Introduction & Discovery

Microsorum grossum is the great coloniser of the Pacific's volcanic landscapes, a robust, leathery-fronded fern that stakes its claim on black lava flows while the rock is scarcely cooled, building the first living layer of vegetation on substrates that appear utterly hostile to plant life. In Hawaii, where it is known colloquially as the Lava Fern, M. grossum is one of the earliest vascular plants to appear on new lava, its wind-dispersed spores settling in cracks and vesicles of the rock, germinating in the minimal moisture trapped by condensation and rain, and producing tough, coriaceous fronds that withstand the intense sun, heat, and wind of the open volcanic terrain. This pioneering ability has made it a symbol of botanical resilience in the Hawaiian cultural and ecological imagination. The species belongs to Polypodiaceae and has been treated under various names in the complex taxonomic history of Microsorum, including Phymatosorus grossus and Phymatosorus scolopendria in part, though modern molecular phylogenetic work supports its recognition as a distinct species within a broadly defined Microsorum. The simple, entire, glossy fronds can reach a metre in length in sheltered forest conditions, and the thick rhizome creeping across rock surfaces or through forest litter is densely clothed in scales and capable of storing sufficient water to survive the droughty conditions of its pioneering habitat. For the tropical fern grower, M. grossum offers an easy, rewarding species with strong architectural presence, suitable for garden beds, rock features, and containers in warm climates, and for warm greenhouse or conservatory culture in cooler regions.

Discovery & Naming

The species has been known to Western science since the early nineteenth century, collected by botanical explorers visiting the Pacific islands. It has been treated under numerous names reflecting the complex taxonomic history of the Microsorum-Phymatosorus complex: Polypodium grossum, Phymatosorus grossus, Microsorum grossum, and Microsorum scolopendria in part. The epithet grossum means thick or coarse, referring to the robust, coriaceous frond texture. Modern molecular phylogenetic studies have clarified relationships within the complex but taxonomic opinions on generic boundaries continue to differ, with some authors placing the species in Phymatosorus and others retaining it in Microsorum sensu lato.

Native Range & Distribution Map

Distribution map showing the native range of Microsorum grossum.

Biology & Frond Morphology

Microsorum grossum belongs to the genus Microsorum in the family Polypodiaceae, producing evergreen, monomorphic to slightly dimorphic, simple entire fronds of moderate to large size, typically 30 to 80 centimetres long and 4 to 10 centimetres wide, occasionally exceeding one metre in sheltered humid sites. blades are broadly lanceolate to strap-shaped, simple and undivided with entire to slightly undulate margins, tapering gradually at both base and apex. texture is thick-coriaceous, leathery and firm, with a glossy dark green upper surface and a paler matte underside. the stipe is short, stout, dark brown at the base grading to green, one-tenth to one-fifth the frond length. fronds arise in dense tufts from the rhizome apex or along a short-creeping rhizome, producing a robust rosette or shuttlecock habit. fertile fronds may be slightly narrower than sterile fronds. the thick, leathery texture is an adaptation to exposure on open lava fields where wind, sun, and intermittent drought stress are significant. 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 Microsorum grossum 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 Microsorum grossum 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

Exposing to frost, which is fatal. Overwatering in heavy, poorly-drained substrate causes root rot. Growing in deep shade when the species tolerates and benefits from moderate light. Under-potting, as the robust rhizome system needs a proportionately large container. Confusing with M. scolopendria and expecting lobed fronds; M. grossum maintains simple entire blades.

Seasonal Considerations

In tropical climates there is no distinct seasonal cycle. Maintain consistent care year-round. Water regularly, feed monthly. In subtropical and warm-temperate climates, growth slows in winter with shorter days and cooler temperatures. Reduce watering and cease feeding from November through February. Resume active care in spring as temperatures rise. For indoor culture in temperate climates, maintain minimum temperature above 15 degrees and provide supplemental light during short winter days.

Diseases & Pests

Common issues affecting Microsorum grossum 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

Microsorum grossum 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

Microsorum grossum 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

Microsorum grossum is globally secure across its extensive Pacific range. It is common and widespread from Australia to Hawaii and is not listed as threatened by any agency. Populations on active volcanic islands are dynamic, naturally colonising new lava surfaces as they form. No conservation concerns apply to this widespread and ecologically resilient species.

Collector Notes

Microsorum grossum is of ecological and biogeographic interest as one of the most successful fern colonisers of volcanic substrates in the Pacific. Hawaiian provenance material from actively volcani landscapes is particularly sought after for its cultural and ecological associations. The species complements collections of Pacific ferns and of Microsorum diversity more broadly. Material is available from Hawaiian and Australian tropical fern nurseries. Provenance documentation with island and habitat data enhances collection value.

Ethnobotany & Cultural Significance

In Hawaii, the Lava Fern is culturally recognized as one of the first plants to colonise new lava, symbolising renewal and the regenerative power of the land after volcanic eruption. Hawaiian cultural practitioners note its significance in the cycle of volcanic destruction and regrowth that defines the Hawaiian landscape. In Polynesian ethnobotany more broadly, Microsorum species have been used for wrapping food, as components of traditional medicine for skin ailments and digestive complaints, and as ornamental plants in village gardens. Specific medicinal uses vary by island group and are not always attributed to M. grossum rather than the closely related M. scolopendria. The rhizome has been used in some Polynesian traditions as a poultice or infusion for skin conditions.

Frequently Asked Questions