Reholttumia hudsoniana
(formerly Pneumatopteris hudsoniana & Cyclosorus hudsonianus)
Thelypteridaceae or Marsh fern family
Synonyms: Aspidium hudsonianum, Cyclosorus hudsonianus, Dryopteris hudsonianum, Dryopteris truncata, Nephrodium hudsonianum, Nephrodium truncatum, Pneumatopteris hudsoniana, Polystichum truncatum, Thelypteris hudsoniana
Laukahi (but see Hawaiian Name below)
Hudson's air fern, Hudson's maiden fern
Natural Range & Environment
Reholttumia hudsoniana is endemic to all the main Hawaiian Islands except for Ni‘ihau and Kaho‘olawe [Flora of the Hawaiian Islands]. It is locally common in mesic to wet forests from (20–) 155 to 760 m (509 to 2,493 ft) [Palmer 2003].
A closeup view of the ripe sori on the undersurface of a laukahi frond; compare these to the still-developing sori in the header (Ripe sori photograph courtesy of Kenneth R. Wood (NTBG) CC BY-NC 4.0; Header photograph courtesy of Susan Fawcett CC BY-NC 4.0.). Upper surfaces of a new and older laukahi frond (Photograph courtesy of Andrew Evans CC By-NC 4.0.). Note the distinctive smaller pairs of pinnae at the base of this laukahi frond (Photograph courtesy of Susan Fawcett CC BY-NC 4.0.). A young cultivated laukahi. Older laukahi can get to be quite large, as seen with this wild plant (Photograph courtesy of Susan Fawcett CC BY-NC 4.0.).
Description
Reholttumia hudsoniana is a medium-sized to large fern (up to about 3 ft [1 m] tall), with a lifespan exceeding five years, and decumbent to erect rhizomes. Its pinnate fronds can be over 4 ft [1.2 m] long, with each pinna (leaflet) being deeply lobed. A unique identifying feature of laukahi is that the two to six pairs of pinnae at each frond's base are much smaller than the rest of the frond's pinnae (see photograph above). Laukahi's large round sori (i.e., reproductive structures where spores are produced) are arranged in two short parallel lines on the underside of each lobed pinna. These darken as they mature.
Growth Requirements
General
Reholttumia hudsoniana is one of the easiest of the large native Hawaiian ferns to grow and maintain. Use it as an accent or plant several to create a prehistoric look to your landscape. It can also be kept in a container, provided you are willing to repot it a few times until it obtains its mature size; plants left too long in a small pot require very frequent watering to avoid drying out.
While laukahi can withstand a couple hours of direct sunlight each day, it does much better in a site with diffuse (i.e., non-direct) but bright light. It can also survive in a more heavily-shaded location.
Laukahi requires moist soil at all times, and its roots can quickly die if the soil (or media) becomes dry for more than a day or so. In fact, this fern seems to grow best in sites that get periodically flooded (but quickly drain). For example: In one of our gardens, there was a laukahi planted near an automated sprinklerhead with a cracked pipe. About once a week, the area surrounding this laukahi would flood (because of the crack) and then quickly drain — this all happened in the span of about an hour. Somewhat surprisingly, not only did this laukahi grow extremely well, it also repeatedly reproduced (from spores), and over time, we had several laukahi at the site where there was once a single plant!
Laukahi in the ground don't seem to require fertilizer. For container plants, 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 laukahi 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.
Pests and Diseases
Healthly laukahi have very few pests. Rarely, a plant will become infested by mealybugs (look near the central growing base of fiddleheads), scale insects, or spider mites (on plants in drier windless locations). Refer to the Pests & Diseases page for ways to deal with these pests.
Uses
Ancient Hawaiian
We could find no recorded ancient uses for this fern. However, see Cyclosorus interruptus, a related species.
Modern
Special Features and Information
General
Thelypteridaceae is one of the largest and most taxonomically complex families of ferns, with over 1,100 species worldwide (with most species being found in tropical regions). Recently the family was reexamined and reorganized by Susan Fawcett and Alan Smith [2021], resulting in many new or resurrected genera. Based on Fawcett's and Smith's examination and some earlier work, there are currently twelve recognized species of Thelypteridaceae in Hawai‘i. This includes: eight endemic species: one indigenous species, neke (Cyclosorus interruptus); and three invasive naturalized species. One particularly notable species is the critically endangered (but not yet federally-listed under the US ESA) Menisciopsis wailele, an unusual fern that grows along fast-flowing water like waterfalls; it is endemic to Kauaʻi [Flora of the Hawaiian Islands]. For fern-lovers who want to learn more about the confusing Marsh fern family (Thelypteridaceae) from the person responsible for the recent (scientific) renaming of laukahi, Dr. Susan Fawcett (who is currently (2025) a postdoctoral researcher at the National Tropical Botanical Garden on Kaua‘i), check out this YouTube video.
Hawaiian Name
It is unclear how this species acquired the name laukahi since it is not referenced in any of the Hawaiian dictionaries we looked at. Rather, the name laukahi is used in Hawai‘i for a Eurasian weed, Plantago major, that is used medicinally [Pukui & Elbert 1986].
Laukahi is the name used for Reholttumia hudsoniana (syn: Pneumatopteris hudsoniana) by Palmer [2003]. Therefore, we will here follow Palmer, and, within this website, use the name laukahi; laukahi is also the name commonly used by local growers of this fern.
Etymology
Reholttumia honors Richard Eric Holttum (1895–1990), "whose keen insights, perseverance, and fundamental contributions to our knowledge of Thelypteridaceae enabled the present work" [Fawcett & Smith 2021]. The former genus name, Cyclosorus, comes from the Greek words kyklos, meaning "circle," and soros, meaning "heap," referring to the round sori on the undersurface of the plant's fronds [Palmer 2003].
The species name, hudsoniana, honors Navy Captain William Leverreth Hudson (1794–1862), commander of the Peacock, one of the U.S. Exploring Expeditions (1838–1842) [Palmer 2003].
BPK