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Weed Alert!

Glyceria maxima (C. Hartm.) Holmb.
(Reed sweetgrass)

Summary: new plant of unknown characters
Glyceria maxima was first found in the USA in Wisconsin (Racine County) in 1975, and again 19 kilometers away four years later (1). The next detection was in 1990, in Massachusetts (Essex County). G. maxima reproduces vegetatively, and has the potential to be a serious invader of wetlands. Quick control action has prevented its spread in the Ipswich River Wildlife Sanctuary in Massachusetts. Active monitoring and rapid control action could prevent its spread into other susceptible areas (1; 2).

(Click on thumbnail images for a closer view)

Young flowers

Flower spike

Flower spike

Many plants

The four photos above were supplied by Swedish Museum of Natural History
The painting was supplied by http://www.mpiz-koeln.mpg.de/~stueber/lindman/

Glyceria maxima (Poaceae -- the grass family) is a perennial, rhizomatous grass. Stems are unbranched and can grow up to 2.5 m high. Leaf sheaths have prominent midribs, visible transverse veins, and are closed to near the top. The unlobed, membranous ligules are 5-7 mm long, smooth and obtuse in shape. Leaf blades are flat, 22-29 cm long and 0.7-2 cm wide. The leaf blades are shallowly grooved, with prominent midribs. The leaf margins have short, stiff hairs that are rough to the touch. The plants are bisexual. The inflorescence is a panicle. The inflorescence can be open (chasmogamous) or contracted and symmetrical. The inflorescence branches have short, stiff hairs similar to those on the leaf margins (6).

Scientific and Common Names:
Synonyms for G. maxima (C. Hartm.) Holmb. include "Glyceria aquatica (L.)", "Molinia maxima C. Hartm.", and "Poa aquatica L.". It is commonly known as reed sweetgrass (5). A cultivar G. maxima 'Variegata' is grown in North America.

Impacts and Considerations:
Glyceria maxima has the ability to form huge stands in wetlands. It is an aggressive plant that has been invasive in Ontario for over 50 years. Even in its native range, conservationists are concerned with the ability of reed sweetgrass to form monocultures under different levels of disturbance. Reed sweetgrass has a competitive advantage because growth starts early in the spring. Glyceria maxima reduces plant species diversity (1, and references therein), in particular, seed producing plants that provide food for wildlife. Glyceria maxima is a poor food-plant and nesting substrate for wetland wildlife (1). G. maxima has been used as a forage crop. However, several instances of cattle poisoning have occurred due to cyanide production in the young shoots (4).

Native Range:
Glyceria maxima is native to temperate Europe and Asia, specifically from the British Isles to Japan and Kamchatka (1).

Range As An Invader:
In the USA, Glyceria maxima has been documented twice in Wisconsin and once in Massachusetts. The majority of the New World collections occur in Ontario, Canada. Other stands occur in British Columbia, Newfoundland, and Alaska (7). Overseas, it has been found invading Tasmania, New South Wales, Victoria, Queensland, as well as New Zealand.

Little is known on the control of this new invader.
1)If removing G. maxima by hand, concentrate on removing all pieces of the root or resprouting may occur.
2)Black plastic used to smother the grass was 100% effective in Massachusetts. However, this method is not feasible over large areas (2).
3)A 3% solution of glyphosate (Rodeo) during early summer and late summer months has been effective. Follow-up treatments the year after application is recommended (2).
4)Cutting may reduce populations of reed sweetgrass by allowing sunlight to reach other, competitive plants. Multiple cuttings (more than three) may reduce the amount of carbohydrates stored in the rhizomes. Cutting during the fall months when carbohydrates and nutrients are stored for the winter may affect spring regrowth (3).
5)Flooding cut stubble may drown G. maxima (4).
6)Since several instances of cattle poisoning have occurred due to cyanide production in the young shoots, grazing is not recommended (4).

1)Anderson, J.E. and Reznicek, A.A. 1994. Glyceria maxima (Poaceae) in New England. Rhodora 96(885):97-101.
2)Rawinski, T.J. Prevention Strategies in Wetlands. Massachusetts Audubon Society in house memo.
3)Sundblad, K. and Robertson, K. 1988. Harvesting Reed Sweetgrass (Glyceria maxima, Poaceae): Effects of Growth and Rhizome Storage of Carbohydrates. Hydrobiology 340:259-263.
4)Sundblad, K. and Wittgren, H.B. 1989. Glyceria maxima for Wastewater Nutrient Removal and Forage Production. Biological Wastes 27(1):29-42.
7)Alaska Botanical Biodiversity: Fort Wainwright vascular plant species

--Tunyalee Martin/Wildland Invasive Species Team; July 2000

Updated January 2005
©The Nature Conservancy, 2000