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Report an Invasive Species

Giant Mannagrass

Giant Mannagrass

Giant Mannagrass

(Glyceria maxima)

Priority: -  Prevent

Tags: Aquatic

Identification and Reproduction


  • Giant mannagrass is a tall aquatic grass that can grow up to 2.5 m tall. 
  • Stems grow erect, hollow and often reddish on the lower stem. 
  • Leaves are stiff, rough textured, triangular at base and can reach up to 40 cm long. 
  • Leaf margins are covered in short stiff hairs. 
  • When leaf is folded back from stem a translucent ligule is exposed. 


  • Inflorescence is openly branched and made up of yellow to green spikelets that mature to purple. Flowers are between 10 and 40 cm long. 


  • This plant can reproduce vegetatively or by means of seed production. 
  • Rhizome or root fragments can break and establish new plants. 
  • Root systems can extend a metre deep and can make up half of the plants biomass. 
  • It is dormant during the winter and stores energy in its root system. It will send out new shoots during the new growing season. 

Habitat & Ecology

  • It grows in wet areas and in shorelines that have deep waters up to 2 m deep.
  • It thrives in waterlogged soils that have direct sun. 
  • When bordering deep waters it can form a floating mat that attaches to the bank.  
  • This grass grows along the edges of lakes, rivers and streams. 



  • Its dense root system can out-compete and suppress native wetland plants. 
  • Creates a monoculture taking over the wetland. 
  • It can also alter the ecosystem's food chain by altering wildlife habitat and fish migration patterns. 
  • Can contain high levels of cyanide that can cause wildlife or cattle poisoning. 
  • Dense monocultures can block water flow leading to flooding. 



  • Small infestaions can be dug up and removed. 
  • Dense infestations can be covered by black tarp during the growing season to smother plants. 
  • To prevent further spread mow and cut prior to seed set. This treatment will need to be repeated at least 3 times in a growing season. 
  • Treatments will need to be applied for at least 2-3 years to ensure regenerated sprouts are removed. 
  • Dry plant fragments before disposal. 


King County provides a good resource for identification and control of Glyceria maxima. Note this is a US resource and Canadian guidelines and regulations may differ. Be sure to read product labels carefully. 

Header photo (Harry Rose).