Family: Costaceae
Costus dubius, commonly known as costus, is a tropical, spiral-stemmed plant native to parts of Africa. It is known for its lush, upright growth, broad glossy leaves arranged in a distinctive spiral, and showy flowers that add a bold, ornamental look to landscapes. Because of its attractive foliage and tolerance of warm, humid conditions, costus has been widely planted in gardens as an ornamental and sometimes used in tropical landscaping to create a dense, leafy accent in shaded or partially shaded areas.
In Hawaiʻi, Costus dubius is naturalized on Oʻahu and possesses traits that are cause for concern. This species can spread beyond cultivation, form dense clumps, and potentially displace native plants by competing for light, space, and resources. Such behavior could detrimentally impact tropical island ecosystems, which are especially vulnerable to invasive species. To help protect Hawaiʻi’s unique natural environments, it is recommended to avoid planting costus and instead choose a low-risk or native alternative that supports local biodiversity and ecological health.
High Risk Traits:
- High climate match for tropics/subtropics.
- Naturalizes readily where introduced (e.g., Hawaii arboretum).
- Invasive congener (Costus speciosus).
- Forms dense thickets vegetatively.
- Prolific seed producer.
- Spreads via rhizome fragments.
- Short time to maturity (1–2 years).
- Intentionally planted & sold ornamentally.
- Resprouts vigorously after damage.
Low Risk Traits:
- Not reported as a weed in agriculture/forestry.
- Not harmful: non-toxic, non-spiny, not allelopathic.
- Low unintentional dispersal risk (not wind, traffic, or contaminant-dispersed).
