Variable Leaf Milfoil (Myriophyllum heterophyllum)

Description

Variable leaf milfoil (VLM) is a submerged aquatic plant with fine, feather like leaves whorled around a main stem. There are generally 4-6 leaves per whorl and commonly 5-14 pairs of leaflets per leaf. Stems are heavy and stiff, a characteristic that helps to differentiate the plant from native northern milfoil or invasive Eurasian water milfoil. The stems can reach heights of 15 feet and are usually a green to reddish hue. M. heterophyllum will flower around July and produce a 3-6 inch emergent stalk. The flowers are small and white and occur in the axils of the emergent bract.

Native Range and Distribution

VLM is native to the Southeastern and Midwestern United States. It was first observed in New England in Bridgeport, Connecticut in 1932 and is now widely distributed across the New England States. The first introduction was likely a result of an aquarium release or from a viable fragment carried overland by a boat or trailer. Variable leaf milfoil is found in numerous water bodies across New York State and is widespread across the Adirondack Park.

Threats and Impacts

Variable leaf milfoil can cause numerous ecological, cultural and economic impacts. VLM spreads quickly via fragmentation and can easily displace beneficial native aquatic plants. Dense beds of variable leaf milfoil degrade water quality for numerous species of fish and wildlife. In large mats, dissolved oxygen levels can be reduced to zero, making the area completely uninhabitable to game fish. Thick growths of VLM can impede fishing, swimming and boating, thus, indirectly impacting tourism and the economic activity of lake towns.

Management

Management has been sought to control established populations and condense the distribution of variable leaf milfoil because of its damaging impacts on the ecosystem and water based industry. Some management strategies include:

• Chemical - some aquatic herbicides, such as Triclopyr, can effectivley reduce populations of VLM

• Biological - aquatic plant managers have experimented with Triploid grass carp as a biological control, but found the carp will selective graze on several other species before feeding on VLM. Milfoil weevils have also been used a means of control, but a very high quantity is needed for success.

• Mechanical Harvesting - Most widely used method of control 
o Proven successful in eradication if populations are caught early
o Hand pulling can be successful for small areas and bathymetric mats can be employed for larger infestations