Rough mannagrass (Glyceria maxima)

Synonyms: Glyceria aquatica, Molinia maxima, Poa aquatica

Common Names: Reed mannagrass, tall mannagrass, reed sweet-grass, great water grass

Description:

Rough mannagrass (Glyceria maxima) is a tall, semi-aquatic grass from Europe and Asia that invades wetlands, ditches, and shorelines across the Great Lakes and Northeast [1][2]. It spreads by thick underground stems into dense single-species stands that crowd out native wetland plants [2][4]. MISIN tracks it because it degrades wetland habitat and can poison cattle that graze it [2].

Habit:

Rough mannagrass is a stout perennial wetland grass that grows roughly 3 to 8 feet tall in spreading patches [2][4]. Its stems are hollow and ribbed and often have a reddish tint near the base, and they rise from a thick mat of creeping underground stems [2]. The long, flat leaf blades (up to about 2 feet) have a prominent midrib and feel rough because of stiff, short hairs along the edges, and there is usually a reddish-brown band where the leaf joins the stem [2][4]. In summer it produces a large, branched, fairly upright seed head at the top of the stem [2][4].

It is easiest to spot in summer, from June through August, when the tall stems and large seed heads stand above the surrounding marsh [4]. Look for dense, tall stands along the edges of ponds, ditches, streams, and marshes, sometimes forming floating mats out into shallow water [2].

At-a-glance cues:

1. Tall (3?8 foot) wetland grass forming dense, spreading stands [2][4]
2. Hollow, ribbed stems with a reddish tint near the base [2]
3. Long, flat leaves rough along the edges, with a reddish-brown band where leaf meets stem [2][4]
4. Large, branched, fairly upright seed heads in summer [2][4]

Leaves:

1. Arrangement: in two ranks along the stem [2]
2. Shape: long, flat blades, roughly 12 to 24 inches long and 0.25 to 0.8 inch wide, with a prominent midrib [2]
3. Margins: rough to the touch from stiff, short hairs [2][4]
4. Sheath: leaf sheaths wrap the stem and show a reddish-brown band where the leaf joins the stem [2][4]

Stems:

1. Habit: a stout, erect, herbaceous perennial grass, about 3 to 8 feet tall, in spreading patches [2][4]
2. Stem: hollow and ribbed lengthwise, often with a reddish tint near the base [2]
3. Roots: thick, creeping underground stems (rhizomes) that reach up to about 3 feet down and hold up to half the plant's mass below ground [2]

Flowers:

1. Arrangement: tiny grass flowers in spikelets that form a large, open, branched seed head (panicle) at the top of the stem, held fairly upright [2][4]
2. Size: the seed head is roughly 6 to 12 inches long [4]
3. Bloom time: summer, about June through August [4]
4. Pollination: wind-pollinated, as in other grasses [2]

Fruit and seeds:

1. Type: a small, dry grain (the typical seed of a grass) [2]
2. Color: seeds ripen dark brown in late summer [2]
3. Viability: seeds can remain alive in the soil for several years [2]
4. Main spread: in North America the plant spreads mostly by creeping rhizomes rather than by seed [2][4]

Habitat:

Rough mannagrass is native to temperate Europe and Asia and was introduced to North America, where it is now established in wetlands of the Great Lakes region and the Northeast [1][2]. It is a plant of wet ground and shallow water [2].

It grows in marshes, wet meadows, the banks of streams and rivers, ditches, and the edges of ponds and lakes, in full sun to partial shade [2][4]. Special air-filled tissue in its stems lets it survive in shallow water (around a foot deep) and tolerate long periods of flooding, and it can even form floating mats out from the shore [2].

Reproduction:

Rough mannagrass spreads mainly by its thick, creeping underground stems (rhizomes), and this is the main way it moves onto and across a property [2][4]. A clump widens year after year into a dense stand, and broken pieces of rhizome ? carried by water, by animals such as muskrats and beavers, or on equipment ? can take root and start new patches [2]. It also produces seed that can stay alive in the soil for several years [2].

Plants regrow from rhizome buds in spring, grow through the season, and set seed in summer [2]. Because the plant stores so much energy in its roots and regrows readily from fragments, the key to control is removing every piece of rhizome and not chopping it in ways that scatter fragments [2][4].

Impact and Damage:

The most important homeowner-and-livestock concern is that rough mannagrass can be poisonous to grazing animals: it can cause cyanide poisoning in cattle and other livestock that eat it, so it is a hazard where animals have access to wet pastures [2]. It is not known to harm people. On a property, it forms tall, dense stands that take over pond edges, ditches, and wet corners and are hard to clear [2][4].

In natural wetlands the damage is serious. Rough mannagrass forms impenetrable single-species stands that crowd out native wetland plants and lower plant diversity, and these stands are poor food and nesting habitat for wetland wildlife [2][4]. By replacing native plants it also changes the community of insects and other small animals, which ripples through the wetland food web [2]. It can even act as an "ecosystem engineer," its dense growth helping turn flowing, oxygen-rich streams into stagnant, low-oxygen swamps [2].

Similar species:

Rough mannagrass is confused with native mannagrasses and with other tall wetland grasses.

American mannagrass (Glyceria grandis) is the closest match ? a native grass in the same group with similar leaves and seed heads, but it is shorter (usually around 5 feet) and its seed head nods and droops, unlike the taller, more upright rough mannagrass [2].

Reed canarygrass (Phalaris arundinacea) is another tall invasive wetland grass that forms dense stands, but its seed head is denser and more compact when young, and its leaves lack the rough edges and reddish leaf-junction band of rough mannagrass [4].

Fowl mannagrass (Glyceria striata) is a smaller native mannagrass of wet ground, but it is more slender and delicate with a fine, open, drooping seed head, far less robust than rough mannagrass [2].

Common reed (Phragmites australis) also forms tall dense wetland stands, but it is much taller with tough, almost woody stems and a large, feathery, plume-like seed head, unlike the leafier, smaller rough mannagrass [4].

Monitoring and rapid response:

Signs to watch for:

1. Tall, dense stands of robust grass along marshes, ditches, stream banks, and pond or lake edges, sometimes as floating mats [2][4]
2. Hollow ribbed stems with a reddish base and rough-edged leaves with a reddish-brown band at the leaf junction [2][4]
3. Large, branched, fairly upright seed heads in summer (June?August) [4]

What you can do:

1. For a small patch, dig out the whole plant including all of the underground stems when the soil or muck is workable, and remove every fragment, since broken rhizome pieces re-root [2][4].
2. Do not chop or till the plant in ways that scatter rhizome pieces; repeated cutting or mowing through the season can gradually drain the roots' reserves [4].
3. Smothering a small patch with black plastic can help where that is practical [4].
4. Keep livestock away from rough mannagrass, since it can cause cyanide poisoning in cattle and other grazing animals [2].
5. Natural area crews avoid fragmenting the rhizomes, work outward from the edges and target small outlying patches first, and remove cut or dead material from the water within a few weeks so it does not rob the water of oxygen as it rots [4].

When to call a professional:

1. Large or well-established stands, or plants growing in standing or flowing water and connected wetlands where digging is not practical [2][4]
2. Sites in sensitive wetlands where work could disturb habitat or water quality [4]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. USDA NRCS Plants Database. Glyceria maxima (Hartm.) Holmb. (reed mannagrass). plants.usda.gov
2. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Glyceria maxima (reed mannagrass) ? Species Profile. nas.er.usgs.gov
3. Plants of the World Online (Royal Botanic Gardens, Kew). Glyceria maxima (Hartm.) Holmb. powo.science.kew.org
4. Vermont Invasives. Reed Manna Grass (Glyceria maxima). vtinvasives.org

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