An Eye on Plants – Koʻokoʻolau (Bidens wailele)

September 26, 2023

By Jon Letman, Bulletin Editor

The Hawaiian flora is comprised of a remarkable variety of rare flowering plants, many of which grow nowhere else. But the islands are also home to one of the most prolific plant families, Asteraceae, which includes chrysanthemums, daisies, and sunflowers.

Among the approximately 25,000 Asteraceae species worldwide is the genus Bidens which has between 150 and 235 species, with 42 in Polynesia alone. Called koʻokoʻolau in Hawaiian, Bidens is considered a prime example of adaptive radiation in Hawaiʻi. The genus has been found on seven of the eight high Hawaiian Islands and probably once grew on Niʻihau. The islands claim 19 endemic species, seven of which occur on Kauaʻi, three of them single-island endemics. Bidens are highly variable in form, growing in habitats ranging from coastal dunes, lava flows, and cliff faces to scrubland, bogs, and forests over 2,000 meters high.

In 2020, when University of Hawaiʻi-Hilo botany professor Matthew Knope returned to his family home on Kauaʻi during the pandemic, he and NTBG research biologist Ken Wood agreed the time was right to collaborate on describing a highly unusual Bidens limited to remote parts of Kauaʻi’s rain-soaked interior. The Bidens in question had been found growing in small populations near the rim’s edge of the Blue Hole crater and in scattered talus along the base of cliffs below Mt. Waiʻaleʻale. An additional colony was located at the back of the Wainiha Valley, Kauaʻi’s deepest drainage. Both areas are known for thousand-foot-high cliffs and ribbon-like waterfalls.

Botanists have documented this rare Bidens for years, but its lineage was uncertain and additional fertile material was needed for its description. This new Bidens species caught the botanists’ attention with its low, spreading habit and very unusual inflorescence which had long stalks terminating with a nodding flower head, similar to the bird-pollinated Bidens cosmoides. This undescribed Bidens was considered most closely related to B. valida, a Kauaʻi endemic found in several remote mountain regions of southeastern Kauaʻi

Over multiple field trips, Matthew and Ken made herbarium vouchers and collected viable seeds which have since been grown at NTBG’s conservation nursery. Utilizing collections from various botanists over the years, Matthew and Ken proceeded with a taxonomic assessment, describing its distinctive morphology, habitat, and conservation status. The formal written description was submitted for peer review along with photos, maps, and an illustration by NTBG partner and botanical artist Wendy Hollender.

Over a period of nine months — relatively fast in the world of taxonomic publication — the new Bidens was described and accepted as a new species. Matthew and Ken gave it the name Bidens wailele (lit. “leaping waters”) in recognition of its habitat. In June, the paper was published in the International Journal of Plant Sciences in a special issue honoring the legacy of noted Asteraceae specialist Vicki A. Funk.

With an estimated 700-800 remaining individuals, B. wailele meets the criteria to be listed as Critically Endangered on the IUCN Red List of Threatened Species. According to Matthew, Bidens represents “one of the most rapid and explosive plant diversification events in the Pacific.” What contributes to this diversity and wide dispersal? Ken explains that the tiny barbs and bristly nature of the seeds easily stick to birds, making them more likely to be carried long distances. He adds that Bidens probably evolved alongside Hawaiʻi’s yellow-faced bees and various diptera (flies) which served as pollinators and the plants may have been a favorite food of the flightless geese and duck-like fowl that once lived in Hawaiʻi. Surely, the first humans to reach Hawaiʻi admired Bidens for its delicate green foliage and bright yellow flowers. Bidens muʻo (branch tips) have long been drunk as a medicinal tea, consumed to fight colds and promote general health. Indeed, other Bidens species are used medicinally around the world. Lei Wann, director of Limahuli Garden and Preserve, began drinking koʻokoʻolau tea as a child. She describes the taste as mildly sweet and earthy and says there are still families who visit the Limahuli Valley to gather Bidens, a plant that is both rare and yet familiar to many.