The Nottawasaga Valley Conservation Authority

Rough Manna Grass​

What is Rough Manna Grass?

Rough Manna Grass (Glyceria maxima) is one of Ontario’s most unwanted invasive plants. 

Also known as reed or giant manna grass, rough manna grass is commonly found spreading into wetlands. Patches of rough manna grass are currently expanding in the internationally significant Minesing Wetlands!

  • Rough manna grass can grow up to 2.5 m (8 ft) tall. 
  • Their flowering stems are hollow and ribbed lengthwise. 
  • Leaves are quite long, up to 60 cm (23 in). 
  • The seed heads at the top of the plant are upright compared to our native manna grass species where the seed heads tend to bend over. 
  • Rough manna grass is found in moist soils, open wetlands and shorelines of rivers and streams.
Rough Manna Grass

Where is Rough Manna Grass found in our watershed?

Rough manna grass was first reported in Minesing Wetlands in the 1970s. Since then it has colonized hundreds of hectares of former forested swamp. Although rough manna grass has reduced plant biodiversity in the Minesing marshes, marsh birds such as American bittern, least bittern, sora and Virginia rail continue to frequent these marsh habitats.

Why is it of concern?

Rough manna grass grows best in very moist and nutrient rich areas which are often associated with human disturbance.Once established, patches of this wetland grass form dense strands crowding out native marsh species and eventually displacing them. Infestations can impair wetland diversity and even impair flow and water quality in wetland streams.

How to control the spread

  • Removal of rough manna grass can be quite difficult. Herbicides are not suggested since this species is mainly found in inundated marshes where herbicides are not permitted and where they may do more harm than good.
  • In small infestations on private property, the grass can be pulled manually.
  • In places like the Minesing Wetlands where stands are large and densities are heavy, we can only focus on preventing spread of the rough manna grass into new areas.
  • Remember to clean your canoe or vehicles after being in an area that contains rough manna grass.
  • Keep pets on a leash to avoid further spread.

Early detection is key

Early detection and rapid response is essential for the control and/or eradication of invasive species in an area. If you spot Rough Manna Grass, please report the sighting to EDDmapS.

More information

Invasive species on private property

NVCA does not offer a service to remove invasive species on private property.  If you are looking for tips on dealing with invasive species on your property, see the links above, or visit Ontario’s Invading Species Awareness Program.

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