Native iliau a striking symbol of Hawaii botany
By Duane Choy
Earlier this year, I visited one of my favorite places in the world, Waimea Canyon, on Kaua'i. The native Hawaiian plant iliau (Wilkesia gymnoxiphium) grows naturally only in this magical environment.
Iliau was first documented in the wild by non-Hawaiian botanists from the United States Exploring Expedition of 1838 to 1842. Asa Gray, professor of botany at Harvard University, designated the new genus Wilkesia, after Captain Charles Wilkes, the leader of the expedition. The appellation gymnoxiphium relates to the plant's leaves, from the Greek root "gymno," meaning naked, and "xiph," signifying sword-like. Iliau is considered descended from a single species related to a North American tarweed, possibly from the Baja California area.
Populations of endemic iliau are generally on the western rim of Waimea Canyon, on dry ridges, or open sites in dry to mesic forests, at elevations varying from 425 to 1,100 meters. Feral goats are a nemesis to iliau because of their destructive grazing.
Iliau is the spotlight plant across the Iliau Loop Trail (which doubles as the starting trailhead for the canyon-accessing Kukui Trail) in Waimea Canyon. A tiny population of iliau can also be found on the elevated ground at the Y-junction of routes 550 and 552.
Another community of iliau flourishes at the end of the dazzling Awa'awapuhi Trail (my favorite), which is 1.5 miles south of the Kōke'e State Park headquarters.
Outside of its Waimea Canyon wild habitat, iliau can be viewed and enjoyed by everyone in numerous botanical gardens and public landscaping throughout the state.
Iliau is an unbranched, monocarpic species, indicating that once it flowers, and sets seeds, it perishes. Reproductive maturity may take up to two decades. The rosettes of light-green leaves emanate from the apex of a stalk elevated on woody stems. Ultimately, the blooming structure rises from the center of the tuft of leaves.
Flowering usually occurs between May and July. The flower stalk can be up to 5 feet high, with hundreds of cream-greenish, gummy, daisy-like blooms, positioned in spirals surrounding the central blooming stalk. Toward evening, the blooms emit a delicate, pleasing scent. The flowers reveal the relationship of this exceptional Hawaiian plant to the sunflower family (Asteraceae). As mentioned, the iliau dies after flowering, but if it branches into multiple stalks (resulting when the top is broken), each spike will flower and demise separately.
For me, iliau resembles a fireworks explosion of green blades. Iliau is a stunning symbol of our great botanical heritage here in Hawai'i. With its striking structure , the iliau is a vivid accent in our local landscape.
Plant it singularly or in mass, in well-drained, cindery soil and exposure to full sun. Moist locations, with poor drainage, will kill the iliau by root-rot. Feed with a balanced, slow-release fertilizer every six months. Foliar feed every three to six months with fish or kelp emulsion, or a water-soluble fertilizer, diluted by one-third to one-half of the recommended application. No pruning is needed, except to remove dead leaves off the main stalk. Water once a week for a month, until established, and then only after prolonged dryness. Iliau is wind- and drought-tolerant.
Ants are sometimes problematic with iliau. Spittle bugs can be washed off by hose water or hand-rubbed off. Root nematodes should be treated by nematocide amended into the soil, or a neem oil soil drenching. The hazardous agromyzidae fly stem miner should be controlled by a systemic pesticide.