An evergreen branched shrub growing up to 9 feet tall (3.5 feet more commonly) is densely covered in hair. Native to Tropical America, Clidemia hirta, commonly known as Kosters curse or simply Clidemia, is an invasive weed in many tropical locations. Its pathway into Hawai’i is unknown, ornamental, or accidental thru contamination. It’s hard to imagine that someone would intentionally grow this vile plant! The first official record of invasiveness occurred in 1949; however, evidence suggests it was naturalizing as early as 1941.
Its invasive characteristics are astounding: clidemia reaches reproductive maturity in 6 months, it’s self-compatible, it grows back vigorously after cutting or mutilation and the seeds remain viable yet dormant in the soil for up to four years. Its seed production is perhaps the most alarming. An average of 10,000,000 seeds can be found in a 1 square meter area of a clidemia thicket. Laid end to end, the tiny seeds would make a line 3.5 miles long.
It is next to impossible to eradicate this pest once it starts producing viable seeds. Furthermore, the tiny seeds are dispersed accidentally by humans as a contaminant. Evidence suggests that marijuana growers, hikers, and hunters are partly responsible for intra-island and inter-island dispersal via contaminated clothes, gear, or soil. Incipient patches were found close to abandoned marijuana patches and along trail sides. Other infestations occurred alongside 4 x 4 roads suggesting contaminated tires were the cause. Pig hunters are thought to be the disperser for the Laupāhoehoe forest invasion. Clidemia is ugly. It interferes with lawns and gardens, it provides endless weeding tasks, and it has no uses. All despise Clidemia. No one in their right mind would intentionally spread this plant.
Besides long-distance dispersal through contamination, seeds are eaten and dispersed by birds, rats, and mongoose. The Clidemia invasion has been well documented; it happens right before our eyes, almost in real-time. Hurricane Iwa hit Oahu in 1981. The storm disturbed and opened the vegetation along the Contour trail in the Waiʻanae Mountains. In two short years, an impenetrable thicket of Clidemia ten feet tall dominated the forest. This disturbance oriented plant takes no time establishing a population that excludes all other species, including mosses! In the 1990 edition of “Manual of Flowering Plants,” Wagner suggested that Clidemia be controlled on Hawai’i Island, as the invasion was new and still possible to eradicate. Clidemia is listed as a noxious weed in Hawaii and as on the “100 of the World’s Worst Invasive Alien Species”.
Description and Dispersal:
- An evergreen branched shrub growing up to 9 feet tall (3.5 feet more commonly) and densely covered in hair
- Leaves are light green in color, are arranged opposite each other and are finely toothed on the edges
- Leaves have 5 prominent longitudinal veins, a descriptor for the Melastomataceae family, along with horizontal veins gives a scalloped appearance in the leaves
- The upper leaf is sparsely covered in pink hair, fold a leaf and look closely to observe
- Flowers are born from a cup, are densely covered in hair, are white, have 5 petals along with 5 stamens, are not showy and are born at the nodes or branch tips
- Fruit is dark blue/purple in color, also covered in small hairs and with a persistent calyx
- Each fruit contains 200 to 900 small seeds, fruit comes in numerous bunches and is also covered in hair.
- Seeds are spread by fruit eating birds
High Risk Traits:
- Elevation range exceeds 1000 m, demonstrating environmental versatility
- Thrives in tropical climates
- Widely naturalized tropical weed
- Weed of agriculture and pastures
- Environmental weed
- Possibly toxic to browsing animals
- Tolerates many soil types
- Forms dense thickets and prevents native plant regeneration
- Reproduces by seeds
- Able to produce seeds through agamospermy
- May be able to spread vegetatively (but primarily by seed)
- Reaches maturity in <1 year
- Seeds dispersed by birds, water and and unintentionally as a result of human activity
- Prolific seed production (>1000/m2)
- Forms a persistent seed bank (at least 4 years)
- Able to resprout after cutting to the base
- No effective biocontrol agents present in the Hawaiian Islands
Low Risk Traits:
- Unarmed (no spines, thorns or burrs)
- Herbicides may provide effective control
- In some locations, biocontrol has been effective