Native Plants of Hawai‘i - Wilkesia gymnoxiphium

Wilkesia gymnoxiphium

Asteraceae or Sunflower family [Five-minute families]

Synonyms: Argyroxiphium gymnoxiphium

Iliau

Kauaʻi greensword, Kauaʻi wilkesia, Wilkesia, Nakedsword

Natural Range & Environment

Wilkesia gymnoxiphium is a spectacular and rare plant (although surprisingly considered "secure" and without federal listing as endangered) that is endemic to Kauaʻi. It grows on dry ridges or in open sites in dry to mesic forests from about 425 to 1,100 m (1,394 to 3,609 ft) and is most common in Waimea Canyon [Wagner et al. 1990].

Whorls of florets and bracts on a wild iliau (Photograph courtesy of G.D. Carr ©). A young cultivated iliau. Closeup of variegated leaves. Older and much taller cultivated iliau (Photograph courtesy of Daderot CC BY-SA 3.0). A wild iliau in bloom (Photograph courtesy of KarlM CC BY-SA 3.0).

Description

Wilkesia gymnoxiphium is a rare short to tall (15-plus feet [4.6-plus m]) unbranched (or occasionally sparsely branched) rosette shrub with a lifespan greater than five years. The sword-shaped leaves of iliau, from 6 to 20 inches (15 to 51 cm) long, are silky and a bit tacky. They are generally light green with a grayish hue but can also be somewhat variegated. A distinctive feature of iliau is that the whorls of 9 to 15 leaves have a "sheathing base" in which the leaves are fused at their base.

Most iliau bloom only once in their lifetime and then die unless the plant has multiple stems, in which case any non-blooming stems will live on until they bloom; multi-stem plants are most commonly the result of some injury. Iliau produces a huge inflorescence, often with hundreds of yellow to cream-colored florets (i.e., daisy-like flowers). According to Rick Barbosa, co-owner of Hui Ku Maoli Ola, near the evening, iliau gives off a slight and pleasant scent resembling that of a ginger flower. While iliau flowers are perfect (bisexual), they are also self-incompatible, meaning that an iliau flower cannot pollinate itself to produce seeds [Carr et al. 1986]. Each pollinated flower develops into an elongated dry cypsela (a type of dry single-seeded fruit). Wild iliau bloom from May to July with the fruits ripening about a month later [St. John 1971].

Growth Requirements

General

Botanist and conservationist Otto Degener (1899–1988) stated that iliau "is a magnificent plant, worthy of cultivation" [Degener 1932-1963]. Today, you can see iliau in some botanical gardens but rarely in anyone's front yard. This is a shame because it is such a beautiful plant that is relatively easy to grow and does well even at elevations well below its natural habitat.

Iliau is best displayed as an accent in your landscape or as a container plant. Plant it in a site with full sunlight for all or most of the day, and water it once a week until it is established (i.e., visible growth). After that, water your iliau only during prolonged dry periods; it does best with infrequent deep waterings rather than frequent short showers. Obviously, if you keep your iliau in a container, you'll need to water it more often. However, let the media surface become dry before giving your plant a drink.

Iliau is drought-tolerant and can endure moderate winds. The planting site or container should easily drain away excess water. To keep your iliau green, healthy, and growing, consider using a balanced controlled-release fertilizer with minor elements every six months, or foliar feed it monthly with a kelp or an organic or inorganic water-soluble fertilizer diluted to one-third the recommended strength. Avoid pruning your plant except to remove any dead brown leaves. Take care not to damage the other leaves or the stem when doing this.

Unfortunately, in most cases, iliau only blooms once, shortly before its death; multi-branched plants sometimes have an extended lifespan. Still, even without its spectacular floral display, this distinctive-looking plant is worth having in your landscape. And, of course, when it does finally flower, you'll be amazed at its beauty.

While iliau flowers are perfect (bisexual), Carr et al. [1986] discovered they are also self-incompatible, meaning, a single plant cannot produce viable seed on its own. Therefore, if you wish to keep iliau in your landscape for more than one generation, you need to grow two or more plants, AND at least two of those plants must develop their beautiful inflorescence at the same time. (This may explain why we rarely see cultivated iliau.)

Our experiences with iliau in cultivation are currently limited to plants that have not yet flowered, and, therefore, we cannot comment further on self-incompatibility. However, if your experience exceeds ours, and you have grown iliau for more than one generation, we hope you will take the time to contact us (EickhoffandKoebele@gmail.com) and share your insights so we can better inform others via this website.

Pests and Diseases

Iliau is occasionally injured by ants, leaf-miners, and spittlebugs. It is also susceptible to attacks by root-knot nematodes. Refer to the Pests & Diseases page for treatments.

Uses

Ancient Hawaiian

Otto Degener in Flora Hawaiiensis states that the "Kauai Wilkesia ...is said to have been used medicinally by the ancient Hawaiians" [Degener 1932-1963].

Modern

Special Features and Information

General

Asteraceae is a very large family of mostly herbaceous plants with about 32,000 species within 1,900 genera with a subpolar to tropical worldwide distribution. Nearly all Asteraceae have what appears to be a single "daisy"-type flower that is actually a composite of several to many much smaller flowers called florets [WikipediA:Asteraceae]. There are about 250 native and non-native Asteraceae in Hawai‘i, with a little more than half being native [Flora of the Hawaiian Islands].

Iliau, or the Kaua‘i greensword, is closely related to the famous Hawaiian silverswords and greenswords (Argyroxiphium spp.) found at high elevations on Maui and Hawaiʻi Island. Although iliau is closely related to naʻenaʻe (Dubautia spp.), no natural inter-genetic hybrids have been found (yet). In contrast, silverswords appear to cross easily with Dubautia species as well as other Argyroxiphium species; more than thirty crosses have been recorded [Ziegler 2002].

The genus Wilkesia is an endemic Hawaiian genus with only two species, iliau and the dwarf iliau (W. hobdyi). Like iliau, the dwarf iliau is only found on Kauaʻi and is extremely rare today. The species is named after its discoverer, Robert Hobdy, now deceased, botanist and former resident of Maui. Smaller than iliau, Wilkesia hobdyi has only been seen on a few very dry ridges in Waimea Valley from about 275 to 400 m (902 to 1,312 ft). It typically blooms later than iliau, from September to October, or even in December [Ziegler 2002].

Some people, having read the Dr. Seuss book or seen the recent animated movie, have given iliau the name "Lorax tree."

Hawaiian Name

Etymology

The genus name, Wilkesia, is in honor of Charles Wilkes (1798–1877), the commander of the United States Exploring Expedition which visited Hawai‘i [Wagner et al. 1990]. The species name, gymnoxiphium, comes from the Greek words; gymno, meaning "naked or bare," and xiphos, meaning "sword" [etymonline], likely a reference to iliau's elongated sword-shaped leaves.

BPK