Family: Verbenaceae

Origin, Description & Uses
Verbena bonariensis (tall verbena or purpletop vervain) is a tall, upright perennial wildflower native to South America, ranging from Brazil and Bolivia south to Argentina and Chile. It produces clusters of small lavender-purple flowers atop long, slender stems that bloom over an extended season and attract butterflies, bees, and other pollinators. Because of its airy growth habit, drought tolerance, and long-lasting floral display, it is widely cultivated as an ornamental in gardens, borders, cottage landscapes, and pollinator plantings. It readily reseeds under favorable conditions and thrives in sunny, well-drained sites.

Risks & Threats
In Hawaiʻi, Verbena bonariensis is naturalized on Kauaʻi, Molokaʻi, Lānaʻi, and Maui. This species possesses traits that are cause for concern, including prolific seed production, the ability to self-sow and spread beyond cultivation, and successful establishment in disturbed habitats. These characteristics allow it to form persistent populations that may compete with native vegetation and detrimentally impact tropical island ecosystems. Because it is already naturalized in the Hawaiian Islands, gardeners and landscapers should avoid planting it in or near natural areas and help prevent its spread into islands and regions where it is not yet widespread. When selecting plants for landscaping, choose a low-risk or native alternative whenever suitable options are available.

High Risk Traits:

• Broad climate suitability
  
• Thrives and spreads in regions with tropical climates
  
• Naturalized on Kauai, Molokai, Lanai. Maui and controlled on Oahu (Hawaiian Islands) and widely naturalized elsewhere
  
• A common weed of roadsides, pastures, grasslands, open woodlands, riparian vegetation, crops, orchards, gardens, disturbed sites, and waste areas
  
• A potential agricultural and environmental weed (although impacts are generally unspecified)
  
• Other Verbena species are invasive weeds
  
• May have low palatability (although some sources state it is browsed by cattle)
  
• May be toxic to grazing animals (unconfirmed)
  
• Tolerates many soil types
  
• Can form dense, localized stands that may exclude other vegetation
  
• Reproduces by prolific seed production
  
• Self-compatible
  
• An annual or perennial, capable of reaching maturity in one growing season
  
• Seeds dispersed externally and internally by animals, by wind, in water, as a seed contaminant and through intentional cultivation
  
• Seeds may form a persistent seed bank (longevity not determined)

Low Risk Traits:

• No reports of serious detrimental impacts in the Hawaiian Islands
  
• Unarmed (no spines, thorns, or burrs)
  
• Grows best in high light environments (dense shade may inhibit spread)
  
• Herbicides may provide effective control