Alligator Weed

Scientific name: Alternanthera philoxeroides
  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Low-growing, non-woody perennial
  • Long horizontal stems (stolons), up to 10 metres long, can float on the surface of water, forming rafts, or grow onto banks, forming closely matted clumps
  • Leaves are shiny, spear-shaped, in opposite pairs or whorls, 2-7 cm long and about 1-2 cm wide
  • Small white papery flower heads, similar to clover heads, 1-2 cm in diameter, on 2-7 cm stalks generally appear from November to March
  • Stems grow to 60 cm high and have large, hollow internodes
  • On land, adventitious roots and thickened taproots form, stems are shorter, and internodes smaller and less hollow.


  • Originally from Brazil, and now a major weed problem in parts of the United States, Australia, China, India, Thailand and Indonesia, this plant has been described as one of the world’s worst aquatic and terrestrial weeds
  • First discovered in New Zealand in the northern Wairoa River, near Dargaville, in 1906. It is believed to have arrived with ship’s ballast water
  • Now widespread in Northland as a serious weed of waterways and cropping land. Common in Auckland waterways and there are several infestations in a range of habitats in the Waikato.


  • Low-growing herb that grows very quickly in summer
  • Mats may extend for 15 m over the water surface and become so robust they can support the weight of a person
  • Tends to be more prostrate on land, developing a very deep root system
  • Does not produce viable seed, so reproduction is only vegetative
  • Frost and ice kill exposed stems and leaves, but  protected stems can survive to support the next season’s growth
  • Responds to high levels of nutrients and withstands some salinity in flowing brackish water.

  • As a terrestrial weed it grows rapidly, particularly under warm, wet conditions, and can out-compete crops and pastures
  • Alligator weed will take up heavy metals from the soil and is known to be toxic to some livestock
  • Aquatic alligator weed can rapidly spread in streams and drainage canals, clogging them with vegetation and trapping sediments, thus increasing risk of flooding
  • In natural wetland areas alligator weed is a threat to native flora and fauna and is extremely difficult to eradicate once established in such a habitat
  • Alligator weed is listed in the National Pest Plant Accord and it is therefore illegal to propagate and knowingly spread this plant.

  • Ensure that alligator weed fragments are not carried on vehicles or equipment
  • Farmers should protect their properties by:
    • insisting all contractors clean their equipment before entering the farm
    • ensuring supplementary feed brought onto the farm is weed free
    • ensuring aggregates, soil and sand brought onto the farm are weed free
  • To prevent the spread of alligator weed and other aquatic weeds such as didymo, boat operators should clean their boats and trailers when leaving waterways.