Short description of Myriophyllum heterophyllum, Twoleaf Watermilfoil

A submerged evergreen aquatic perennial herb that produces emergent flowering spikes (5-35 cm) consisting of flowers in whorls of 4(-6), the upper male, the lower female and the intermediate bisexual; the bracts leaf-like, lanceolate the margin toothed. Submerged leaves are 4-5 whorled with 5-12 feather-like segments. Emergent leaves lanceolate to elliptical, entire or toothed. The tiny reddish flowers have 4 sepals, tepals and stigmas and are produced from May to October. Flowering is rarely observed in its native or invasive range (OEPP/EPPO, 2016). In Europe M. heterophyllum can only be reliably told from M. verticillatum when in flower and so is probably frequently misidentified (Gross et al., 2020).

Impact summary: Myriophyllum heterophyllum, Twoleaf Watermilfoil

Myriophyllum heterophyllum produces 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.

Habitat summary: Myriophyllum heterophyllum, Twoleaf Watermilfoil

Lakes, ponds, slow-moving rivers, irrigation channels, ditches, canals and swamps where conditions are moderately alkaline and conductivity is high. Although it usually grows in open water (to a depth of 9.5 m), it can survive as a semi-terrestrial plant on the muddy margins of water-bodies with variable water-levels. In GB it has been recorded in a canal and a series of artificial ponds.

Overview table

Environment	Freshwater
Species status	Non-Native
Native range	South-Central U.S.A., Ontario, Alabama, Arkansas, Delaware, Florida, Georgia, Michigan, Missouri, North Carolina, Ohio, Oklahoma, South Carolina, Tennessee, Virginia, Wisconsin
Functional type	Land plant
Status in England	Non-Native
Status in Scotland	Non-Native
Status in Wales	Non-Native
Location of first record	Halifax
Date of first record	1941

Origin

North America (Canada, southeast USA) & Mexico.

First Record

The Calder Hebble Navigation, between Halifax and Salterhebble, Southwest Yorkshire in 1941.

Pathway and Method

Myriophyllum heterophyllum is used in aquaria and as an ornamental plant in outdoor ponds and has long been available for sale in Europe under a variety of names. Early escapes in North America are thought to have originated from ornamental cultivation with subsequent spread via vegetative propagules (CABI, 2019). In Europe details regarding its introduction and spread are lacking although they too are likely to have come from escapes from ornamental plantings. The origin of the Sussex plants is unknown although the ponds in which it occurs were planted with aquatics in 2011 and so it is possible that it was introduced (as a contaminant) with plants imported from Europe. The origin of the Yorkshire plants is unknown.

Species Status

Considered invasive throughout New England and northwest USA and as an emerging invader in Europe, where it has been classified as invasive in Germany, France and the Netherlands (CABI, 2019; Gross et al., 2020). It has also been introduced to China and Guatemala.

Dispersal Mechanisms

There appears to be no seed production in Europe and so spread is entirely via clonal reproduction and fragmentation (OEPP/EPPO, 2016). Like many other invasive aquatic species, very small vegetative fragments (< 1cm) have the capacity to regenerate new plants and extend infestations. Spread is therefore likely to be enhanced by disturbance caused by human (including manual control operations), boat, fish and water-bird activity.

Reproduction

Myriophyllum heterophyllum appears to flower very infrequently in Europe and no seed production has been reported and consequently most spread is via clonal reproduction and the dispersal of vegetative fragments. However, the species was photographed flowering in the UK in 2015 and 2016 (see front cover of BSBI News 134).

Known Predators/Herbivores

Although a range of insects are known to feed on the foliage of Myriophyllum heterophyllum in the USA (OEPP/EPPO, 2016) none have been recorded for plants in its introduced range.

Resistant Stages

None known.

Habitat Occupied in GB

In 2015 it was discovered in 3 ‘receptor’ ponds dug in 2011 as part of ecological mitigation scheme for the translocation of Great Crested Newts (Smith et al., 2017). In the 1940s it was recorded from a former canal in Yorkshire that had been drained by 1948.

Native in North America (Canada, southeast USA) and Mexico. In Europe it has been introduced to Austria, Belgium (first record 1993), France (first record 2011), Germany (first record 1960s), Hungary, the Netherlands (first record 1999), Spain, Switzerland and UK. It has also been introduced to China and Guatemala.

Environmental Impact

Dense stands of M. heterophyllum reduce light to other aquatic plants and reduce oxygen levels thereby impacting fish and invertebrates. Dense stands can also alter water chemistry and waterflow thereby altering conditions for other taxa.

Health and Social Impact

Dense stands of M. heterophyllum can clog ponds and waterways thereby reducing their aesthetic value and impeding water-related activities such as boating, swimming and fishing.

Economic Impact

Dense stands of M. heterophyllum can increase suspended organic matter and sedimentation potentially increasing risks of flooding. They can also reduce wildlife leading to a potential loss of recreational revenue and in the USA dense infestations has been reported as causing house prices by 20-40% along lake shores with severe infestations (OEPP/EPPO, 2016).

Identification

Aids to identification including links to websites. https://www.invasive.org/browse/subthumb.cfm?sub=12803

Gross, E.M., Groffier, H., Pestelard, C., Hussner, A. 2020. Ecology and environmental impact of Myriophyllum heterophyllum, an aggressive invader in European waterways. Diversity 12, 127; doi:10.3390/d12040127

Biology, ecology, spread, vectors

CABI, 2019. Datasheet: Myriophyllum heterophyllum (broadleaf watermilfoil). https://www.cabi.org/isc/datasheet/34940

Gross, E.M., Groffier, H., Pestelard, C., Hussner, A. 2020. Ecology and environmental impact of Myriophyllum heterophyllum, an aggressive invader in European waterways. Diversity 12, 127; doi:10.3390/d12040127

OEPP/EPPO, 2016. Datasheets on pests recommended for regulation: Myriophyllum heterophyllum Michaux. Bulletin OEPP/EPP Bulletin 46, 20-24.

Management and impact

CABI, 2019. Datasheet: Myriophyllum heterophyllum (broadleaf watermilfoil). https://www.cabi.org/isc/datasheet/34940

Gross, E.M., Groffier, H., Pestelard, C., Hussner, A. 2020. Ecology and environmental impact of Myriophyllum heterophyllum, an aggressive invader in European waterways. Diversity 12, 127; doi:10.3390/d12040127

OEPP/EPPO, 2016. Datasheets on pests recommended for regulation: Myriophyllum heterophyllum Michaux. Bulletin OEPP/EPP Bulletin 46, 20-24.

General

Smith, C., Fennell, M. & Wade, M. 2017. Myriophyllum heterophyllum (Variable-leaved Watermilfoil) in ponds near Horsham, West Sussex (v.c.13). BSBI News 134, 51-53.