Family: Haloragaceae

Origin, Description & Uses
Myriophyllum aquaticum (Brazilian water milfoil, parrot’s feather) is a submerged to emergent aquatic plant native to Central and South Tropical America. It is easily recognized by its feathery, bright green leaves that resemble small bottle brushes or “parrot feathers,” often forming dense mats in freshwater ponds, ditches, and slow-moving streams. In ornamental water gardens, it has been widely used for its attractive foliage and ability to provide cover for aquatic life, as well as for erosion control and water filtration in managed systems.

Risks & Threats
In Hawaiʻi, Myriophyllum aquaticum is naturalized on Kauaʻi, Oʻahu, and Hawaiʻi Island. This species possesses traits that are cause for concern, including rapid vegetative spread and the ability to form thick, floating mats that block sunlight, reduce oxygen levels, and disrupt native aquatic ecosystems. These dense infestations can degrade water quality, interfere with recreation, and outcompete native aquatic plants. Because of its persistence and aggressive growth, it could detrimentally impact tropical island freshwater habitats. Choosing low-risk or native aquatic plants is recommended whenever suitable alternatives are available.

High Risk Traits:

• Broad climate suitability (tropical to temperate; tolerates shade, variable water levels, and ice)
  
• Naturalized outside native range (e.g., Hawaii, Europe, Australia, New Zealand, Japan)
  
• History of repeated introductions (escaped aquarium/ornamental trade)
  
• Agricultural weed (invades rice fields, irrigation systems)
  
• Environmental weed (forms dense floating mats, displaces native species, reduces oxygen, alters habitat)
  
• Congeneric weed (e.g., M. spicatum is also invasive)
  
• Forms dense thickets
  
• Aquatic (facilitates spread in water bodies)
  
• Reproduces by vegetative fragmentation (fragments readily regenerate)
  
• Minimum generative time = 1 year (rapid vegetative reproduction)
  
• Propagules dispersed unintentionally (boats, waterfowl, flowing water, human activities)
  
• Propagules dispersed intentionally (aquarium trade, ornamental use)
  
• Propagules water-dispersed (fragments move via streamflow)
  
• Propagules dispersed by other animals externally (waterfowl, vertebrates)
  
• Tolerates mutilation/cultivation (mechanical removal can spread fragments)
  
• Herbicide control often requires multiple applications or specific combinations

Low Risk Traits:

• No spines, thorns, or burrs
  
• No evidence of toxicity to animals or humans
  
• Does not tolerate very low nutrient soils (prefers nutrient-rich sites)
  
• No viable seed production outside native range (only female plants present)