Help us to identify and report, but avoid infested sites

Glyceria maxima (a.k.a. Tall manna grass or Reed sweet grass) is an exotic, perennial grass capable of invading wetlands such as swamps, ditches, and marshes as well as edges of lakes, ponds, rivers, and creeks. Tall manna grass often forms huge monotypic stands that crowd out all native vegetation, ultimately eliminating habitat for native wildlife species.

The grass was first reported in Wisconsin in 1975 in Racine County, and is currently confirmed in only 9 southeastern counties, though several populations have been reported elsewhere in the state. Under NR40 regulations it is therefore restricted in the southeast region and prohibited in the rest of the state where few stands might allow elimination and/or prevention. Nationally, its distribution has been limited to two east coast states, Washington State, and Wisconsin, but reports suggest it is starting to spread.

This invasive grass can cause unusual problems and will be especially difficult to control since it can grow on shallow stream beds up to 4 feet deep, often from bank to bank. In several southeastern Wisconsin streams it now grows so thickly that it slows or even blocks water flows. Ensuing problems can include reduced fish populations and recreation, increased stagnant water (more mosquitoes) and raised water levels upstream capable of killing adjacent upland vegetation. Controlling such sites will be difficult since the primary control option is use of herbicides, but use in flowing water systems is almost impossible.

Thus, preventing further dispersal by its several mechanisms, especially along streams, is critical. It readily produces seeds, especially right after initial invasion of a new site. Then it fills suitable locations vegetatively through extensive rhizome development. Seeds are easily transported by water and humans, but both rhizome and stem fragments can also grow roots to become new stands, especially when they are carried downstream and catch on new stream banks or beds.  Learn more by downloading this G. maxima fact sheet.

The limited spread of Tall manna grass has enabled WDNR to obtain limited federal funding to prevent its further spread, and eliminate stands where possible. To do so, the Department needs the help of all resource users, such as hunters, fishers, bird watchers, and boaters - everyone who accesses typical sites. All should learn to identify and report stands, and even more importantly, avoid aiding its dispersal by staying out of infested sites to avoid unknowingly picking up and moving seeds or releasing plant fragments into waters.

Recognize Tall manna grass by its typical locations, very early green-up in spring, 7+ foot flowering stems with a very wide, stiff  panicle, extensive and monotypic growth habit, and other physical traits, including leaf blades 8-18 mm wide, and leaf sheaths with a sticky feel when sliding a finger back and forth on it.

If you suspect this species, contact WDNR staff. Email:  or ; 608-266-2554.