Glyceria maxima (Hartm.) Holmb.

Common Name: Reed mannagrass

Synonyms and Other Names:

Catabrosa hydrophila, Exydra aquatica, Festuca aquatica, Glyceria altissima, Glyceria aquatica, Glyceria spectabilis, Heleochloa aquatica, Hydropoa spectabilis, Molinia maxima, Panicularia aquatica, Poa aquatica, tall mannagrass, English water grass, giant manna grass, reed manna grass, reed-meadow grass, reed sweet grass, sweet reedgrass, water-meadow grass

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Identification: Glyceria maxima is a perennial, helophytic, rhizomatous grass with unbranched stems. Leaf sheaths have prominent midribs, visible transverse veins, and are closed to near the top. The unlobed, membranous ligules are 1.2-6 mm long, smooth, and obtuse in shape. Leaf blades are flat, 30-60 cm long, and 0.6-2.0 cm wide. The leaf blades are shallowly grooved, with prominent midribs. The leaf margins have short, stiff hairs that are rough to the touch (Campbell et al. 2010, Forest Health Staff 2006). 

These are bisexual plants with panicles that can be either open (chasmogamous) or contracted and symmetrical. The inflorescence branches have short, stiff hairs similar to those on the leaf margins (Boos et al. 2010, 2008).

Glyceria maxima could be confused with large specimens of native Glyceria grandis, but that species typically only grows up to 1.5 m, has nodding (rather than upright) inflorescences, and has shorter glumes and lemmas (parts of the grass spikelet) (Boos et al. 2000). It could also be mistaken for Puccinellia because of their similar spikelet structure and preference for wet habitats, but G. maxima is distinguished by its inability to tolerate highly alkaline soils, typically more flexible panicle branches, closed leaf sheaths, and single-veined upper glumes.

Size: up to 2.5m

Native Range: Glyceria maxima is native to temperate Eurasia.

Great Lakes Nonindigenous Occurrences: The first North American record of Glyceria maxima came from Cootes Paradise, at the far west end of Lake Ontario, in the mid 1940s (Wei & Chow-Fraser 2006).  It subsequently spread to other areas of Ontario, where it has overtaken native cattails and other species.

Glyceria maxima was first found in the United States in the 1970s in Wisconsin's Racine and Milwaukee counties.  Cultivated populations have also been documented in both Door and Wood Counties, and an un-vouchered specimen is noted from Calumet County.  In the early 1990s it was found at three sites in Massachusetts' Ipswich River Wildlife Sanctuary in Essex county; these sites were subject to aggressive control measures, and only one site required re-treatment as of 2005.

New discoveries have occurred in recent years, with two new states reporting small infestations.  In 2005, small population was detected in a residential pond near Monroe, Washington; control measures are planned for in 2007.  In the fall of 2006, a dense circular patch was detected at Illinois Beach State Park, seemingly growing outward from a recently-replaced manhole cover.  This population was treated in 2006 and monitoring will continue for a number of years (D. Nelson, pers. comm.). 

Other stands occur in British Columbia, Newfoundland and Alaska. Overseas, it has been found invading Tasmania, New South Wales, Victoria, Queensland, as well as New Zealand.  

Table 1. Great Lakes region nonindigenous occurrences, the earliest and latest observations in each state/province, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Glyceria maxima are found here.

Full list of USGS occurrences

State/ProvinceFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
MI201620172Flint; St. Joseph
WI197920178Bad-Montreal; Door-Kewaunee; Fox; Lower Fox; Manitowoc-Sheboygan; Milwaukee; Northwestern Lake Michigan; Pike-Root

Table last updated 9/26/2023

† Populations may not be currently present.

* HUCs are not listed for areas where the observation(s) cannot be approximated to a HUC (e.g. state centroids or Canadian provinces).

Ecology: Glyceria maxima is typically found in open wetlands such as marshes, meadows, shrub-carrs and along shorelines (Campbell et al. 2010, King County 2012). It performs better in waterlogged soils that have direct sunlight, but can be found in partially shaded areas adjacent to woodlands as well (Forest Health Staff 2006, van der Putten et al. 1997).

Glyceria maxima can expand into shallow water (~ 30 cm) and survive prolonged flooding because of its aerenchyma tissue and superficial root system (Lawniczak et al. 2010, Studer-Ehrensberger et al. 1993). The root system and rhizomes can extend 3 feet down into the soil (King County 2012). When growing near open water, reed mannagrass can form floating mats attached to the shore (King County 2012).

This species primarily reproduces vegetatively via rhizomes in North America (Campbell et al. 2010, Forest Health Staff 2006). Reed mannagrass emerges early in the year and concentrates up to 50% of its biomass in its root system (Westlake 1966). The energy stored in the roots and rhizomes enable this species to produce new shoots through the growing season (Buttery and Lambert 1965). Muskrats and beavers may aid the expansion of G. maxima. While foraging, plants may become uprooted and portions of the rhizomes may break off, float down stream, and re-establish (Forest Health Staff 2006).

Glyceria maxima also has florets that can bloom and produce viable seed (IPANE 2004). Individuals are in bloom between June and August. Once the inflorescences are mature, the panicle opens and rises above the other foliage (Campbell et al. 2010, Forest Health Staff 2006). The dark brown seeds are 1.5-2 mm in length, egg-shaped, and smooth except for a deep, slender furrow down the middle (IPANE 2004). Seeds dispersed in the fall will likely germinate the following spring; however, seeds can remain dormant and viable in the soil for several years (King County 2012).
During the winter, reed mannagrass becomes dormant. In early spring, regrowth occurs from rhizomes buds (King County 2012).

Means of Introduction: This species is thought to have been introduced intentionally as a forage species in some cases (Barkworth et al. 2000, USEPA 2008).  Alternative pathways may include ornamental introductions or seeds hitchhiking with packing material, migrating waterfowl or workers and/or their equipment.

Status: Established in the Great Lakes region.

Great Lakes Impacts:  

Glyceria maxima has a moderate environmental impact in the Great Lakes.

Glyceria maxima invades numerous wetland ecosystems: swamps, lakes, ponds, slow-moving rivers and creeks, ditches, and wet meadows (Boos et al. 2010). Early emergence in spring and rapid growth enables this species to outcompete other wetland plants (Buttery and Lambert 1965, King County 2012). Glyceria maxima can form monospecific stands and reduce plant diversity along the shore to a depth of about 15 cm (Andersson 2001, Boos et al. 2010, Forest Health Staff 2006). Reed mannagrass is not a suitable food source or nesting site for many wetland species. Expansion of G. maxima degrades the ecological dynamics in the wetland (Forest Health Staff 2006). The displacement of native vegetative often leads to an altered macroinvertebrate community, which can impact the entire food web for the ecosystem (King County 2012).

Glyceria maxima has an extensive system of roots and rhizomes (King County 2012).Dense populations of this species create rhizomal mats that can trap sediment faster than native species. This increased sedimentation can alter the flow of water, restrict or clog small waterway and drainages, and cause flooding (Forest Health Staff 2006, King County 2012).

The availability of organic material and denitrifying capacity is high in G. maxima dominant ecosystems (Kallner Bastviken et al. 2007). Glyceria maxima also uptakes available ammonium, which further decreases nitrifying activities (Bodelier et al. 1998). As G. maxima increases in a habitat, the availability of nitrogen in the soil could decrease.

Glyceria maxima may also be a competitive threat to native species of mannagrass. Native species listed as threatened or endangered in at least one Great Lakes state include G. acutiflora Torr., G. arkansana Fernald, G. borealis (Nash) Batchelder, G. grandis S. Watson, and G. obtusa (Muhl.) Trin (PLANTS Team 2012).

There is little or no evidence to support that Glyceria maxima has significant socio-economic impacts in the Great Lakes.

Glyceria maxima has been used as forage, however cattle may experience cyanide poisoning if allowed to graze on young shoots (Boos et al. 2010, King County 2012).

Large populations of G. maxima can impede water flow, alter hydrology, and restrict access to natural waterways, irrigation, and drainage channels. Reduced flow rates in waterways from siltation and debris build-up also creates breeding habitat for mosquitoes. In Tasmania, populations of G. maxima have created so much additional silt (from reduced water flow) that shallow dams have become useless (Department of Primary Industries 2012).

There is little or no evidence to support that Glyceria maxima has significant beneficial effects in the Great Lakes.

Glyceria maxima is sold and used as an ornamental plant (King County 2012).
In areas where G. maxima begins growth early in the season, it can out-compete Phragmites australis (Studer-Ehrenseberger et al. 1993).

Glyceria maxima tolerates low oxygen concentrations and thrives in eutrophic environments, making it an ideal species for artificial wetland systems (Sunblad and Robertson 1988). Glyceria maxima has been used to treat the wastewater from swine farms in integrated constructed wetlands (ICW) in Ireland. During an 18-month study, the ICW successfully removed 98.1-99.9% of the ammonia-nitrogen (Harrington et al. 2012). In other ICWs planted with only G. maxima, there was significant reduction in total organic nitrogen, ammonia-nitrogen, nitrate-nitrogen, and molybdate reactive phosphorus (Harrington and Scholz 2010). In experiments by Sundblad and Robertson (1988) in the Czech Republic, harvesting G. maxima increased nutrient recovery from wastewater.

Management: Regulations (pertaining to the Great Lakes region

Glyceria maxima poses a high ecological threat to ecosystems; therefore, the New York Invasive Species Council recommends that this species be prohibited (New York Invasive Species Council 2010). The New Invaders Watch Program lists G. maxima on its “watch list” for Illinois (Maurer 2009). Glyceria maxima is prohibited from transport, transfer, or introduction in Wisconsin; however, there are exceptions made for several counties (Bureua of Plant Industry 2012).

The Great Lakes Indian Fish and Wildlife Commission (GLIFWC) has not detected G. maxima within its territories. To keep it from invading, the GLIFWC recommends controlling any individuals of this species immediately (Falck and Garske 2003).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Glyceria maxima is a perennial species; therefore, populations may require treatment for two to three years for complete control (USACE 2011a).

There are no known biological control methods for this species.

Small infestations of G. maxima can be dug up; care should be taken to remove all parts of the roots and rhizomes (Forest Health Staff 2006). Subsequent removal of seedlings germinated from the seed bank or missed rhizomes pieces may be necessary (King Country 2012). Small, dense communities of reed mannagrass can also be controlled by being covered with black plastic for 5 or 6 weeks during the growing season (Forest Health Staff 2006).

The vegetative spread of larger populations can be controlled by repeated mowing, cutting, harvesting, roto-tilling, or rotovating (Sundblad and Robertson 1988, USACE 2011b). Where applicable, these treatment methods can be supplemented with artificially created flood conditions (Hroudová and Zákravský 1999). Mowing or cutting two to three times a summer may deplete the energy reserves in the roots and rhizomes. This may reduce G. maxima’s ability to compete and allow other vegetation to expand into the site (King County 2012).

A foliar spray of glyphosate (3% solution) applied early to late summer will control populations of G. maxima (King County 2012, USACE 2011a). Rhizomes may survive after initial spraying (USACE 2011a). Braverman (1996) found that glyphosate at 2 kg ai/ha and dalapon (2,2 dichloropropanoic acid) at 10 kg ai/ha controlled G. maxima. Imazapyr is most effective on reed mannagrass when applied in summer or early fall and when water levels are low and plant stems are not submerged (King County 2012, USACE 2011a).

In floating fens in the Netherlands, sulfate was experimentally added to the soil. This caused the free sulfide concentration to increase and resulted in a decrease in the growth of G. maxima (Loeb et al. 2007).

For more information on chemical control methods please see the Pacific Northwest Weed Management Handbook (

For large populations, herbicide treatment will be an effective option. If the decaying plant material falls into a nearby body of water and decomposes, the dissolved oxygen levels could decrease. To avoid this, dead plant material should be removed two to four weeks after herbicides have been applied (King County 2012).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions

References (click for full reference list)

Author: Berent, L., and V.M. Howard

Contributing Agencies:

Revision Date: 8/22/2012

Citation for this information:
Berent, L., and V.M. Howard, 2023, Glyceria maxima (Hartm.) Holmb.: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI,, Revision Date: 8/22/2012, Access Date: 9/26/2023

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.