Reed sweetgrass or Glyceria maxima is a tall aquatic grass from temperate Europe and Asia that grows along the margins of lakes, rivers, and streams. Reed sweetgrass is sold as an ornamental plant and has been introduced to several countries outside its native range including the United States and Canada. In North America, it is most widely distributed in Ontario, Canada but it is also spreading in a few other provinces and states. In Washington State, it is only known to occur in a few isolated locations in Snohomish and King Counties, although it is possibly present elsewhere since it has been sold as an ornamental. It can be found both in its variegated and non-variegated forms. The variegated form is the one most often sold as an ornamental and is named Glyceria maxima var. variegata. Reed sweetgrass is also sometimes called tall mannagrass or reed grass.
Because of the difficulty in distinguishing this plant from other tall grasses, we recommend contacting the noxious weed program for a positive identification before removing. There are currently very few records of this plant in King County, so if you do find reed sweetgrass or any suspicious patches of Glyceria in King County, please report the location right away.
Legal status in King County, Washington
Reed sweetgrass is a Class A Noxious Weed in Washington State due to its limited distribution in the state and the potential for significant impact to environmental and economic resources. Public and private landowners are required to control this plant when it occurs on their land. This species is also on the Washington quarantine list (known as the prohibited plants list) (external link) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts of this species, into or within the state of Washington. It is further prohibited to intentionally transplant wild plants and/or plant parts of this species within the state of Washington.
Identification (see below for more photos)
Tall aquatic emergent perennial grass, sometimes variegated
Grows up to 8.5 ft tall, has unbranched stems
Variegated form has distinctive green and creamy white stripes
Leaves stiff, shallowly grooved, with prominent midribs
Leaf blades flat, up to 16 inches long, about 1/2 to 3/4 inch wide
Leaf margins rough with short stiff hairs
Leaf sheaths rough in texture and have reddish-brown band at leaf junction
Ligule rounded, pointy at the apex (a ligule is the translucent tissue found where the leaf blade attaches to the stem, visible when leaf is folded away from stem)
Stems often reddish on lower portion
Inflorescence (flower stem) is open and branched (a panicle), up to 18 inches tall, made up of many yellow to green or purple-tinged narrow spikelets
Seeds are small (1.5-2 mm, 0.07 in), obovoid, smooth, dark brown, with a deep and narrow central furrow
Flowers in June to August
Can be confused with northwest native American mannagrass (Glyceria grandis), but G. grandis is shorter (up to 5 feet tall), has drooping infloresence branches, and smooth sheaths at the base of the infloresence brances (G. maxima has rough sheaths).
Habitat and impact
Reed sweetgrass (Glyceria maxima) grows in wet areas, from the shoreline edge to fairly deep water (up to 6 feet deep). It can be found along the margins of streams, rivers, lakes, and wetlands. In deep water it can form floating mats attached to the bank. The dense root system and large size of the plants enable it to out-compete other wetland plants. Once established, reed sweetgrass can completely dominate a wetland to the exclusion of all other vegetation. Monotypic stands of reed sweetgrass reduce plant diversity. This grass is an inferior food and nesting source for watefowl compared with the species it displaces and it also changes the macroinvertabrate community structure, which can impact the food chain for fish and wildlife. Dense rhizomes accumulate sediment faster than native species and this can clog up small streams and drainages.
In some parts of the world, reed sweetgrass has been used as forage but it can contain dangerous levels of cyanide and has caused cattle poisoning. Cyanide concentrations are highest in the young shoots that occur in the spring and autumn. It is also sometimes used as an ornamental, but it can escape out of intentional plantings and invade natural wetlands and shoreline habitats.
Growth and reproduction
Reed sweetgrass (Glyceria maxima) has extensive rhizomes that produce a thick mat of stems. The massive root system can extend 3 feet down and the rhizomes can make up about half of the plants total biomass. Reed sweetgrass grows fastest in high nutrient soils and fresh water.
Reed sweetgrass typically goes dormant in the winter, overwintering as small green shoots and regrowing from buds along the rhizomes in early spring. Rapid early spring growth gives reed sweetgrass a competitive advantage over other wetland plants.
Stems may be vegetative (no flower heads) or flowering and on older plants most of the stems are typically non-flowering. Reed sweetgrass colonizes new areas by seed or root and rhizome fragments and then spreads vegetatively by rhizomes to increase in size and exclude other vegetation. Seeds mostly germinate the following spring but some remain dormant in the soil for many years. It is thought that North American reed sweetgrass plants are reproducing mostly by vegetative means and that most seeds are not viable.
Prevention: Do not plant reed sweetgrass (Glyercia maxima). Even in an upland area, seeds can disperse into natural shorelines and wetlands. If you are unsure of the name of an ornamental grass you are considering, please ask for help from the nursery and avoid it if they can't tell you its Latin name.
Small patches: Digging up the entire root mass can be effective for small patches if follow-up is done to remove shoots emerging from seeds and broken bits of rhizomes left behind. Mowing plants short right before the water rises can also be somewhat effective. Small patches that are not flooded can be controlled by tarping with heavy duty black plastic or non-woven geotextile fabric.
Larger patches: Permits will be needed since this plant grows in water. Please check with your local permitting office for more information. For information on aquatic weed control in Washington State visit the Department of Ecology website. Please refer to herbicice labels for site specific control information and refer to the PNW Weed Management Handbook for additional information on herbicide use.
Using an aquatic formulation of glyphosate (such as Aquamaster, Aquaneat and other products) or imazapyr (such as Habitat) will be most effective in the summer or early fall. If more than one third of the stems are flooded, effectiveness may be reduced, so it is best to spray when the water level is low. Also, plant decomposition following the treatment of a large area can reduce dissolved oxygen in the water, so it is best to remove the dead plant material after the herbicide has had a chance to work (2 to 4 weeks typically). Established populations will usually require at least 2 to 3 years of follow-up treatment.
Selectively mowing the reed sweetgrass at least two to three times a season will reduce the vigor of the stems and decrease belowground biomass. This may be enough to allow surrounding vegetation to move in and outcompete the reed sweetgrass, although it is not likely to completely kill it off.
Additional information on reed sweetgrass
What to do if you find this plant in King County, Washington
Please notify us if you see reed sweetgrass growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. Also, because reed sweetgrass is not established in King County, we have an opportunity to stop it from spreading if we act quickly. We map all known locations of regulated noxious weeds such as reed sweetgrass in order to help us and others locate new infestations in time to control them.
Reed sweetgrass photos - click thumbnail for larger image