Kidney Fern

Botanical name: Hymenophyllum nephrophyllum
Synonyms: Trichomanes reniforme, Cardiomanes reniforme
Common name(s): Kidney fern, Konehu, Kopakopa, Raurenga

About:
A distinctive endemic filmy fern immediately recognised by its glossy, undivided kidney-shaped fronds. Unlike most New Zealand filmy ferns, the frond is not divided into pinnae or fine segments, but forms a single rounded blade with conspicuous veins and a row of sori around the margin.

It spreads by long-creeping, much-branched rhizomes to form extensive interwoven mats over forest floors, banks, rocks, fallen logs and tree bases. The fronds are exceptionally thin and sensitive to drying, but can shrivel during dry conditions and later expand again when moisture returns.

Natural habitat:
Coastal to montane forest, especially moist, sheltered environments.

Commonly found on forest floors, damp banks, fallen logs, mossy rocks, boulders, cliff faces, lava fields and the bases of trees. It may also grow as a low epiphyte on trunks.

Occurs in kauri, podocarp, broadleaf and beech forest, old kānuka scrub and other humid native vegetation.

Growing environment:
Shade, High Humidity, Moist Soil & Sheltered tolerant. Requires consistently humid conditions and protection from drying winds and prolonged direct sunlight.

Endemic distribution:
Endemic to New Zealand.

Found throughout the North Island and in western and southern parts of the South Island, as well as Stewart Island/Rakiura and the Chatham Islands.

It is largely absent from the drier eastern South Island.

Height:
Fronds typically stand approximately 5–25 cm above the substrate.

Width:
Individual frond blades approximately 3–12 cm long and 4–13 cm wide.

Colonies may spread extensively through creeping rhizomes.

Flowering:
Not applicable — fern species reproducing by spores.

Fruiting:
Not applicable — spore producing.

Numerous sori develop around the frond margin, with fertile receptacles extending beyond the surrounding indusia as they mature.

Uses:
• Shaded fern gardens
• Native forest-floor planting
• Moss gardens
• Humid ferneries
• Shaded rock gardens
• Log and stump planting
• Epiphytic displays
• Native forest restoration in suitable habitat

Appearance:
Growth habit:
A creeping terrestrial, rock-growing or low epiphytic fern forming broad, dense mats.

Its slender rhizomes branch repeatedly and weave together beneath and across the substrate, producing numerous individual fronds.

Leaves:
The frond blade is bright yellow-green to dark glossy green and characteristically kidney-shaped to almost circular.

Unlike most members of the genus, the lamina is completely undivided.

Fertile fronds are usually approximately 17–65 mm long and 25–105 mm wide, while sterile fronds may be somewhat larger and more irregular in outline.

The blade is very thin, smooth and hairless, with a conspicuous network of veins.

Margins are entire and bordered by a slightly thickened rim.

The stipe is reddish-brown, brittle and approximately 20–240 mm long, with a narrow wing extending along much of its upper length.

Flowers:
None.

As a fern, Hymenophyllum nephrophyllum produces neither flowers nor seeds.

Fruit:
None.

Spores develop within sporangia grouped into numerous sori crowded around the outer margin of fertile fronds.

The sori are partly to deeply immersed in the lamina.

Each is enclosed by a small cup-shaped indusium approximately 1–2 mm long, while the receptacle may project up to about 4 mm beyond it.

This creates the characteristic row of small peg-like structures around fertile fronds.

Ecology:
Kidney fern is strongly associated with moist, humid forest microclimates.

Its thin fronds have limited ability to regulate water loss, so they respond directly to atmospheric moisture. During dry conditions the fronds can contract and curl, greatly reducing exposed surface area, then rapidly rehydrate and unfold when rain or high humidity returns.

This ability allows it to survive occasional drying despite its preference for wet forest environments.

The species spreads both by wind-dispersed spores and vegetatively through creeping rhizomes, enabling it to form large persistent colonies.

Dense mats may suppress the establishment of some competing seedlings immediately beneath them.

Associated habitats:
Kauri forest, podocarp forest, broadleaf forest, beech forest, kānuka scrub, moss forest, shaded banks, damp cliffs, lava fields, fallen logs, boulders, rock faces and lower tree trunks.

How to grow:
Difficult in ordinary garden conditions and best left undisturbed in the wild.

Successful cultivation requires a cool, shaded and consistently humid environment that closely resembles the floor of a moist native forest.

Plant on moss-covered logs, shaded rocks, humus-rich banks or other surfaces where moisture remains reliable but stagnant water does not accumulate.

Protect completely from hot afternoon sun and drying winds.

Maintaining high atmospheric humidity is more important than simply keeping the soil wet.

Propagation:
Propagation is by spores or, in cultivation, potentially by carefully established pieces of creeping rhizome.

Spore propagation requires sterile, continuously humid conditions through both the gametophyte and young sporophyte stages.

Because established plants are highly sensitive to disturbance, wild colonies should not be lifted or divided.

Cultural use:
Known by several Māori names including konehu, kopakopa and raurenga.

The name kopakopa is associated with the fern’s ability to fold or contract during dry conditions.

Historical records describe several traditional uses, including as a fragrant or decorative plant and in mourning practices. Medicinal use for internal complaints has also been recorded, although historical accounts vary.

Garden value:
One of New Zealand’s most distinctive native ferns.

Its glossy kidney-shaped fronds are unlike those of almost any other indigenous fern, and fertile plants are particularly striking when the frond margins are lined with sori.

It is best suited to specialist ferneries, shaded moss gardens and naturally humid forest settings where its creeping habit can produce impressive colonies over logs, rocks and banks.