvested the starchy pith (known as korau) from the trunk interior. The pith was extracted by felling the tree fern, splitting the caudex longitudinally, and scraping out the white starchy core, which was then roasted in an earth oven (hangi) or boiled. Korau was a supplementary food rather than a staple, consumed particularly during lean times and on journeys through the forest. The practice of eating mamaku pith is documented in numerous historical accounts and continues in some communities as a cultural tradition. Beyond food, the mamaku holds symbolic significance: its koru (unfurling crozier) represents new life and growth in Maori art, and its massive stature in the forest embodies the strength and generosity of the land. The species' fast growth, often 15 to 30 centimetres of trunk per year under good conditions, is exceptional for a tree fern and contributes to its pioneer ecology, enabling it to colonise forest clearings and grow rapidly toward the canopy.

One of the worlds largest tree ferns, this impressive species is native to lowland rainforests in the South Pacific between Fiji and Pitcairn and south to New Zealand, Stewart Island and the Chatham Islands, where it is a common component of the vegetation and sometimes forms large, monotypic stands. Its large, sturdy trunk can reach a height of 20 m (67 ft.). The spreading, tripinnate fronds can reach a length of 6 m (20 ft.) and are held by thick, black petioles that are covered in dark brown scales especially when young. Even though initially slow to establish from spores, it is eventually very fast growing and is not uncommonly seen in cultivation. It can take only light freezes but will put up with a wide range of temperatures from cool temperate to tropical.

Discovery & Naming

Cyathea medullaris has been known to Maori for centuries before European contact, intimately embedded in cultural practice under its te reo Maori name mamaku. The species was among the first New Zealand tree ferns encountered by European naturalists during the voyages of James Cook in the late eighteenth century. Joseph Banks and Daniel Solander collected fern material during Cook's first voyage, and subsequent botanical expeditions by the Forsters (on Cook's second voyage) and later by Allan Cunningham, William Colenso, and others documented the species in increasing detail. The specific epithet medullaris, from the Latin medulla meaning pith, refers to the starchy central pith of the caudex that was a documented food source for Maori. This connection between the name and the ethnobotanical use is unusually direct in fern taxonomy. Joseph Dalton Hooker's flora of New Zealand in the 1850s consolidated the species' taxonomy, and it has remained stably placed within Cyathea through all subsequent revisions including the molecular phylogenetic work of the early twenty-first century.

Native Range & Distribution Map

Distribution map showing the native range of Cyathea medullaris.

Biology & Frond Morphology

Cyathea medullaris belongs to the genus Cyathea in the family Cyatheaceae, producing bipinnate to tripinnate, very large arching fronds forming a massive dense crown that can span 5 to 6 metres in mature specimens. fronds commonly reach 3 to 6 metres in length, with the largest recorded fronds exceeding 6 metres, making this one of the longest-fronded tree ferns in the world. the most diagnostic feature is the colour of the stipes and rachis: they are a striking, glossy jet black, a character that immediately distinguishes this species from all other new zealand tree ferns and most tree ferns worldwide. the black colouration extends from the stipe base through the rachis and onto the costae of the primary pinnae. stipe scales are dark brown to black, narrowly lanceolate, 15 to 25 millimetres long. blades are dark green above, mid-green beneath, lacking the silver undersurface of cyathea dealbata. croziers are large and dramatically black-scaled during unfurling. 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 Cyathea 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 Cyathea 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 species' eventual size is the most common and consequential error. Cyathea medullaris reaches 8 to 20 metres in height with a crown span of 5 to 6 metres: it is a canopy tree, not a border plant, and planting it within 3 metres of a house, fence, or pathway will create serious management problems within a decade as the massive fronds encroach on structures and walking spaces. Allow a minimum planting radius of 3 to 4 metres from any structure, and ideally 5 metres. Insufficient moisture is the second major error: the species' large crown, carrying 10 to 20 fronds each potentially 4 to 6 metres long, transpires enormous volumes of water, and inadequate watering leads to rapid frond decline, stunted growth, and loss of the vigorous growth rate that is one of the species' most impressive characteristics. Daily trunk hosing is essential in any climate where humidity falls below 70 percent, as the adventitious root system on the caudex surface is the primary pathway for moisture uptake and must remain moist at all times. Planting in cold, exposed gardens without frost protection results in crown death: the species is significantly less frost-hardy than Cyathea dealbata (tolerating only minus 3 to minus 5 degrees Celsius versus minus 5 to minus 8 for dealbata), and since Cyathea species cannot regenerate from the trunk, loss of the crown means loss of the plant. Expecting the silver frond undersurface is a common confusion: mamaku (C. medullaris) fronds are plain green beneath, and the silver fern is Cyathea dealbata (ponga). The two species are frequently confused in popular media and tourism marketing. Neglecting to feed this vigorous species limits its growth potential: unlike most tree ferns, which are slow growers content with light fertilisation, the mamaku benefits from twice-yearly slow-release feeding supplemented by monthly liquid fertiliser during the growing season.

Seasonal Considerations

Spring (September to November): vigorous growth flush begins. New croziers unfurl rapidly. Apply first slow-release fertiliser (50 to 80 grams). Resume deep watering two to three times weekly and daily trunk hosing. Remove winter-damaged fronds. Renew mulch. Summer (December to February): peak growth. Trunk elongation visible. Continue watering, hosing, and monthly liquid feeding. Spore collection possible. Provide shade if temperatures exceed 30 degrees Celsius. Autumn (March to May): growth slows. Apply second fertiliser dose in early autumn. Maintain watering. In cold zones, prepare winter protection. Remove tattered fronds. Winter (June to August): dormant or semi-dormant. Protect crown with mulch and fleece in frost zones. Water sparingly. No fertiliser.

Diseases & Pests

Common issues affecting Cyathea 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

Cyathea 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

Cyathea 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

Cyathea medullaris is classified as Not Threatened under the New Zealand Threat Classification System. It is widespread and locally abundant throughout the North Island and the northern and western South Island, with healthy population trends and vigorous natural recruitment in disturbed forest and regenerating bush. The species also occurs on the Kermadec Islands, Fiji, Samoa, Tonga, and other Pacific island groups where it is generally common in lowland moist forest. All Cyathea species, regardless of their individual conservation status, are listed on CITES Appendix II (Convention on International Trade in Endangered Species of Wild Fauna and Flora), which regulates all international commercial trade in live plants, spore, and processed tree fern products and requires export permits from the country of origin. Within New Zealand, the species benefits from protection within an extensive national park and reserve system covering approximately one-third of the country's land area. The principal long-term conservation concern is habitat loss through continued deforestation in the Pacific islands, where lowland forests are being converted to agriculture at an accelerating rate, and the potential effects of climate change on rainfall patterns and storm frequency in New Zealand, which could alter the moist gully habitats the species prefers. However, the species' vigorous pioneer ecology, prolific spore production, fast growth rate, and ability to colonise disturbed ground provide considerable demographic resilience that sets it apart from the more habitat-sensitive Cyathea species. Unlike the gully specialist Cyathea cunninghamii or the montane Cyathea colensoi, the mamaku is capable of exploiting human-modified landscapes and is likely to persist in the long term even under moderate climate change scenarios.

Collector Notes

Cyathea medullaris is a must-have for any serious tree fern collection, prized for its record height, dramatic jet-black stipes, and deep cultural significance. All Cyathea species are listed on CITES Appendix II. Plants are available from specialist nurseries in New Zealand and Australia, and spore is obtainable through fern society exchanges. Provenance documentation is valuable; material from the Kermadec Islands, the Chatham Islands, or the Pacific island populations may represent genetically distinct stocks. In British collections, the species is grown under glass at Kew, Edinburgh, and in sheltered outdoor sites in Cornwall and the Isles of Scilly. For American collectors, the California coast and Pacific Northwest offer the best climate matches, though winter protection is necessary. The species' fast growth (15 to 30 centimetres of trunk per year) means that a specimen trunk can develop within a decade from spore, faster than any other Cyathea in this encyclopaedia.

Ethnobotany & Cultural Significance

The mamaku (Cyathea medullaris) holds profound significance in Maori culture as a food source, a symbol of the forest's bounty, and a living embodiment of cultural values. The starchy pith of the trunk, known as korau or mamaku, was an important supplementary food for Maori. Harvesting involved felling the tree fern, splitting the caudex longitudinally, and scraping out the white starchy core from within the fibrous vascular cylinder. The pith was then prepared by roasting in a hangi (earth oven), boiling, or steaming. The taste has been described as bland and starchy, comparable to a fibrous potato. Korau was particularly valued as a food during forest journeys and during times of scarcity, and its availability was a factor in the management and valuation of forest resources. The practice is documented in numerous historical accounts from the earliest European contact period and continues to be practised in some communities as a cultural tradition. Beyond food, the mamaku features in Maori oral tradition, carving, and the broader cultural relationship with te ngahere (the forest). The koru, the spiral of the unfurling crozier, is among the most important motifs in Maori art, symbolising new life, growth, strength, and peace. The koru of the mamaku, being the largest of any New Zealand fern, is particularly dramatic and powerful as a visual symbol. The black colouration of the stipes has also been noted in the naming of the New Zealand women's rugby team, the Black Ferns, which draws on the mamaku's distinctive appearance.

Frequently Asked Questions