Family: Acanthaceae

Origin, Description & Uses
Ruellia simplex (commonly known as Mexican petunia, Mexican bluebells, or Britton’s wild petunia) is native to Mexico and tropical regions of the Americas. It is a fast-growing perennial plant valued in landscaping for its showy, trumpet-shaped purple to bluish flowers and its ability to bloom repeatedly throughout the year in warm climates. Because it is hardy, drought-tolerant once established, and able to thrive in a wide range of conditions, it has been widely used as an ornamental plant in tropical and subtropical gardens, including hedges, borders, and mass plantings.

Risks & Threats
In Hawaiʻi, Ruellia simplex is naturalized on Oʻahu and Maui and is potentially naturalizing on Molokaʻi, indicating it is already establishing outside of cultivation in parts of the state. This species spreads aggressively through both prolific seed production and vegetative growth, allowing it to form dense stands that displace native vegetation and reduce habitat quality for native species. It is considered a species of concern because its persistence and spread can alter natural ecosystems, particularly in moist and riparian habitats where it can dominate understories and streamside areas. Because it is naturalized and possesses traits that increase its invasive potential, it could detrimentally impact tropical island ecosystems if its spread continues. When available, low-risk or native plant alternatives are strongly recommended, and in areas where it is not yet widely established, its use should be carefully managed or avoided to help prevent further spread and ecological harm.

High Risk Traits:

• Broad climate suitability
  
• Thrives in tropical climates
  
• Naturalized in Australia and the southeastern US, but no evidence in the Hawaiian Islands to date
  
• An environmental weed (competes with native vegetation)
  
• Other Ruellia species are invasive
  
• Shade-tolerant
  
• Tolerates many soil types
  
• Forms monotypic stands that exclude other vegetation
  
• Reproduces by seeds and vegetatively by rhizome and stem fragments
  
• Self-compatible
  
• A perennial, but able to reach maturity in one growing season
  
• Seeds dispersed by explosive dehiscence, by water, and by adhering to animals and possibly people and equipment
  
• Vegetative fragments dispersed by water and as garden waste
  
• Seeds may persist in the soil for 13-15 months
  
• Able to resprout after cutting

Low Risk Traits:

• Unarmed (no spines, thorns or burrs)
  
• Non-toxic
  
• Sterile cultivars may reduce invasiveness
  
• Herbicides may provide effective control