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...
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.
Recent phylogenetic analysis identifies the species as being a member of a clade of Myriophyllum endemic to North America including: Myriophyllum pinnatum Britton, Sterns & Poggenb., Myriophyllum hippuroides Nutt. ex Torr. & A. Gray, Myriophyllum farwellii Morong, Myriophyllum humile Morong, Myriophyllum laxum Schuttl. ex Chapm., and Myriophyllum tenellum Bigelow (Moody and Les, 2010).
As with most milfoils, M. heterophyllum can be easily confused with other species of Myriophyllum, especially when only vegetative (non-flowering) material is available for identification: M. heterophyllum is known to hybridize with M. laxum (Moody and Les, 2002).
Common names include variable-leaf watermilfoil, two-leaf watermilfoil and broad-leaf watermilfoil. The exact origins of these names are not known, but are presumed to be North American. In the aquarium trade, M. heterophyllum may be sold as ‘myrio’, ‘foxtail’, or ‘parrotfeather’.
M. heterophyllum is an aquatic plant that has submerged vegetation with emergent flowering spikes. Plants hermaphroditic, occasionally monoecious. Stem stout, to 100 cm; internodes crowded. Submerged leaves 4- or 5-whorled or scattered, pectinate, oblong in outline, (1.5-)2-4 × 1-3 cm; segments in 5-12 pairs, filiform, 0.5-1.5 cm. Inflorescence a terminal spike of 4-whorled flowers, 5-35 cm, in monoecious plants lowermost flowers female, uppermost ones male; bracts persistent, eventually reflexed, lanceolate to oblong or obovate, 4-18 × 1-3 mm, margin sharply serrulate; bracteoles ovate, ca. 1.2 × 0.6 mm, margin serrate. Petals 1.5-3 mm. Stamens 4. Fruit 4-loculed, subglobose, 1-1.5 mm; mericarps with 2 finely tuberculate ridges abaxially, apex beaked (Flora of China Editorial Committee, 2012).
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).
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
Although native to eastern North America, M. heterophyllum is not considered native to northeastern USA. The first recorded observation of the species in northeastern USA was from Bridgeport, CT in 1932. Les and Mehrhoff (1999) presume the original introduction can be attributed to escape from cultivation, with subsequent spread by vegetative propagules escaping cultivation. Recent genetic analyses suggest that M. heterophyllum may have been independently introduced to northeastern USA on several occasions (R Thum, Grand Valley State University, USA, personal communication, 2010).
Very little is known about the history of introduction to western USA. The Washington State Noxious Weed Control Board (2007) claims that only four populations of M. heterophyllum have been found in the state, all within private lakes, while the Aquatic Plant Monitoring project at the University of Washington has 5 locations listed throughout the state of Washington as having M. heterophyllum (see http://www.ecy.wa.gov/programs/eap/lakes/aquaticplants/).
In Europe, M. heterophyllum was first recorded in Germany in the 1960s, but details regarding its introduction and spread are lacking (Hussner, 2005). The species is also reportedly naturalized in Spain (Cirujano et al., 1997). It was first reported from France, in a private pond in Haute-Vienne department, in 2011 (Lebreton, 2013).
M. heterophyllum has also been reported in China (Yu et al., 2002), but details regarding its history and spread are lacking.
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.'
M. heterophyllum occurs primarily in lakes, ponds, rivers and swamps, but can also grow in a semi-terrestrial form when stranded on mudflats.
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.
M. heterophyllum exhibits potentially important genetic variation as revealed by chloroplast DNA sequences and ITS DNA sequences. Moody and Les (2002) identified three different ITS lineages, and cpDNA structure across M. heterophyllum’s native range has been investigated (R Thum, Grand Valley State University, USA, personal communication, 2010). Furthermore, northeastern and western invasions are comprised of multiple cpDNA and ITS haplotypes from these different geographic origins.
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.
In general, aquatic invasive weeds in the USA cost about $110 million annually (Pimentel et al., 2000). In the specific case of M. heterophyllum, it has been associated with up to a 40% decrease in property values in New Hampshire (Halstead et al., 2003) in addition to costs associated with control.
M. heterophyllum is highly competitive and able to outcompete other aquatic plants. It forms dense mats of submergent vegetative material throughout the water column and at the water surface, which can prevent water flow, reduce sunlight and reduce oxygen availability. The resulting low oxygen conditions can harm or kill aquatic organisms (EPPO, 2012).
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).
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.
M. heterophyllum is listed as a prohibited plant in several New England states and Washington state, USA, in efforts to prevent its introduction via escape from the aquarium trade.
‘Small, recently detected infestations may be successfully eradicated through careful and thorough hand-pulling or using a tarpaulin.’ However, this can cause M. heterophyllum to fragment and spread further, so care must be taken (EPPO, 2014).
Eradication after establishment is not likely.
Measures include mechanical harvesting, hand harvesting, suction harvesting, blanketing – a large blanket is placed over the population, limiting light and stressing the plants, ‘drawdown’ – water is drawn down in a reservoir area and milfoil beds are exposed to dry substrate, heat, cold, ice, etc. (Massachusetts Department of Conservation and Recreation http://www.mass.gov/dcr/).
Movement control is through inspection of boats and boat trailers on public waterbodies, also signs posted to inform boaters of the risks of transporting milfoil. Wash stations are available at some boat launches to provide a means of ridding milfoil fragments from boats.
2,4-dichlorophenoxyacetic acid (2,4-D) – A systemic herbicide that is absorbed by vegetative tissue disrupts cellular growth, ultimately killing the plant.
Fluridone – A systemic herbicide that is absorbed by vegetative tissue, causing bleaching. Fluridone inhibits the synthesis of carotenoid pigments, which play a protective role in the plant. Lack of carotenoid pigments leads to slow death via reduced food production and damage by sunlight.
Triclopyr – A systemic herbicide that is absorbed by vegetative tissue. Triclopyr inhibits the synthesis of important plant enzymes, which causes plant death.
In general, the taxonomy and geographic distribution of M. heterophyllum is not well understood. In particular, additional taxonomic study is necessary for the development of reliable identification methods, particularly of vegetative forms. This is especially relevant where native, rare and endangered status’s overlap among species that may be confused. For example, M. pinnatum is considered historically native to northeastern USA and may currently be rare. M. heterophyllum in contrast is considered invasive, and misidentification has serious implications for control versus conservation.
In addition, further study of the habitat requirements and development of ecological niche models would be useful for this species. 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.
Finally, while economic and recreational impacts are clear, scientific studies documenting the impacts on biota and ecosystems are absent.