A vine-like runner of Koster's Curse (Clidemia hirta), Keanae Peninsula, east Maui.
If ever a plant belonged on the world’s Ten Most Wanted list of Dangerous Criminals, it would most likely be Clidemia hirta, AKA “Koster’s Curse“. This aggressively invasive pest species is considered to be “armed and dangerous” by range managers and plant scientists throughout the world, and has earned for itself the botanical equivalent of a standing order of “shoot to kill” on all of the Hawaiian Islands.
How it got its Name
The common name for Clidemia hirta is Soapbush. However, it is frequently referred to as “Koster’s Curse” in places where it grows as a noxious weed; this includes Hawaii, where it is sometimes simply referred to as “the curse” for the damage it caused to coconut plantations. Sometime between 1880 and 1886, the hapless Mr. Koster inadvertently introduced Clidemia hirta seeds to coffee nursery seed stocks in Fiji. Since then, it has spread inexorably throughout the islands of the Pacific Basin, including Australia.
The distinctive five-veined leaf of Clidemia Hirta. Note the fine hairs covering the leaves, twigs, and fruits. Nahiku, east Maui.
Description
Clidemia hirta is a coarse perennial shrub that can grow as tall as 5 m under ideal conditions, but its average height is about 2 m. Its branches, vine-like runners, stems, leaves, and fruit are covered with numerous reddish-green bristles (hence its Latin species name “hirta”, which means “hirsute” or “hairy”).
The opposite, roughly ovate leaves are 5-15 cm long, 4-7 cm wide, and covered with fine bristles on both surfaces. The leaf margins are scalloped or toothed, and quite hairy. Leaf surfaces display five deep veins that originate at the base of the leaf and extend to the apex; numerous smaller lateral veins between give leaves a characteristically pleated or “chessboard” appearance. The upper surfaces are pronouncedly more glossy than the dull under surfaces.
The flowers, fruit, and foliage of Clidemia hirta, Nahiku, east Maui.
The flowers are small (five petals, each about 9 mm across), colored off-white, and form panicles (clusters of flowers) that emerge in the leaf axils or at the ends of branches. Clidemia hirta will set flowers and fruits throughout the year if there is enough moisture in the soil.
When they mature, the fruits are deep-blue to black in color, hairy, four-celled, and very small (about 8 mm long). The flesh is sweet with a flavor a bit like that of a blueberry, only stronger. A large mature plant will produce as many as 500 fruits a year. Each fruit contains about 100 small seeds (5-6 mm long) that can remain viable in the soil for as long as four years. The fruits are poisonous to goats due to their high concentrations of tannin.
Ripening and immature fruits of Clidemia hirta, Nahiku, east Maui.
Reproduction and Seed Propagation
In rain forest habitats where there is no dry season, vegetative growth and sexual reproduction of Clidemia hirta continue throughout the year. However, in habitats with seasonal dry periods, the rate and quantity of flowering and fruiting are greatly reduced. Growth is rapid after germination: seedlings may grow into mature fruiting plants within 6 to 10 months.
Seeds most commonly are dispersed by birds, feral pigs (Sus scrofa), mongooses (Herpestes auropunctatus), and humans. Humans are the dominant means of spreading the seeds over long distances. Hunters, hikers, marijuana growers, plantation workers, and their motor vehicles are the main vectors. The bristle-covered fruits are able to cling tenaciously to clothing, equipment, and muddy tires, and so may be transported many miles from the original infestation.
Habitat and Ecology
In Hawaii, Clidemia hirta usually grows in areas that receive more than 127 cm (50 in) of annual rainfall. Its failure to spread to the islands of Kaho’olawe and Ni’ihau may be due to their arid climates and geographic isolation. Although it requires high rainfall and humidity, Clidemia hirta is otherwise able to tolerate a wide range of environmental conditions, which partly accounts for its success as an invasive weed. It prefers forests, plantations, and pasture lands, but it grows equally well in both the full sunlight of exposed ground and the deep shade of a rain forest canopy. It will grow in undisturbed ground, but spreads very rapidly (usually by vegetative growth) in soils that have been recently disturbed by storms, fires, landslides, and feral pigs. Its preferred altitude ranges from a few meters above sea level to more than 1,200 m (about 3940 ft), though it will grow in areas as high as 1,500 m under favorable conditions. When it becomes well established, Clidemia hirta develops dense, monotypic stands that effectively shade out understory plants. In habitats with ideal soil-moisture conditions, mature plants can reach heights in excess of five meters, extending well into the subcanopy of typical Hawaiian forests. However, in more exposed areas mature plants rarely grow more than a meter or two in height.
Geographic Range and Distribution
Clidemia hirta is a native species of the tropical Americas, specifically Central and South America, and the island groups of the Caribbean. It has become a pest species throughout Oceania, Southeast Asia, and the Indian Subcontinent. Its first documented appearance in Hawaii was on O’ahu in 1941; by 1957 it was officially classified as a “noxious weed” on the island. Subsequently, it spread to the Big Island (1972), Moloka’i (1973), Maui (1977), Kaua’i (1982), and Lana’i (1988). It is believed that many of the most recent serious infestations of Clidemia hirta on Moloka’i, Maui, and Kaua’i were inadvertently initiated by local marijuana growers.
A mature stand of Koster's Curse doing what it does best: choking off the sunlight to smaller plants of the rain forest's understory.
A Weed with all the “Right Stuff”
The success of Clidemia hirta as an invasive weed in Hawaii is due to a set of unique circumstances, traits, and behaviors, including the following:
- Absence of natural enemies and fast-growing competitors
- Ready availability of several dispersal vectors (birds, mammals, and humans) that aid in seed propagation
- Adapts readily to a wide range of environmental conditions
- Quickly takes advantage of being introduced into to a region with a favorable climate
- Prolific seed production
- Rapid growth and maturation
- Reproduction occurs throughout the year under favorable conditions
- Spreads rapidly into gaps in a habitat opened by fires, storms, and soil disturbance
- Forms dense, impenetrable thickets under forest canopies and shades out understory vegetation beneath them.
Tags: Clidemia, Clidemia hirta, Hawaiian plants, invasive plant species, invasive plants of Hawaii, invasive species, invasive weed, invasive weeds, Koster's Curse, linkedin, Maui's weeds, noxious weed, noxious weeds, plants of maui, Soap Bush, Soapbush, the curse, weed, weeds
