Other Common Names: variable watermilfoil, two-leaved watermilfoil, broadleaf watermilfoil
Weed class: A
Year Listed: 2008
Native to: Eastern United States
Is this Weed Toxic?:
not known to be
Why Is It a Noxious Weed?
It’s an invasive plant that can alter aquatic ecosystems. It forms mats that shade out native plants and inhibits water flow and recreational activities. It may be able to hybridize with the native watermilfoil resulting in a more aggressive hybrid.
How would I identify it?
It is a submersed, rooted aquatic plant, having both submerged and emergent leaves growing from a stout stem up to .18 inches in diameter and 3.25 feet in length.
The infloresence is a spike 1.9 to 13.8 inches long consisting of flowers in whorls of four. Flowers have four stamens, and petals are 0.06 to 0.12 inches.
Emergent spikes are 2 to 6 inches long with persistent, lance shaped to oblong leaves. It has prominently serrated bracts that are reflexed, extend past the flowers. Submerged leaves are whorled and dissected, feather-like, with 7 to 11 leaflets.
Stems are dark red to reddish-brown.
May Be Confused With
Phenotypically, it is difficult to differentiate between Myriophyllum heterophyllum and the native species western milfoil, Myriophyllum hippuroides. DNA analysis is required. If you need help with identification, please contact your county noxious weed coordinator.
Where does it grow?
It grows in freshwater ponds, lakes, ditches and other still or flowing aquatic systems. Please click here to see a county level distribution map of variable-leaf milfoil in Washington.
How Does it Reproduce?
It primarily reproduces vegetatively by plant fragments but can also reproduce by seeds. It can easily be introduced into new bodies of water through inadvertent transport of plant fragments on boats and boat trailers or deliberate dumping of aquariums.
How Do I Control It?
General Control Strategy
Small infestations may be hand removed or tarped. The entire plant must be removed.
Research has shown that draw down techniques may be effective; although, this technique is non-selective and may impact native species as well.
Because herbicide availability and regulation differ between states, we recommend the Washington Department of Ecology website for information on aquatic weed management and herbicides, or contact your county noxious weed coordinator.
For More Information
See our Written Findings for more information about variable-leaf milfoil (Myriophyllum heterophyllum).
Pierce County NWCB Fact Sheet on variable-leaf milfoil
Control Options for variable-leaf milfoil and Eurasian watermilfoil from King County NWCB