Native Plants of Hawai‘i - Microlepia speluncae

Microlepia speluncae

Dennstaedtiaceae or Bracken family

Synonyms: Davallia andersonii, Davallia speluncae, Dicksonia flaccida, Leptolepia andersoni, Microlepia jamaicensis, Microlepia strigosa var. hirta, Polypodium speluncae

No known ancient Hawaiian name

Palai nui (see Hawaiian Name below)

Limpleaf fern, Cave fern

Natural Range & Environment

Microlepia speluncae is found in South China, the Himalayas, India, Sri Lanka, the Malay Peninsula, Japan, Taiwan, the Philippines to Polynesia, and is the only species in the genus native to Africa. It is also found in the American tropics but is probably not native there [Palmer 2003].

In Hawai‘i, Microlepia speluncae is seen infrequently (but occasionally often in sites within the Wai‘anae Mountains of O‘ahu) in mesic to wet forests from 480 to 1,280 m (1,575 to 4,199 ft) on Kaua‘i, O‘ahu, Moloka‘i, Maui, and Hawai‘i Island [Flora of the Hawaiian Islands].

While this species has a widespread distribution outside Hawai‘i, curiously, it's not as common as its sister species palapalai (M. strigosa) within the Islands.

Ripe sori on the underside of a palai nui frond; compare this to the non-reproductive frond in the header photograph (Sori photograph courtesy of Army Natural Resources via GD Carr©.). Pali nui's tall fiddleheads unfurl one after another along its barely-subterranean rhizomes in this potted plant. An easy way to propagate pali nui is by division (see Growth Requirements below); note how shallow the root system of this species is (Photograph courtesy of Kenneth R. Wood (NTBG) CC BY-NC 4.0.). A still relatively young cultivated palai nui mixed in with some of its native Hawaiian fern friends. Older palai nui can get to be quite large, as you can see in these last two photographs of wild plants (Photograph of lone pali nui courtesy of Andrew Evans CC BY-NC 4.0; photograph of pali nui with hikers courtesy of Kevin Faccenda CC BY 4.0.).

Description

Microlepia speluncae is a medium-sized to large fern (up to about four feet [1.2 m] tall) with a lifespan exceeding five years. It has creeping hairy rhizomes and shallow roots.

Palai nui has large fronds usually more than three feet (1 m) and as much as ten feet (3 m) long. The frond's stipe is shorter than its blade, grooved, sparely hairy, and sometimes reddish brown. The blade is 3-pinnate (i.e., divided three times), and typically is hairless or nearly so. The embedded veins within the pinnule are only slightly visible.

Young palai nui can be mistaken for its sister species, palapalai (Microlepia strigosa). However, a quick way to determine which species is which is to detach one of the fern's fronds. The fronds of palai nui or the limpleaf fern, as its common name implies, go limp soon after being picked, whereas the fronds of palapalai do not. This is why palapalai is commonly used in lei-making and palai nui is not. See Special Features and Information below for other ways to distinguish palai nui from palapalai.

Reproductive palai nui have round sori held in cup-shaped indusia on the underside of their fronds. These are reportedly positioned more toward the center of the pinnule than those of palapalai (Microlepia strigosa). However, it can be hard for non-experts (like us) to see the difference.

Growth Requirements

General

Similar to laukahi (Reholttumia hudsoniana), and unlike our slow-growing native tree ferns (Cibotium spp.), palai nui is a native fern that in just a few years can grow to a significant size (i.e., three [1 m] or more feet tall). Use it as an accent in your landscape or plant several to fill in the empty shaded spaces under your trees or hide an unsightly feature such as an ugly border wall. Consider using palai nui as an alternative to the popular non-native and aggressive lauaʻe haole (Phymatosorus grossus). Palai nui can also be kept as a container plant but will need to be regularly repotted into larger and larger containers to obtain its full size.

Palai nui can withstand one, maybe two, hours of direct sunlight but will do much better in a site with diffuse but bright light for the entire day. It will also grow in a more heavily-shaded site but not as quickly.

Palai nui is somewhat forgiving in that it can tolerate a day or two of dry soil or media, which will likely cause one or more of its fronds to wilt or (partially or completely) die. However, if you act quickly and rehydrate the soil/media, it can recover and will eventually replace the dead fronds with new fiddleheads. Dry soil or media for any longer than a couple of days will almost certainly kill your palai nui for good. Therefore, avoid this by keeping this fern well-watered at all times.

Avoid planting palai nui in windy sites, which can damage the fronds or cause their tips to dry and brown. Additionally, strong winds can topple over a containerized palai nui. Fronds can also be damaged inadvertently by people and pets, so avoid planting palai nui directly adjacent to walkways.

We have grown palai nui in clay and organic-dominant soils and in cinder-rich media. It seems unlikely it would do well in sandy or coralline soils, but that's an experiment for another day.

Palai nui planted in the ground don't seem to require any fertilizer. However, an occasional application of compost may improve growth and color. For containerized palai nui, apply a balanced controlled-release fertilizer with minor elements about every six months, or use a foliar organic or inorganic fertilizer diluted to ⅓ to ½ the recommended strength every couple of months. When applying the controlled-release fertilizer, just sprinkle it on the media's surface and water it in; DO NOT till (i.e., mix) the fertilizer into the media because this can damage the fern's shallow fibrous root system.

To keep your palai nui looking tidy, it's okay to prune away old dead fronds but always be careful not to damage the developing fiddleheads or shallow rhizomes. Rather than discarding the dead fronds, cut them up into small pieces and use them as mulch around your fern (or other plants) to improve the soil.

The easiest way to create new palai nui for your landscape is by division. Wait until the parent plant has three or more distinct rhizomes. Then, with a sharp knife, cut through one of the rhizomes about six inches (15 cm) back from its growing tip to separate it from the rest of the plant. Carefully, dig up the detached rhizome along with its shallow roots (see photograph above), and either immediately replant it in a new location in the landscape, or pot it up in a container containing new (i.e., clean) well-draining media. Drench the new transplant immediately. If the detached rhizome has any fronds attached, cut these halfway down to reduce the transpirational water loss while its roots recover.

Pests and Diseases

To date (2025), we haven't encountered any pests on pali nui. Other growers have reported the occasional slug or caterpillar chewing on its fronds [Quinn 2013]. Refer to the Pests & Diseases page for ways to deter these attacks.

Uses

Ancient Hawaiian

We were unable to find any recorded ancient Hawaiian uses for this species.

Modern

Special Features and Information

General

Dennstaedtiaceae is one of 15 families in the order Polypodiales, currently with about 240 known species within ten genera. The morphological diversity among species within the order has confused past taxonomists; however, more recent molecular studies support the monophyletic nature of the order and the family. The most well-known member of this family is the bracken fern (Pteridium aquilinum), known as kīlau in Hawai‘i, and probably the world's most abundant fern [WikipediA;Dennstaedtiaceae].

There are two indigenous species of Microlepis in Hawai‘i, M. speluncae and the more common and better-known palapalai (M. strigosa). There is also a hybrid between the two species that has been given the interesting Latin name, Microlepia x adulteriana, implying an adulterous relationship between the two parental species. This hybrid forms large colonies where the two species are found together [Palmer 2003].

Microlepia speluncae is morphologically similar to M. strigosa. However, one of the easiest ways to distinguish between the two is the behavior of their fronds after being harvested. The fronds of M. speluncae soon go limp after being picked, whereas those of M. strigosa do not. This makes fronds of M. speluncae unsuitable for lei-making or decorations. Other differences between the two species include:

M. strigosa, typically and sometimes extremely, has bristlelike hairs on its fronds which you can often tell just by touch; M. speluncae fronds are normally hairless.
Mature M. speluncae are normally larger, sometimes much larger, than mature M. strigosa.
M. speluncae often have reddish brown stipes and rachises (i.e., the lower stem of the frond and the middle stem portion of the body of the frond).
And, if you look very closely, the veins within the pinnule of M. speluncae are less prominent than those of M. strigosa.

Hawaiian Name

We've been unable to find any known ancient Hawaiian name for this species. Locally, the name palai nui is most commonly used for this fern. Therefore, this is the name we will use within this website. Palai nui translates as "large fern," specifically the fern, palapalai (Microlepia strigosa) [Pukui & Elbert 1986].

Etymology

The genus name, Microlepia, is derived from the Greek words, mikros, meaning "small," and lepis, meaning "scale," likely referring to the small cuplike indusia (spore covering) of plants in this genus [Palmer 2003].

The species name, speluncae, is Latin for "caves," possibly referring either to the sporangia being sheltered in a cup-shaped indusium, or to the deeply shaded habitat of this fern [Pteridophytes of Africa].

BPK