Introduction & Discovery

Phymatosorus grossus, the lava fern, is a conspicuous tropical Polypodiaceae of the Indo-Pacific region whose thick fleshy rhizomes and large pinnate fronds make it one of the most architecturally distinct and ecologically versatile ferns in tropical cultivation. The common name lava fern records the plant's notable habit of colonising recent basalt lava flows in parts of its range, particularly in the Hawaiian Islands and elsewhere in the volcanic Pacific, where it establishes on young lithic substrates alongside Metrosideros and other pioneer vegetation. This ecological aptitude for harsh new surfaces reflects the plant's adaptations for lithophytic and epiphytic life: the thick water-storing rhizome anchors to rock or bark with tough fibrous roots, the coriaceous fronds resist desiccation, and the scaly stipe bases tolerate exposure to wind and sun. The species belongs to a complex taxonomic group centred on the genus Phymatosorus as segregated from the broader Microsorum sensu lato, and in many current treatments the plant is referred to as Microsorum grossum or as a synonym of Microsorum scolopendria, depending on whether Phymatosorus is recognised as a distinct genus. Recent molecular work has supported the recognition of Phymatosorus as a monophyletic lineage separable from Microsorum on the basis of sorus placement (sunk in pits or depressions in Phymatosorus, superficial in typical Microsorum), frond architecture and rhizome anatomy, and this generic separation is reflected in current editions of standard floras. For the gardener the nomenclatural complexity matters less than the distinctive look and cultivation of the plant: the stout creeping rhizome with its dense dark scales, the glossy pinnate fronds, and the ease with which the species grows as an epiphyte or lithophyte on bark, cork, tree fern slabs or lava rock make it a standout in tropical fern collections. The sister species P. scolopendria is more widely cultivated and culturally significant in Polynesia (laua'e, monarch fern, maile-scented fern), but P. grossus has its own devoted following among collectors of coarse-textured architectural tropical ferns. Native across the Indo-Pacific from Sri Lanka and southern India through Southeast Asia and Australasia to the Pacific islands, the species is a familiar presence in tropical gardens, conservatory collections and upscale houseplant displays worldwide.

Discovery & Naming

The species now known as Phymatosorus grossus has a tangled taxonomic history reflecting the broader uncertainty of the Microsorum-Phymatosorus-Polypodium complex throughout the nineteenth and twentieth centuries. The plant was first formally described by Langsdorff and Fischer in their work on Brazilian ferns in the 1810s as Polypodium grossum, based on material from tropical origins collected during early nineteenth century plant exploration. Subsequent treatments moved the species through Polypodium sensu lato, Microsorum as broadly defined by Link and later Copeland, and Phymatodes before the genus Phymatosorus was established by Pichi Sermolli in 1973 to accommodate a group of Indo-Pacific species characterised by sunken sori in distinct pits in the blade tissue. The combination Phymatosorus grossus (Langsd. & Fisch.) Brownlie was published by Brownlie in 1977. Recent treatments of the Polypodiaceae following the Pteridophyte Phylogeny Group I classification have variously accepted Phymatosorus as a distinct genus separate from Microsorum or subsumed it within a broader Microsorum. Molecular phylogenetic studies by Schneider, Schuettpelz, Janssen, Kreier and others in the 2000s and 2010s have generally supported the recognition of Phymatosorus as a monophyletic lineage separable from Microsorum, and the current standard treatment in many floras and taxonomic databases maintains the distinction. The alternative name Microsorum grossum Brownlie is encountered in some recent literature and horticultural references reflecting treatments that subsume Phymatosorus. The species has been cultivated continuously in European botanical collections since the nineteenth century and in tropical Pacific horticulture for centuries longer, and it has a permanent place in tropical fern culture regardless of the current consensus generic placement.

Native Range & Distribution Map

Distribution map showing the native range of Phymatosorus grossus.

Biology & Frond Morphology

Phymatosorus grossus belongs to the genus Phymatosorus in the family Polypodiaceae, producing evergreen, pinnate or pinnatifid fronds of thick coriaceous to subcoriaceous texture, deep glossy green above and matte paler green beneath, arising at intervals along a stout long-creeping rhizome that extends across the substrate producing loose colonies rather than tight crowns. mature fronds typically measure 30 to 80 centimetres long in cultivated specimens and occasionally exceed 100 centimetres in optimal wild conditions, bearing 3 to 8 pairs of large lanceolate pinnae plus a terminal pinna. pinnae are 10 to 25 centimetres long and 2 to 5 centimetres wide, with entire or undulate margins and acuminate apices, joined to the rachis by a broadly decurrent base. the overall frond architecture suggests a simplified nephrolepis or a miniature polypodium rather than the finely divided ferns common in conservatory culture. stipes are stout, dark brown to nearly black at the base fading to tan above, 10 to 30 centimetres long, firmly attached to the creeping rhizome by an articulation (phyllopodium). rhizome is conspicuously thick (5 to 12 millimetres in diameter), densely clothed in dark reddish-brown lanceolate scales, and visible creeping across the substrate. 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 Phymatosorus grossus 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 Phymatosorus grossus 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

Several recurring errors compromise success with Phymatosorus grossus. Burying the creeping rhizome beneath substrate is the most frequent structural mistake; the rhizome is adapted to surface creeping with roots penetrating the substrate below, and burying it invites rot. Plant with the rhizome exposed at the surface. Exposure to frost or sustained temperatures below 10 degrees Celsius causes damage or death in most situations; the plant is not cold-hardy and outdoor cultivation is confined to zones 10 and warmer. Allowing the substrate to waterlog is fatal; the plant requires excellent drainage and tolerates brief drying better than sustained wetness. Using a heavy poorly-aerated substrate such as fine peat or garden soil produces root decline; the species demands a free-draining epiphyte-style mix or substrate. Placement in direct sun through south or west windows in indoor culture scorches fronds; use bright indirect light. Attempting to grow the plant as a conventional terrestrial houseplant in a standard pot often produces poor results; the creeping rhizome requires either a shallow wide pot, a hanging basket, or an epiphytic mount to display its natural habit and perform well. Spacing individual rhizome sections too closely in mass plantings leads to overcrowding within 2 to 3 years; allow 30 to 50 centimetre spacing. Expecting fine variegation or elaborate cultivar variation is unrealistic; the species is grown essentially in its wild form with little cultivar development. Purchasing the plant mislabelled as Microsorum or as a common houseplant fern and then treating it as such can produce disappointing results; understand the epiphytic creeping habit for best success.

Seasonal Considerations

In tropical outdoor cultivation the plant follows a seasonal rhythm driven largely by wet- and dry-season rainfall patterns rather than temperature. Maintain consistent watering during any dry season and allow the plant to benefit from natural rainfall in the wet season. In temperate indoor and conservatory culture a modest seasonal rhythm improves plant performance. Spring from March through May is the period of most active new frond production; resume monthly feeding at quarter to half strength, increase watering frequency, and consider repotting or re-mounting if the plant has outgrown its support. Summer from June through August brings maximum growth rates; maintain consistent watering, feeding, and humidity; outdoor plants benefit from regular misting or light showers. Watch for spider mite colonisation on indoor plants as household humidity drops in air-conditioned settings. Autumn from September through November brings a modest slowing of growth; reduce watering frequency slightly and cease feeding by mid-October. Spore production is ongoing through this period and mature sori may be collected for propagation. Winter from December through February is the rest period for indoor culture; maintain minimum temperatures above 12 degrees Celsius, water less frequently but do not let the substrate dry completely, withhold fertiliser, and avoid cold drafts. The species continues slow frond production through winter in warm conditions but the pace is much reduced. Resume full cultural schedule from late February as daylight lengthens. Mounted epiphytic plants require regular misting year-round adjusted for seasonal temperature variation.

Diseases & Pests

Common issues affecting Phymatosorus grossus 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

Phymatosorus grossus 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

Phymatosorus grossus 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

Phymatosorus grossus is considered globally secure across its wide Indo-Pacific range, with populations widespread and locally abundant in suitable habitat. The species has not been formally evaluated in detail on the IUCN Red List at the species level, but would qualify for Least Concern based on extent of occurrence, population size, habitat availability and ecological flexibility. No international trade controls apply under CITES. The plant occupies a sufficient range of primary, secondary and anthropogenic habitats that population persistence is essentially assured so long as tropical forest, rocky coastal habitats and garden landscapes persist; the species is not dependent on any narrow or vulnerable habitat type. Local pressures from rapid urbanisation and tropical deforestation affect individual populations but do not threaten the species globally. Some Pacific island populations, particularly those that participate in early colonisation of volcanic lava flows, have special biogeographic and ecological significance but are not otherwise of conservation concern. The species occurs in many tropical protected areas across its range. Commercial trade is essentially entirely from nursery stock (rhizome division or spore-raised); wild collection is not a conservation concern. Climate change projections suggest potential range shifts at high-elevation tropical margins and possible range contraction in drying tropical zones, but the broad climate tolerance of the species buffers against significant range reduction on a century timescale. The sister species P. scolopendria is more widely distributed and similarly secure.

Collector Notes

For collectors of tropical Polypodiaceae and architectural epiphytic ferns, Phymatosorus grossus is a foundational species and a reliable specimen for long-term display. The plant is widely available in the specialist tropical fern nursery trade and in well-stocked tropical houseplant retailers worldwide. Specialist suppliers in Europe include Farnwerk in Germany, Dutch and Belgian nursery operations, and UK tropical plant specialists; in North America Glasshouse Works, Asiatica Nursery, Black Jungle Terrarium Supply and various tropical houseplant mail-order sources stock the species regularly. In Australia, New Zealand and Pacific tropical regions the plant is widely available through native and tropical nursery trade. No significant named cultivars are in commerce; the species is grown essentially in its wild-type form. Occasional variant forms with slightly different pinna shape, rhizome thickness or frond texture have been noted from particular Pacific island or Asian collection localities but none have become established named cultivars. Collectors interested in provenance variation can sometimes source material of documented origin through botanical garden exchange networks and specialist tropical fern society connections including the International Tropical Fern Society and regional Pacific plant societies. The plant performs well as a long-term specimen in tropical conservatories and can be trained into impressive mounted or basket displays over 10 to 20 years of culture. Notable mature specimens in public collections include the tropical fern displays at the Royal Botanic Gardens Kew Temperate and Palm Houses, the Fairchild Tropical Botanic Garden, the Hawaii Tropical Botanical Garden, Bogor Botanic Garden in Indonesia, and numerous Pacific and Southeast Asian botanical collections.

Ethnobotany & Cultural Significance

Phymatosorus grossus has a modest ethnobotanical record in parts of its Indo-Pacific range, generally less prominent than the closely related P. scolopendria which occupies a central cultural position in Polynesian tradition. In parts of Southeast Asia including Malaysia, Indonesia and the Philippines, the rhizome has been used locally in traditional medicine for preparations applied to wounds and as general tonics, though specific species identification in ethnobotanical records is often uncertain given the overlap with related Phymatosorus and Microsorum species. In tropical Pacific cultures the species has been used variously for decorative greenery, as a component of lei and other floral arrangements (though typically less favoured than P. scolopendria for its less fragrant dried foliage), and as an ornamental plant in home gardens. No significant food use is documented. The plant has no single concentrated ethnobotanical or cultural association comparable to those of some other Polynesian plants but appears generally in the broad inventory of useful and ornamental species maintained by indigenous tropical communities. In modern horticulture the species has been continuously cultivated in tropical botanical collections and the upscale houseplant trade since the late nineteenth century, and it has experienced renewed popularity since the 2010s as interest in architectural epiphytic plants has grown among collectors and commercial interior plantscapers. Its ecological association with lava flows in the Pacific and the evocative common name lava fern have contributed to its appeal in volcanic-themed garden designs in Hawaii and similar settings. The species is occasionally mentioned in discussions of Polynesian ethnobotany in reference to the broader use of Pacific ferns in traditional culture.

Frequently Asked Questions