Clidemia hirta is a small shrub that looks unremarkable in its native tropical America. In other parts of the world it has been named among the most damaging invasive plants of tropical forests, and its common name, Koster's curse, says plenty about its reputation.
Quick facts. Family: Melastomataceae. Native range: southern Mexico, the Caribbean and tropical South America. Introduced range: Hawaii and many other Pacific islands, parts of tropical Asia, Africa and Australia. Growth form: hairy shrub, typically one to three metres. Fruit: small blue-black berries.
What it looks like
Clidemia hirta is covered in stiff hairs, as the species name hirta (hairy) suggests. The opposite leaves show the conspicuous longitudinal veins typical of the family and are fringed with hairs. Small white flowers give way to berries that ripen blue-black and contain a great many tiny seeds.
How it spreads
The traits that make Clidemia hirta effective as an invader are worth understanding, because they recur in other invasive melastomes.
- Prolific, tiny seeds. A single plant can produce huge numbers of seeds, and the seeds can remain viable in the soil for years, forming a persistent seed bank.
- Bird dispersal. Fruit-eating birds, along with other animals, carry seeds far from the parent plant.
- Shade tolerance. Unlike many weeds that need open ground, Clidemia hirta can establish in the shady understory of intact forest, which makes it unusually hard to exclude.
- Rapid growth. It forms dense thickets that shade out native seedlings and shrubs.
Its spread was assisted by people. Plants and seeds travelled on boots, vehicles, livestock and in soil, and the species was moved between islands, sometimes deliberately for ornamental use.
Impact
In Hawaii, where it was first recorded in the 1940s, the shrub invaded native forest on several islands and altered the understory. On islands with many unique plants and few competing shrubs, an aggressive newcomer can have disproportionate effects on native biodiversity.
Management
Control of Clidemia hirta has been one of the better-known biological control efforts for an invasive shrub. In Hawaii, insects and a fungal disease from its native range were introduced to attack the plant. A thrips introduced from Trinidad reduced the plant in sunny areas, though shaded forest populations proved harder to suppress. Other tools include manual removal of small plants, targeted herbicide use by land managers, and cleaning boots and gear to avoid spreading seeds.
What gardeners should know
Clidemia hirta is not a plant to grow. If you live in a warm, humid region, check local invasive species lists before planting any melastome. The family also includes well-behaved ornamentals, so it is easy to choose safely. For the general ecology of the problem, read why some melastomes are considered invasive weeds and the profile of Miconia.
Key takeaways
- A native of tropical America, now invasive across the Pacific and beyond.
- Spread by tiny, numerous, bird-dispersed seeds that persist in the soil.
- Its shade tolerance lets it invade intact forest.
- Biological control has helped in open areas but not solved the problem in deep shade.