Scientific name: Myriophyllum heterophyllum
Taxanomic position: Haloragaceae
Common names: variable water milfoil, twoleaf water milfoil, variable-leaf milfoil, red foxtail, Verschieden-blättriges Tausendblatt (German).
EPPO region: Austria, France, Germany, Great Britain, the Netherlands, Spain, Switzerland.
North America: Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Louisiana, Massachusetts, Maryland, Maine, Michigan, Minnesota, Missouri, Mississippi, North Carolina, North Dakota, New Hampshire, New Jersey, New Mexico, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Virginia, Wisconsin, West Virginia.
Canada: Alberta, British Columbia, New Brunswick, Ontario, Quebec.
M. heterophyllum is a perennial, aquatic plant with both submerged and emergent leaves. Reproduction is vegetative and less seed-propagated.
M. heterophyllum is a submersed, rooted macrophyte with both submerged and emergent leaves growing from a stout stem. The stem is coloured from dark red to reddish-
brown and up to 3mm in diameter and 100cm in length.
The submerged leaves are feather-like, green, 2-5 cm long and 2-4 cm wide. They are dissected into 7-11 leaflets and arranged into whorls of 4-5 leaves.
The emergent leaves develop during late summer and range in length from 5-15 cm above water surface. They are 0.4-3.0 cm long and 1.5-5.0 mm wide.
Inflorescence is a spike 5-35 cm long, consisting of flowers in whorls of four. Flowers have 4 stamens and petals are 1.5-3.0 mm long. Fruits are 1.0-1.5 mm in length, round, with 4 chambers.
Biology and ecology
M. heterophyllum colonizes still and slow-moving freshwater bodies and ecosystems such as ponds, lakes, channels, ditches and also spring-fed swamps and sloughs. The plant grows in acidic and alkaline water and can tolerate a wide range of calcium concentrations. It grows on fine textured sediments such as silt, sand and gravel and prefers high ammonium nitrogen levels.
The areas M. heterophyllum invades are humid warm or cold temperate with average warm summers. The plant is able to overwinter.
Climatic and vegetational categorization
M. heterophyllum is typically associated with Cf (a,b), Cs (a,b) and Df (a-c) in Köppen’s climate classification, i.e. warm temperate climate, fully humid or summer dry with hot or warm summer (“C-climate”) and snow climate, fully humid with hot, warm or cool summer.
The aquatic weevil Eubrychius velutus is distributed throughout Europe and Asia. It is a Myriophyllum specialist with native hosts M. verticillatum and M. spicatum and has expanded its host range to include M. heterophyllum. The weevil can complete all life stages on the plant by living on and consuming the meristem and leaves during larvae stage up to pupation on the stem. After hatching the adults begin feeding on the meristem and leaves (Newman et al. 2006).
Uses and benefits
M. heterophyllum is sold in garden centres and aquarium stores as an ornamental plant for aquaria and garden ponds.
Pathways of movement
The main pathway of movement is the distribution and sale of the plant in aquarium trade. Spread from the intended to the unintended habitat is caused by dumping of aquaria in the wild and cleaning of ponds. Fragments of the plant are transported by flowing water to new locations.
Effects on plants
Dense and extensive populations of M. heterophyllum cause loss of light and reduction in dissolved oxygen content. This results in a change of water quality and modifies the suitable habitat for other species. The plant out-competes and displaces the aquatic flora.
Environmental and social impact
Monospecific populations of M. heterophyllum can negatively affect wildlife (e.g. predator/prey relationship among fish by impeding predation or covering spawning areas).
A dense growth of M. heterophyllum can reduce light and oxygen levels in water bodies and can lead to an eutrophication with higher nutrient levels and a resulting plant dying. Decay of large plant masses produces toxic substances such as ammonia, hydrogen sulphide and methane. This leads to fish avoidance or fish kills.
An extensive growth of the plant can limit a recreational use of water body by humans such as boating, swimming, diving and fishing.
Summary of invasiveness
M. heterophyllum is an aquatic plant which is traded as an aquarium plant and used both indoors and outdoors in garden ponds. The plant was introduced into Europe as an ornamental plant and is meanwhile partly naturalized. The spread is caused by human activities. The plant is released from the intended to the unintended habitat by dumping of aquaria in the wild or cleaning of ponds, by uncleaned fishing gear or diver’s equipment and also boats and trailers benefit spreading.
In the north-eastern United States M. heterophyllum is not native, but has spread from the south and reached the New England region where it is considered invasive now. In New England the plant has already a negative effect on the aquatic ecosystem by generating dense mats. These extensive populations out-compete and displace the aquatic flora and limit a recreational use of water body by humans.
M. heterophyllum has the ability to hybridize with other Myriophyllum species, e.g. M. pinnatum. Via DNA analysis the hybrid M. heterophyllum x pinnatum was confirmed in Maine (USA). This hybrid is considered to be possibly more aggressive in invasiveness than the parent, M. heterophyllum.
The climatic requirements in the current areas of distribution are largely similar to each other as well as suitable habitats.
Cultural and mechanical control
A mechanical removal of water plants by harvesting them via mowing/collector boats, suction appliances or divers is popular. These measures have to be repeated periodically for a persistent success.
Laying out benthic barriers as it is practiced in the United States is another method to destroy invasive water plant infestations. They cover the sediment like a blanket and compress water plants while blocking light.
The most applied systemic herbicide for control of water milfoil species in infested areas in the United States is the fast-acting and selective 2,4 D (2,4-Dichlorophenoxyacetic acid). Other popular systemic herbicides are Fluridone and Triclopyr. In some European countries, e.g. Germany, an application with herbicides on aquatic ecosystems is prohibited.
The aquatic weevil Eubrychius velutus feeds on the meristem and leaves of the plant (see above) and has potential as a biological control agent. A further research is necessary.
Possibilities for eradication
An early detection and repeated harvesting may keep the plant’s annual growth to a minimum and may lead to an eradication of the plant. In later stages of plant infestation eradication may become difficult and a success partly depends whether an application with herbicides is allowed. After eradication a re-introduction is likely because the plant is furthermore sold in aquarium trade. If control costs are high and eradication with less success a prohibition of import/cultivating in-country, selling, planting and holding of the plant could be necessary.
In Europe there are no regulations. M. heterophyllum is present in the wild and partly naturalized but has not shown an invasive behaviour yet.
In the north-eastern United States such as Connecticut and Maine where M. heterophyllum is considered invasive legislature banned the sale, purchase, cultivation, distribution and transplantation of the plant in addition to eradication measures such as harvesting, benthic barriers and an application with systemic herbicides.
CGR Connecticut General Assembly,
OLR Research Report (2008), http://www.cga.ct.gov/2008/rpt/2008-R-0251.htm, 18/02/2009.
Department of Ecology (2009), Aquatic
Plant Management – Bottom Screens, http://www.ecy.wa.gov/programs/wq/plants
Department of Ecology (2009), Aquatic Plant
Management – Aquatic Herbicides, http://www.ecy.wa.gov/programs/wq/plants
ENSR International (2005), A Rapid Response
Plan for Variable Watermilfoil (M.
heterophyllum) in Massachusetts.
Hussner, A. (2009),
Lakes Environmental Association (2009),
Maine VLMP (2009), Maine Volunteer Lake
Monitoring Program, http://www.mainevolunteerlakemonitors.or
Newman et al. (2006), Life History and
Developmental Performance of the Eurasian Milfoil Weevil, Eubrychius velutus