Identification and Reproduction

Identification:

• 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. 

Reproduction: 

• 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. 

Impacts

Ecological: 

• 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. 

Management

Mechanical/Manual: 

• 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. 

Resources

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).