Family: Fabaceae

Origin, Description & Uses
Senna septemtrionalis (arsenic bush, smooth senna, kolomona) is a shrub or small tree native to Mexico and Central America. It produces bright yellow flowers, compound green leaves, and slender seed pods, making it an attractive ornamental plant for tropical and subtropical landscapes. The species has been cultivated for use as a hedge, screen, ornamental planting, and in some regions for traditional medicinal purposes. In Hawaiʻi, it has been cultivated since the 19th century and escaped from cultivation as early as 1871. Today, it is naturalized on Kauaʻi, Oʻahu, Molokaʻi, Lānaʻi, Maui, and Hawaiʻi Island, where it occurs primarily in low-elevation dry and disturbed habitats, but can also be found in pastures and along the margins of wetter forests.

Risks & Threats
Senna septemtrionalis is naturalized in Hawaiʻi and possesses traits that are cause for concern. It produces abundant seeds, establishes readily in disturbed environments, and can spread beyond cultivation into natural and semi-natural ecosystems. Its ability to thrive across a range of environmental conditions, from dry lowlands to higher-elevation pastures and forest edges, increases its potential to expand into new areas. Dense populations may compete with native vegetation and contribute to changes in plant community structure, potentially impacting tropical island ecosystems. Because this species is already widespread in Hawaiʻi, landowners and landscapers should avoid planting it in areas where it is not yet common and take steps to prevent further spread from cultivated sites. When selecting plants for landscaping or restoration projects, choosing a low-risk or native alternative is recommended to help protect Hawaiʻi’s unique ecosystems and reduce future invasive species impacts.

High Risk Traits:

• Elevation range exceeds 1000 m, demonstrating environmental versatility
  
• Thrives in tropical climates
  
• Naturalized on main Hawaiian Islands and elsewhere
  
• Environmental weed in Australia and Africa, competing with desirable vegetation (impacts in Hawaii not as well documented)
  
• Other Senna species are invasive
  
• Reported to be unpalatable to animals (but seeds used in fodder)
  
• Unconfirmed reports of toxicity in seeds
  
• Tolerates many soil types
  
• Unclear whether this species in N-fixing
  
• Reproduces by seeds
  
• Reaches maturity in 2 years
  
• Seeds dispersed by water, mud sticking to animals, humans, machinery, and vehicles and as a contaminant of agricultural produce
  
• Seeds form a persistent seed bank

Low Risk Traits:

• Unarmed (no spines, thorns, or burrs)
  
• Seeds may provide fodder for livestock (palatable despite reports of toxicity)
  
• Light-demanding
  
• Not reported to spread vegetatively
  
• Herbicides provide effective control