Myriophyllum heterophyllum in the EPPO region: addition to the EPPO Alert List
Myriophyllum heterophyllum (Haloragaceae, common name variable watermilfoil) is an aquatic perennial plant native to the Southeast of the USA. The species is used in aquarium and for ornamental purposes in ponds. Within the EPPO region, its distribution is still limited. Because this plant has shown invasive behaviour where it has been introduced elsewhere in the world and is still limited in the EPPO region, it can be considered an emerging invader in Europe.
EPPO region: Austria, Germany, Spain.
North America: Canada (British Columbia, New Brunswick, Ontario, Quebec), USA (Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Michigan, Minnesota, Mississippi, Missouri, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Virginia, Washington, West Virginia, Wisconsin).
Note: there is no consensus on the native distribution of this species, however, USDA considers the species indigenous in Eastern North America (Canada and USA), and exotic in Western North America.
M. heterophyllum is a submersed, rooted macrophyte. It has both submerged and emergent leaves growing from a stem up to 3 mm in diameter and 100 cm in length. Stems are dark red to reddish-brown. Submerged leaves are feather-like, green and 2-5 cm long and 2-4 cm wide, dissected into 7-11 leaflets and arranged into whorls of 4-5 leaves. The highly variable emergent leaves develop during late summer and can reach 5-15 cm above the water; they are 0.4-3 cm long and 1.5-5 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 mm long. Fruits are 1-1.5 mm in length, round, with 4 chambers.
Biology and ecology
Reproduction occurs primarily through vegetative fragmentation and rhizome division, although the plant may also reproduce by seeds remaining in lake and ponds sediments. The flowers and fruits appear from June to September. M. heterophyllum over-winters in the frozen lakes of northern climates and can thrive in warm southern water bodies. It has also been found growing under a wide range of water temperatures and chemical conditions: it can be found calcium-rich waters, but tends to prefer acid pH waters. Vegetative parts of the plant may be spread by animals and human activities (e.g. fishing, movement of boats). Waterfowl can also facilitate the spread of the plant by eating seeds.
M. heterophyllum’s habitats are freshwater ponds, lakes, ditches, standing and slow flowing waters. It can grow in waters up to 1.8 m deep. According to the Corine Land Cover nomenclature, these habitats correspond to “continental waters (water courses, water bodies)”.
M. heterophyllum is used as an aquarium plant and as an ornamental plant in garden ponds.
M. heterophyllum is highly competitive and can grow and spread rapidly, and is able to displace other submerged macrophyte species. It produces dense mats that reduce sunlight and can restrict water movement, and particularly when decomposing, that reduce water quality and available oxygen. The low oxygen conditions can kill fish and harm other aquatic organisms. The dense mats can impede swimming, boating and fishing. Moreover, dense mats along lake shorelines have been reported to reduce property values by 20-40%.
In Eastern USA, the species may hybridize with the native M. pinnatum, resulting in a more aggressive hybrid Myriophyllum heterophyllum x pinnatum.
Small, recently detected infestations may be successfully eradicated through careful and thorough hand-pulling or using a tarpaulin. Great care should be taken with such methods since they cause fragmentation of the plant and therefore its spread. Dense stands occurring in shallow lakes in Nordrhein-Westfalen (Germany) have been mechanically controlled. Benthic barriers may be used in small areas (swimming beaches, boating lanes, around docks) to restrict light and upward growth. Nevertheless, barriers can have a negative impact on benthic organisms and need to be properly maintained. Drawdown can also be used to control M. heterophyllum where applicable, if it is extensive enough to prevent re-growth from seeds, but this control method could have a negative impact on native plants and animals (fish, reptiles, amphibians, etc.).
Herbicide control (e.g. diquat-dibromide and 2,4-D) is recommended in some States of USA to manage this species.
So far, no biological control agents have been identified.
This plant is thought to have the potential to become invasive, especially in shallow lakes and channels accross the whole EPPO region.
Commonwealth of Massachusetts ~ Department of Conservation and Recreation ~ Office of Water Resources ~ Lakes and Ponds Program - Variable Milfoil: An Invasive Aquatic Plant - Myriophyllum heterophyllum. http://www.mass.gov/dcr/waterSupply/lakepond/factsheet/Variable
Delivering Invasive Alien Species Inventories for Europe (DAISIE) Database. http://www.europe-aliens.org/
Hussner A (2005) [Distribution of alien aquatic plants in the river Erft (North Rhine-Westphalia)] (Zur Verbreitung aquatischer Neophyten in der Erft, Nordrhein-Westfalen). Frankfurter Geobotanische Kolloquien 19, 55-58 (in German).
Invasive Plant Atlas of New England (IPANE) - Myriophyllum heterophyllum.
USDA – Plant profile for Myriophyllum heterophyllum
Washington State Noxious Weed Control Board Website