Native Plants of Hawai‘i - Asplenium kaulfussii

Asplenium kaulfussii

Aspleniacaeae or Spleenwort family [Five-minute families]

Synonyms: Asplenium bipinnatum, Asplenium enatum, Asplenium enatum var. appendiculatum, Asplenium enatum var. caudatum, Asplenium enatum var. dareoides, Asplenium enatum var. gemmiparum, Asplenium gemmiferum, Asplenium gemmiferum var. enatum, Asplenium gemmiferum var. flexuosum, Asplenium hillebrandii, Asplenium kauaiense, Asplenium kaulfussii f. bipinnatum, Asplenium kaulfussii f. dareoides, Asplenium kaulfussii var. dareoides, Asplenium kaulfussii f. gemmiparum, Asplenium kaulfussii var. gemmiparum, Asplenium kaulfussii var. membranaceum, Asplenium kaulfussii f. paradoxum, Asplenium lucidum, Asplenium lydgatei, Asplenium mannii, Asplenium mannii var. dareoides, Asplenium mannii var. gemmiparum, Asplenium mannii var. kauaiense, Asplenium meiotomum, Asplenium obliquum, Asplenium obtusatum, Asplenium parallelum, Asplenium protensum, Asplenium sectum

Kūau

Kaulfuss' spleenwort, Queen spleenwort, Spleenwort

Natural Range & Environment

Asplenium kaulfussii is endemic to all the main Hawaiian Islands except Ni‘ihau and Kaho‘olawe from 150 to 1,200 m (492 to 3,937 ft) [Flora of the Hawaiian Islands]. It is locally common in mesic to wet shady forests as a terrestrial fern but also occasionally epiphytic (growing on other plants) or epipetric (growing on rocks). Kūau is most common in the Waiʻanae Mountains of O‘ahu, rare in the Koʻolau Mountains (of O‘ahu), common in some places on Kauaʻi, and much less common on Molokaʻi, Lānaʻi, Maui, and Hawaiʻi Island [Palmer 2003].

The ripe sori of kūau are arranged diagonally on the underside of its pinnae (Photograph courtesy of Kyhl Austin CC BY-NC 4.0.). Kūau displays a lot of variability in frond shape; here are two extremes (Photograph of thin-pinnae kūau courtesy of Kairigney CC BY-NC 4.0; Photograph of short-pinnae kūau courtesy of Kevin Faccenda CC BY 4.0.). A mature cultivated kūau.

Description

Asplenium kaulfussii is a small to medium-sized fern normally less than two feet [61 cm) tall, with a lifespan greater than five years. It has decumbent rhizomes with the tips bearing many persistent stipe (the stalk of a frond) bases.

Kūau has fronds that vary significantly in size and shape; some of these differences are described below in Special Features and Information. However, generally, the fronds of kūau are 10 to 33 in (25 to 85 cm) long and 4 to 14 in (10 to 36 cm) wide, with hairless pinnae that may have none to many proliferations (a vegetative bud that is the beginning of a new plantlet) and entire or toothed margins. The stipe of the fronds is dark brown to black and shiny [Palmer 2003].

The elongated sori of kūau are arranged diagonally on the undersurface of the pinnae and turn a rusty brown when mature (see photograph above).

Growth Requirements

General

Kūau does best is cool, humid, shady sites, and suffers under any direct sunlight (except, maybe, some brief morning sun) or the drying effect of strong winds and extreme heat. It's the perfect plant, one or several (planted about two feet apart), for that spot under your tree or eave where the grass doesn't grow. Kūau can also be kept as a container plant.

Kūau requires continuously moist soil or media. However, that soil or media should also drain water well. In the ground or in a container, kūau grows at a slow to moderate pace, and under optimum conditions will be full size (about a foot tall and wide) in about a year. We have grown this species in clay and organic-dominant soils, and media mixes of sphagnum peat moss (≈75%) with perlite or black cinders (≈25%).

In-the-ground kūau don't seem to require any fertilizer (although, a sprinkle of compost once a year would likely help). For kūau in pots, apply a balanced controlled-release fertilizer with minor elements 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 kūau looking tidy, it's okay to prune away old dead fronds, but always be careful not to damage the central growing portion of your fern.

Most ferns are difficult to propagate from spores (but see Reholttumia hudsoniana, an exception). Therefore, fern growers typically propagate their plants by division, and division is a good way to produce more kūau. However, you'll be more successful if you keep and divide a kūau kept in a pot rather than one planted in the ground (because once this fern reaches it mature size it doesn't expand much thereafter). Here's how we make more kūau:

Start out with a small kūau in a medium-sized pot (i.e., about twice as wide as the fern).
Let the kūau grow and fill the pot. Then, transfer it to a larger pot.
Continue this process until your kūau fills a one- to two-gallon pot.
Using a sharp knife to cut through the roots, divide your kūau into four approximately equal-sized plants and repot each in its own container. Try to cut through the roots along natural breaks visible between the fronds of the large uncut kūau.
Allow you newly-divided ferns a month or so to recover, and then plant them out into your landscape — or give them away to friends!

Pests and Diseases

We have had few pest problems with our kūau. Infrequently, ants will import and protect scale insects on the fronds, and container plants sometimes become infested with root mealybugs. Ocassionally and carefully remove your fern briefly from its pot to look for these. Refer to the Pests & Diseases page for how to deal with these three pests.

Uses

Ancient Hawaiian

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

Modern

Special Features and Information

General

The composition and classification of Aspleniaceae have been subject to considerable changes over the years, with some botanists treating the family as being comprised of a single genus, Asplenium (spleenwort), while others divide the family into nine small genera. There are approximately 800 species worldwide [Britannica; Aspleniaceae].

There are currently (2025) 16 endemic and 10 indigenous species of Asplenium in Hawai‘i [Flora of the Hawaiian Islands], making it the fern genus with the most native species in the Islands.

Asplenium kaulfussii is a morphologically variable species, formerly divided into numerous species, varieties, and formae (see Synonyms above). Palmer [2003], in Hawai‘i's Ferns and Fern Allies, considered the current "Bible of Hawaiian Ferns," recognized four distinct forms (see below); however, these formae are no longer recognized by all taxonomists, and some consider the species to be a single very diverse species.

f. bipinnatum. Latin bis, meaning "two or twice," and pinnate, in reference to the 2-pinnate fronds. This form has fronds that are fully 2-pinnate, with the pinnules usually resembling small, normal pinnae. The form is extremely rare and known only from Niu Valley, Oʻahu, and has not been collected for decades.
f. dareoides. The name is from Darea, an archaic and currently unused genus name for some species of ferns that have been moved to other genera (including Asplenium), plus -oides, meaning "like, resembling." This form has 1-pinnate-pinnatisect to fully 2-pinnate fronds, with the ultimate segments linear with 2 (–4) obtuse teeth on expanded tips. An exceedingly rare form that was last collected in Kuliʻouʻou Valley on Oʻahu in 1922. However, earlier collections may support a much wider past distribution.
f. gemmiparum. Latin gemma, meaning "bud," plus parvum, meaning "too little, not enough," possibly in reference to the small proliferations (a vegetative bud that is the beginning of a new plantlet) on the pinnae. This form often has pinnae with plentiful proliferations on its lateral veins; however, there are also variants with very few proliferations. One plant on Maui was observed to have fronds with proliferations at one time and fronds without proliferations at another. This is an uncommon localized form known from Kauaʻi, Oʻahu, Lānaʻi, and Maui.
f. kaulfussii. The most common and typical form, having pinnae without proliferations and found on all the main Hawaiian Islands except Ni‘ihau and Kaho‘olawe.

Hawaiian Name

Pukui & Elbert [1986] notes that the name kūau is also used for a "sea creature (perhaps Aplysia sp.)," which is a type of sea slug called a sea hare.

Etymology

Asplenium is from the Latin word asplenum, meaning "spleenwort." Ancient Greeks believed this fern could cure diseases of the spleen [Thomas et al. 2025].

The species name, kaulfussii, honors George Friedrich Kaulfuss (1786–1830), a German botanist and professor of forestry and botany at Halle, who described more than 20 new Hawaiian fern species based on Chamisso's collections from 1816 to 1817 made during the Russian Kotzebue expedition [Palmer 2003]. Adelbert von Chamisso (1781–1838), was a French-born German explorer, naturalist, author, poet, and plant collector [WikipediA;Adelbert von Chamisso].

BPK