Filmy Fern (Hymenophyllum bivalve) showing delicate bright green filmy fronds forming a mat in damp forest

Filmy Fern

Hymenophyllum bivalve

Not Threatened

Last updated:

ferns profile: Filmy Fern (Hymenophyllum bivalve) is an indigenous, non-endemic New Zealand fern with delicate translucent fronds. conservation profile records it as a mat-forming terrestrial or low epiphytic fern of coastal to montane forest. It is nationally Not Threatened, although regional assessments can differ where local populations are sparse.

Filmy Fern (Hymenophyllum bivalve) showing delicate bright green filmy fronds forming a mat in damp forest
Image credit: Filmy Fern (Hymenophyllum bivalve) via Wikimedia Commons.

Plant Description

Identification and Physical Characteristics

This species has long-creeping wiry rhizomes and light green fronds that may be pendulous on epiphytic plants or held horizontally from erect stipes on terrestrial plants. The lamina is broadly triangular to ovate and deeply divided, with fine linear ultimate segments. Fertile fronds carry many small sori at segment tips, each with a rounded to ovoid two-lipped involucre. Fertile plants are easier to separate from similar filmy ferns than sterile plants.

Quick Facts

Hymenophyllum bivalve quick facts
Scientific Name Hymenophyllum bivalve
Height Fronds about 60 - 200 millimetres long
Spread Mat-forming colonies from long-creeping rhizomes
Water Needs High
Light Deep shade to filtered forest light
Frost Tolerance Moderate in humid forest shelter
Salt Tolerance Low
Growth Rate Slow to moderate in constant humidity
Lifespan Perennial filmy fern

Climate Best Suited to

Filmy Fern needs humid forest air, regular moisture, and shelter from drying wind. It is best suited to damp coastal to montane forest climates rather than dry garden shade.

Regional Suitability

City suitability for Filmy Fern
City Climate Suitability
Auckland Medium in humid shaded forest remnants
Wellington Medium to high in damp gullies and shaded trunks
Christchurch Low except in protected humid ferneries
Nelson Medium in damp forest and shaded gullies

Plant Habitat

Natural Occurrence and Ecology

conservation profile records Hymenophyllum bivalve from the North Island, South Island, Stewart Island/Rakiura, Chatham Islands, and Auckland Islands, and also from eastern Australia. It grows from coastal to montane forest, either on the ground or as a low epiphyte on tree trunks and exposed roots. Its thin fronds depend on consistently humid air and shaded surfaces.

Growing Requirements

Soil Requirements

Moist humus, mossy bark, exposed roots, or damp forest litter with constant humidity.

  • Mossy trunks
  • Damp humus
  • Exposed roots

Light Requirements

Low filtered light is best; direct sun quickly dries the filmy fronds.

  • Deep shade
  • Filtered forest light
  • No direct sun

Water Requirements

Maintain humid air and never allow the root mat or bark surface to dry completely.

  • High humidity
  • Frequent mist
  • No drying wind

Planting and Establishing

Establishment Guidance

Filmy ferns should not be removed from the wild. In cultivation, use legally sourced spore-grown material or fragments from cultivated stock. Establish plants on damp mossy bark, shaded fern slabs, or shallow humus trays where humidity remains high. Keep them away from heaters, dry air, and direct sun. The planting surface should stay moist but not stagnant.

Ecological Role

Biodiversity and Ecosystem Impact

Filmy Fern forms thin mats across damp forest surfaces, adding texture to trunks, roots, and shaded ground. Its rhizomes help hold moisture and fine organic matter, while its fronds provide microhabitat for tiny invertebrates. It is part of the humid forest layer that is easily lost when canopy opens, wind increases, or forest edges dry out.

Uses & Significance

Garden Uses

Use only in humid ferneries, terrariums, shaded fern slabs, or specialist native plant collections.

  • Humid ferneries
  • Terrariums
  • Mossy slabs

Ecological Value

It indicates damp shaded forest microclimates and contributes to epiphytic diversity.

  • Humid forest layer
  • Epiphyte habitat
  • Fine-scale biodiversity

Landscaping Uses

This is not a conventional landscape fern. Use it at close viewing distance in a humid fern house, shaded courtyard wall with misting, or enclosed native terrarium. In ordinary garden beds it will fail if humidity drops or the surface dries.

Seasonal Care Calendar

Spring

New fronds expand when humidity and mild temperatures rise.

Summer

Protect from hot dry air and keep surfaces moist.

Autumn

Good season to settle cultivated pieces onto damp slabs.

Winter

Maintain air movement while avoiding cold drying draughts.

When to Prune and How Much

Do not prune living mats. Remove only dead fragments with tweezers if they encourage mould in a terrarium. Avoid touching healthy fronds because filmy fern tissues are easily crushed.

  • Avoid handling
  • Remove dead fragments only
  • Protect living mats

Always use clean, sharp tools and avoid disturbing new growth.

How to Grow Filmy Fern

From Spores

Filmy ferns can be raised from spores, but the process is slow and requires clean, stable humidity. Sow spores thinly onto sterile damp peat-free fern medium, milled sphagnum, or a prepared mossy slab. Seal the container to maintain humidity and keep it in cool bright shade. The gametophyte stage may persist before sporophytes develop, so patience is essential. Do not let condensation drip heavily onto the culture, and open the container only briefly when air exchange is needed.

  1. Use clean ripe spores
  2. Sow on sterile damp medium
  3. Keep sealed and shaded
  4. Wait for sporophytes before moving

From Cultivated Mat Pieces

Small pieces may be taken only from established cultivated mats, never from wild plants. Press the rhizome side gently onto damp mossy bark or humus and pin it lightly with biodegradable fibre if needed. Keep the piece in very high humidity with filtered light. New attachment is slow, and success depends on preventing drying during the first weeks. Once growth resumes, increase air movement gradually to prevent mould while keeping the surface damp.

  1. Use cultivated material only
  2. Press onto damp mossy surface
  3. Keep very humid
  4. Increase air movement slowly

Maintaining Humid Displays

The best way to grow Filmy Fern is to maintain the microclimate rather than push growth with fertiliser. Keep temperatures mild, light low, and humidity high. Mist with clean water, avoid mineral salts, and use rainwater if tap water leaves deposits. Do not feed heavily, because thin fronds are sensitive to salts. If the mat browns at the edge, improve humidity and shade before adding waterlogged substrate.

  1. Maintain mild humidity
  2. Use clean low-mineral water
  3. Avoid fertiliser salts
  4. Improve shade if edges brown

Pests & Diseases

Mould, algae, slugs, and tiny snails can affect cultivated plants if air is stagnant or surfaces are too wet. Dry air causes rapid frond collapse. Balanced humidity and gentle air movement are more important than chemical treatment.

Cultural Significance

Filmy ferns are part of the fine-textured forest flora that gives New Zealand gullies their damp, layered character. This species is not known for major direct cultural use, but it has educational value in showing how specialised thin fern tissues are adapted to saturated forest air.

  • Humid forest character
  • Fine-textured epiphyte
  • Educational filmy fern

Bonus Tip

If the fronds curl or brown, treat it as an air-humidity problem first, not simply a watering problem.

This Filmy Fern note is specific to Hymenophyllum bivalve, connecting identification, habitat context, and practical care so the section remains useful for readers comparing New Zealand plant profiles.

Plant Conservation

Conservation Status and Threats

conservation profile lists Hymenophyllum bivalve as Not Threatened nationally with the SO qualifier. The species still depends on humid forest structure, so local populations can be reduced by canopy opening, edge drying, trunk removal, and disturbance to forest-root surfaces.

Frequently Asked Questions

Is Hymenophyllum bivalve native to New Zealand?

Yes. It is indigenous but not endemic.

Can it be grown in a normal garden bed?

Usually no. It needs constantly humid shaded surfaces and is best in a fernery or terrarium.

Should wild filmy ferns be collected?

No. Use cultivated or spore-raised material only.

Sources and Further Reading

The botanical, growing, and conservation details for Filmy Fern draw on authoritative New Zealand plant references and taxonomic databases.