rough mannagrass
Glyceria maxima (Hartman) Holmb.

Overview

Appearance

Glyceria maxima is a perennial, semi-aquatic, rhizomatous grass. Stems are unbranched and can reach 2-8.2 ft. (0.6-2.5 m) in height.

Foliage

Leaves are 12-23.6 in. (30-60 cm) long, 0.2-0.8 in. (6-20 mm) wide, with an acute apex and a prominent midrib. The leaf margins have stiff, short hairs. The leaf sheaths are rough in texture with a reddish-brown band at the leaf junction.

Flowers

Flowering occurs from June through August, when flowers appear in 6-12 in. (15-30 cm) long open panicles. The panicle branches have short, stiff hairs like those on the leaf margins.

Fruit

The small seeds are 0.06-0.08 in. (1.5-2 mm) long, obovoid in shape and smooth in texture. This plant reproduces primarily vegetatively through rhizomes.

Ecological Threat

Glyceria maxima occurs in sunny to semi-shady wetlands, where it can form dense impenetrable monocultures that crowd out native species. It is native to northern Eurasia and was first found in North America in 1940 on the edge of Lake Ontario.

Selected Images

Maps

Invasive Listing Sources

• Connecticut Invasive Plant List
• Connecticut Invasive Plant Working Group
• Connecticut Noxious Weeds
• Maryland Aquatic Nuisance Species Plan
• Massachusetts Invasive Plants Advisory Group List
• Massachusetts Noxious Weeds
• Massachusetts Prohibited Plant List
• New Hampshire Prohibited Aquatic Species
• New Hampshire Prohibited Invasive Species
• New York Regulated and Prohibited Invasive Species - Regulated
• Ontario’s Invading Species Awareness Program Tracked Species List
• Pacific Northwest Exotic Pest Plant Council, 1998
• Rhode Island Natural History Survey
• To be proposed for MISC
• Washington Noxious Weeds
• Wisconsin's Invasive species rule – NR 40

References