Invasive Freshwater Plants and Algae

Alligator Weed

• Scientific Name

  Alternanthera philoxeroides - Visit ITIS for full scientific classification. 

• Description

  

  Alligator Weed flower. Courtesy CDFA IPC Program. © 2005 Regents, University of California

  

  Alligator Weed plants at shoreline. D.O. Clark. © 2007 Regents, University of California

  

  Alligator Weed stem with shoots and roots close-up. Courtesy IPC Program. © 2005 Regents, University of California

   

  • Dense, floating mats that are rooted in soil of shallow water.
  • Floating form has hollow stems and terrestrial form has solid stems.
  • Stems can reach over 3 feet long.
  • Leaves are in pairs along stem.
  • Leaves are narrow: up to 4 1/3" long and up to 1" wide.
  • Flower spikes are fragrant, pearly and up to 3 1/2" long. 
• Habitat

  • Shallow, slow moving water or wet (or sometimes dry) soil.
  • Tolerates up to 10% salinity.
  • Needs warm summer.
  • Can tolerate cold winter but not a long freeze. 
• Invasion Pathways and Distribution

  • Believed to have been introduced from South America.
  • Also occurs in Central America.
  • In California it is found in the San Joaquin Valley and in Los Angeles, San Bernardino and Riverside Counties.
  • Pieces of mats break off and can infest new areas. 
• Life History

  • Rarely produces seed.
  • Spreads by stolons (a type of stem) that produce new plants. 
• Impacts

  • Dense mats reduce light and displace native species.
  • Especially heavy infestations can reduce oxygen and favor mosquitos. 
• References and Useful Links

  For references by category and links to other useful AIS sites see our LEARN MORE page.