Myriophyllum heterophyllum
(broadleaf watermilfoil)

M. heterophyllum is an aquatic plant native to southeast USA. It is considered invasive throughout New England and northwest USA and as an emerging invader in Europe because of its tendency for uncontrolled growth and subsequent formation of dense mats of submergent vegetative material throughout the water column and at the water surface. These mats prevent water flow, reduce sunlight, reduce oxygen availability, and impede swimming, boating and fishing. M. heterophyllum is banned from sale in some New England states and Washington state.

The distribution of M. heterophyllum is generally known, but not documented in detail. Aiken (1981) lists its distribution in North America as ‘…Virginia to Florida, northward to Ontario and Michigan, and westward to Missouri and Texas’. The species is generally considered as native to the eastern part of the USA, except the northeastern region. It is considered non-native and invasive in New England, where it appears to have been introduced circa 1932 via escape from cultivation with subsequent spread via vegetative propagules (Les and Mehrhoff, 1999). Since its initial introduction to New England, it has spread throughout the region and is the most common invasive aquatic plant in New Hampshire (Thum and Lennon, 2009). It is not clear if M. heterophyllum was historically native to the mid-Atlantic region (Delaware, Maryland, New Jersey, New York, Pennsylvania and Virginia). For example, it is treated as a non-native invasive species in New York and eastern Pennsylvania, but is considered an extremely rare native and threatened species in nearby Delaware. Part of the confusion regarding its historical status (native/non-native) in northeastern USA may result from confusion with the closely related species M. pinnatum, which is considered native along the eastern coast from Florida through eastern Massachussetts (Aiken, 1981). M. heterophyllum is also considered non-native in western USA: while it is not clear when it was first introduced here, its recent spread in the region - especially in Washington state - is causing concern among water resource managers. Two causes for concern include the possible misidentification of M. heterophyllum with the closely-related, morphologically similar, and endemic western watermilfoil, M. hippuroides, and the potential for hybridization between the two species.

In Europe, it is found in Austria, Belgium, France, Germany, Netherlands and Spain (EPPO, 2012, 2014).

M. heterophyllum is a popular plant in the aquarium and water gardening trades and can readily be obtained from any number of aquatic plant vendors under a variety of names. Plants genetically confirmed as M. heterophyllum have been purchased  from a variety of vendors under a variety of common names (myrio, foxtail, and parrotfeather) and scientific names of (M. heterophyllum, M. pinnatum, M. tuberculum, M. aquaticum and M. simulans). Once escaped from an aquarium or cultivated pond, M. heterophyllum is capable of spreading through vegetative fragments. As such, it can be moved around by any number of water and animal vectors and may be commonly transported among water bodies on boats and boat trailers. Seeds may also be dispersed by animal vectors.

EPPO (2012) considers M. heterophyllum to have the potential to become invasive across Europe, 'especially in shallow lakes and channels.'

Gerber and Les (1996) found M. heterophyllum to be associated with water bodies that had higher pH and calcium levels relative to other species of milfoils in Michigan and Wisconsin. Thum and Lennon (2010) found M. heterophyllum to be associated with higher order lakes - large, low elevation lakes with relatively high pH, alkalinity and conductivity - in its introduced range in New Hampshire, USA. However, it is not clear whether these relationships hold true across different geographic areas where the species occurs, or among distinct genetic lineages of M. heterophyllum.

Reproduction may occur through asexual vegetative propagation and also sexual reproduction (seed production). Asexual vegetative propagation is thought to be the dominant mode of reproduction in introduced populations (e.g. Washington State Noxious Weed Control Board factsheet) (http://www.nwcb.wa.gov/). The emergent flowers are wind-pollinated.

Environmental

• Wildlife habitat

General

• Pet/aquarium trade

Morphological keys (such as Aiken, 1981) are available but rely mostly on characters of flowers and fruits, which may not be present, as M. heterophyllum rarely flowers. It can best be recognised by its dense underwater growth and undivided, serrate emergent leaves (Q-bank, 2015).

Genetic identifications using the nuclear ribosomal DNA internal transcribed spacer regions (ITS) have become common (Moody and Les, 2002; Thum et al., 2006).

M. heterophyllum may be confused with any number of Myriophyllum species. In general, Myriophyllum are distinguished by characters of flowers and fruits, which may not be present. Vegetative material of M. heterophyllum may especially be confused with closely related species M. humile, M. farwelli, M. pinnatum, M. laxum, and M. hippuroides. However, misidentifications with more distantly related species also occur (Aiken, 1981; Thum et al., 2006), especially M. verticillatum.

Genetic identifications using the nuclear ribosomal DNA internal transcribed spacer regions (ITS) have become common (Moody and Les, 2002; Thum et al., 2006). However, further work on the reliability of these markers based on much larger sample sizes is needed.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Europe: DAISIE - Delivering Alien Invasive Species Inventories for Europe, Web-based service, http://www.europe-aliens.org

France: European and Mediterranean Plant Protection Organisation (EPPO), OEPP/EPPO, 1 rue Le Notre, 75016 Paris, http://www.eppo.org/

USA: MVLMP (Maine Volunteer Lake Monitors), 24 Maple Hill Road, Auburn, ME 04210, http://www.mainevolunteerlakemonitors.org

01/06/09 Original text by:

Ryan Thum, Grand Valley State University Grand Valley State University, Annis Water Resources Institute, 224 Lake Michigan Center, 740 West Shoreline Drive, Muskegon, MI 49441, 616-331-3989, USA

Matthew Zuelig, Grand Valley State University, Annis Water Resources Institute, 224 Lake Michigan Center, 740 West Shoreline Drive, Muskegon, MI 49441, 616-331-3989, USA