Ka'u Silversword

views updated

Ka'u Silversword

Argyroxiphium kauense

ListedApril 7, 1993
FamilyCompositae (Asteraceae)
DescriptionRosette shrub with gray sword-like leaves and a tall stalk bearing many flowering heads.
HabitatMoist mountain forests and bogs.
ThreatsFeral pigs and goats, low reproduction.


Argyroxiphium kauense (ka'u silversword) is an erect rosette shrub consisting of a woody vegetative stem, 1-27 in (2.5-43 cm) high, and a flowering stem that grows to a height of 2-8.5 ft ( 0.6-2.5 m). The vegetative stem bears narrow, sword-shaped leaves, 7-16 in (18-40 cm) long, which are nearly covered with dense, silky, silver-gray hairs and appear dull when dry. The flowering stem is branched, with each branch bearing a flower head producing 3-11 ray flowers that are 0.4 in (1 cm) long and 50-200 smaller, disk flowers. Individual plants produce about 100-350 flower heads. The white or yellow to wine flowers bloom in August and September. Following its period of bloom, the plant dies.

The species has also been known as Argyroxiphium sandwicense var. kauense. It is closely related to the endangered Mauna Kea silversword (A. sandwicense ssp. sandwicense ) which occurs on bare volcanic cinder on nearby Mauna Kea.


Ka'u silversword is found at elevations between 5,300-7,600 ft (1,615-2,316 m) on the south slope of Mauna Loa on the island of Hawaii, mostly in moist forest openings and bogs. It grows on both flat and sloping ground in pahoehoe lava, sometimes mixed with wet humus. Associated vegetation is scrub and scrub forest, dominated by ohia trees (Metrosideros polymorpha ). At the bog sites the surrounding vegetation consists primarily of sedges.


Ka'u silversword was first collected on the south slope of Mauna Loa in 1911 and has been found nowhere else. It may have once occurred in a band across the south, southeast, and northeast flanks of Mauna Loa at an elevation of about 6,000 ft (1,830 m).

Populations of Ka'u silversword exist in three areas: Upper Waiakea Forest Reserve (South Hilo District), the Ainapo Trail in the Kapapala Forest Reserve and the Kahuku Ranch (Kau District), and at Ke a Pohina on the same ranch. The total species population is estimated at less than 400 plants. In the Upper Waiakea Forest Reserve about 80 plants are found at the Upper Waiakea Bog Plant Sanctuary off Powerline Road where one-third of an acre is fenced. Near the Ainapo Trail, a sparse population of a few dozen plants was scattered over 15 mi (24 km) of habitat and, although not documented since 1984, presumably still exists. The only large population is on private land at Ke a Pohina where five acres of Ka'u silversword habitat, supporting less than 300 plants, are fenced.


The overwhelming threat to the Ka'u silver-sword is destruction of plants and habitat by feral herbivores, including mouflon (wild Mediterranean sheep), pigs, and goats. All graze on the plants and, in addition, rooting pigs inhibit the establishment of seedlings and uproot existing plants. Plants that have been grazed often resprout with branched stems and exhibit reduced vigor. The only large remaining population, at Ke a Pohina, has been greatly reduced by a herd of mouflon. In 1974, when the animals were released the Ka'u silversword population consisted of thousands of plants, many of them fully mature with rosettes 3.3 ft (1 m) in diameter. Two years later only about 2,000 plants were over 3 in (8 cm) wide. The decline has continued and today less than 300 plants remain. Before part of the plant's habitat at the Upper Waiakea Forest Reserve was fenced, that population fell from about 1,000 plants to 20 immature individuals.

As numbers decrease the species' reproductive habit adds to recovery problems. Besides flowering only once in its lifetime, the Ka'u silversword bears flowers that must be cross-pollinated from another plant in order to set seed. A low number of widely separated plants makes such cross-pollination less likely.

A small total population also puts the species in danger from natural catastrophes such as lava flows and wildfires. The only large population is only half a mile from a 1950 lava flow from the active southwest rift of Mauna Loa, and in 1984 a flow came near plants at the Upper Waiakea Forest.

Conservation and Recovery

The Fish and Wildlife Service published a Recovery Plan for the Ka'u silversword in 1995. The three surviving populations of the Ka'u silversword are located in the state-owned Upper Waiakea Forest Reserve, and on the private Kahuku Ranch. One of these populations was artificially created, and consists of only a few individuals. All of these critical habitats must be protected against development. The private site could be acquired and designated an ecological reserve, or a conservation easement negotiated with the landowner. In addition, the critical habitat must be managed to reduce the threats to the Ka'u silversword. In particular, the plants should be enclosed within protective fencing, and the abundance of herbivorous mammals decreased or eliminated. Invasive alien plants must also be decreased or eliminated from the local habitat. The populations of the Ka'u silversword should be monitored, and research undertaken into its biology and habitat needs. The goal of the Recovery Plan is to have ten populations of the Ka'u silver-sword, each supporting more than 2,000 plants. This requires cultivation of the plant in captivity, to provide stock for outplanting to supplement the tiny wild populations, and to create new ones in suitable habitat.


Regional Office of Endangered Species
U. S. Fish and Wildlife Service
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121


Carr, G. D., ed. 1990. " Argyroxiphium." In Manual of the Flowering Plants of Hawai'i, by W. L. Wagner, D. R. Herbst, and S. H. Sohmer. Honolulu: University of Hawaii Press and Bishop Museum Press.

Loope, L. L., A. C. Medeiros, and B. H. Gagne. "Recovery of Vegetation of a Montane Bog in Haleakala National Park Following Protection from Feral Pig Rooting." Honolulu: University of Hawaii, 1990.

Stone, C. P. "Alien Animals in Hawai'i's Native Ecosystems: Toward Controlling the Adverse Effects of Introduced Vertebrates." In Hawai ' i's Terrestrial Ecosystems: Preservation and Management. Honolulu: University of Hawaii, 1985.

U.S. Fish and Wildlife Service. 7 April 1993. "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for Argyroxiphium kauense (Ka'u Silversword). Federal Register 58.

U.S. Fish and Wildlife Service. 1995. "Recovery plan for the Ka'u Silversword, Argyroxiphium kauense. " U.S. Fish and Wildlife Service, Portland, OR.