florida Early Detection Network


Reed mannagrass, Reed sweetgrass


Glyceria maxima (Hartman) Holmb.


Grass family


Glyceria maxima




Synonyms: Glyceria spectabilis Mert. & Koch, Molinia maxima Hartman, Poa aquatica L.


Botanical Glossary

Glyceria maxima is a rhizomatous perennial that grows from unbranched stems that can reach 2.5 m (8.25 ft.) in height. The leaf blades are flat or slightly folded and are 2 cm (0.75 in.) wide and 22-29 cm (8-12 in.) long. The blades are shallowly grooved and have prominent mid-ribs. The apex of the leaf is acute, and the leaf margins have stiff, short hairs. The leaf sheaths are rough in texture and have a reddish-brown band at the junction with the leaf. The ligule is about 5 mm (0.2 in.) long and obtuse at the apex. The inflorescence of Glyceria maxima is in an open panicle, the branches of which have short, stiff hairs similar to those that are on the leaf margins. The panicle measures 15-30 cm (6-12 in.) in length. The spikelets are 5-8 mm (0.2-0.3 in.) long. The glumes are keeled in shape. The ovate, obtuse lemma is 3 mm (0.1 in.) in length; the palea is about equal in size to the lemma, ovate and obtuse in shape and very slightly two-cleft. The small seeds are 1.5-2 mm (0.07 in.) long, obovoid in shape and smooth in texture. They are dark brown and have a deep and narrow central furrow. Flowers appear on this plant from June to August. Page References Crow & Hellquist 271, Magee & Ahles 176. See reference section below for full citations.


Glyceria grandis S. Wats. (American mannagrass)


In North America, Glyceria maxima appears to reproduce and spread primarily by means of rhizomes. The extent to which it is able to reproduce and spread by seed is not clear, though evidence suggests that only a small percentage of its florets set viable seed.


Glyceria maxima is native to northern Eurasia, from the British Isles to Japan and Kamchatka. In North America, it is found in southern Canada, primarily in Ontario, but also in Newfoundland, British Columbia, Wisconsin and Massachusetts.


The first specimen of Glyceria maxima in North America dates from 1940 and comes from a marsh at the edge of Lake Ontario. Between 1940 and 1952 several more populations of the plant were located in this same region. It is possible that Glyceria maxima arrived some time before these records were documented. It may have been introduced intentionally as a forage species, or accidentally as part of packing material. The first record of Glyceria maxima in New England comes from the Ipswich River Wildlife Sanctuary in Essex County, Massachusetts in 1990. This Massachusetts population appears to be under control, and has provided some initial insights into how the species can be held in check.


Herbaceous Wetland,Shrub Wetland. At present, Gyceria maxima is only found at one site in New England - the Ipswhich River marsh system in Essex county, Massachusetts. It clearly favors freshwater wetland habitats.


Glyceria maxima forms large, dense monospecific stands capable of crowding out native wetland vegetation. Because it is both a poor food source and a poor nesting substrate for wetland wildlife, it has significant potential to negatively affect wetland habitat dynamics.


Documentation required: Herbarium specimen or mounted snippet of the plants with inflorescence. Best time for documentation: Summer, fall.


Integrated Taxonomic Information System - Has general taxonomic information about the species.

The PLANTS Database - General information and map


Anderson, J.E. and A.A. Reznicek. 1994. Glyceria maxima (Poaceae) in New England. Rhodora 96 (885): 97-101.

Burgess, N.D., C.E. Evans and G.J. Thomas. 1990. Vegetation change on the Ouse Washes Wetland, England, 1972-1988 and the effects on their conservation importance. Biological Conservation 53: 173-189.

Buttery, B.R. and J.M. Lambert. 1964. Competition between Glyceria maxima and Phragmites australis in the region of Surlingham Broad I. The competition mechanism. J. of Ecology 53: 163-181.

Butcher, R.W. 1961. A New Illustrated British Flora vol. 2. Leonard Hill Books Limited, London.

Clapman, A.L., T.G. Tutin, E.F. Warburg. 1962. Flora of the British Isles. Cambridge University Press.

Crow, G.E. and C.B. Hellquist. 2000. Aquatic and Wetland Plants of Northeastern North America. Vol #2. University of Wisconsin Press, Madison.

Dore, WG. 1947. Glyceria maxima in Canada. Canadian Field Naturalist. 61: 174.

Gleason, H.A. and A.C. Cronquist. 1991. Manual of Vascular Plants of the Northeastern United States and Adjacent Canada. 2nd ed. New York Botanical Garden, Bronx, New York.

Magee, D.W and H.E. Ahles. 1999. Flora of the Northeast. University of Massachusetts Press, Amherst.

Sundblad, K. 1990. The effects of cutting frequency on natural Glyceria maxima stands. Aquatic Botany 37: 27-38.

USDA, NRCS. 2001. The PLANTS Database, Version 3.1. (//plants.usda.gov). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.