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Oxygen Weed

Lagarosiphon major
Home 9 Invaders 9 Aquatic Plants 9 Oxygen Weed

What is Ontario Doing?

To prevent this unwanted invader from coming into the province, Ontario has regulated oxygen weed as prohibited under the Invasive Species Act, 2015. 


Oxygen weed is a perennial, submerged aquatic plant native to South Africa that is popular in the aquarium and pond scaping industries. It can grow in dense floating mats that can outcompete native aquatic plants and remove fish and invertebrate habitat.


The plant reproduces vegetatively outside of its natural distribution and can easily become invasive in the event it is introduced to new waterbodies from illegal dumping or transmittance via watercraft equipment. To date, it has not been found in the wild in Ontario.  

Impacts of Oxygen Weed

  • It can be easily transported from one waterbody to another and grows aggressively to compete with native plants. 
  • It forms dense mats of floating vegetation, blocking available sunlight, reducing water quality, and negatively impacting fish habitat. 
  • Mats can dislodge and cause stagnant waters as they decompose, killing native plants and animals. 
  • Dense mats can hinder recreational activities, such as boating, angling, and swimming. 

How to Identify

  • Submerged perennial that appears dark green with spiraling leaves that have minute serrations along the edges, leaves curl strongly downward and back up. 
  • Stems can grow 6.5 m (20 ft) long and create mats 1 m (3 ft) thick. 
  • Similar in appearance to hydrilla and Elodea species 

What You Can Do

    • Learn how to identify oxygen weed and how to prevent the introduction or spread of this plant with your watercraft and/or fishing equipment. 
    • Never buy, plant, or keep oxygen weed in your aquarium or water garden. It is against the law to buy, sell, trade, possess, or transport oxygen weed. 
    • Never deposit unwanted aquarium or water garden plants into Ontario’s lakes or rivers. Dispose of them in the garbage or away from any waterbody. 
    • Clean, Drain, Dry your boat, trailer, and equipment after each use, be sure to remove all plants, animals, and mud before moving to a new waterbody. 


OFAH/OMNRF Invading Species Awareness Program. (2023). Oxygen Weed. Retrieved from:
This factsheet may be reproduced for non-commercial purposes.

Header photo by susan_marley | iNaturalist 

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