Myriophyllum heterophyllum | Various-leaved Water-milfoil
Regulated invasive species of Union concern under the European Regulation on the prevention and management of the introduction and spread of invasive alien species [1143/2014].
Introduction pathways - 1
Release in Nature
Introduction pathways subclass - 1
Botanical garden/zoo/aquaria, Pet/aquarium species
NAPRA Ireland risk assessed
Submerged rooted evergreen fast-growing perennial aquatic plant. Stems grow to approximately 1m in length with the emergent stem 15-20cm above the water surface (Lafontaine et al., 2013). Submerged leaves are arranged in whorls of 4 or 5 leaves and are deeply dissected giving a feathery appearance. Emergent leaves vary in form from deeply dissected to merely toothed (EPPO, 2016). Stems are red or reddy brown. Rarely flowers even in its native range (GISD, 2015) making identification more difficult. Flowers are small and reddish (EPPO, 2015) and appear from June to September (Lafontaine et al., 2013).
Grows quickly creating dense colonies, which reduce water flow and block sunlight (EPPO, 2016) allowing it to out-compete native submerged species (Lafontaine et al., 2013). Optimum growth is at 18° - 25°C (Lafontaine et al., 2013) but tolerates broad spectrum of temperatures even surviving under ice during cold Northern winters (Brunel et al., 2010). Climate is therefore not a limiting factor for its spread in Europe (EPPO, 2015). Lower oxygen levels in the water column under dense stands may result in an avoidance of the area by fish (EPPO, 2016) and in some cases fish kills (Brunel et al., 2010) particularly when vegetation decomposes. Presence of extensive stands increases sedimentation and reduces available spawning gorunds for fish (EPPO, 2016). Infestations have a negative affect on the predator prey balance among fish (EPPO, 2015). Recreational activities such as swimming, boating and fishing can be severely curtailed in affected areas (Brunel et al., 2010) with a reported 20-40% drop in lakeside property prices in the USA (Brunel et al., 2010).
Flowers rarely even in its home range so reproduction is mainly vegetative (EPPO, 2016). A new plant can generate from a single node. However, the stems are quite stout and so do not fragment as readily as many other submerged species (EPPO, 2016). In the North-eastern USA Myriophyllum heterophyllum may have hybridized with other Myriophyllum spp. producing a more aggressive hybrid (Lafontaine et al., 2013).
Pathway and vector description
Imported as a misidentified aquarium plant (EPPO, 2015). The plant then enters the wider environment through improper disposal. Spread is facilitated by man's activities with its long strands easily catching on boating and fishing equipment (Lafontaine et al., 2013). It may also be spread by flooding or waterfowl (Lafontaine et al., 2013).
Mechanism of impact
There is currently an EU wide ban on the sale, growing and keeping of this plant (European Commission, 2017).
Imported aquatic plants could be screened as many are incorrectly identified (EPPO, 2015). Public awareness campaigns providing information about identification, impacts and targeting farmers, fisheries and those using the water recreationally could be launched (EPPO, 2015).
Handpulling may be the most cost-effective method for small or sparse infestations (GISD, 2015).
Experimental work in Lough Corrib uses benthic barriers (jute mats) to control another submerged species; Lagarosiphon major (Caffrey, 2010) and this may be effective too for M. heterophyllum.
Hydro-venturi may also be effective and may cause less fragmentation that other methods (EPPO, 2015).
Cutting boats can be helpful with large infestations but may be more effective if carried out in Winter to reduce the plant's cometitive advantage in the Spring (EPPO, 2015).
Water draw-down can be used but the negative effects on other plants and animals must also be considered (EPPO, 2016).
Freshwater ponds, lakes, canals, ditches, slow flowing rivers, estuaries, canals, reservoirs and marshes. It thrives in a wide range of temperatures (Brunel et al., 2010, Lafontaine et al., 2013). It can also grow on mudflats if water levels drop (Lafontaine et al., 2013).
North America, South America
Myriophyllum heterophyllum can be confused with Myriophyllum aquaticum. They can be distinguished by their flowers and leaves. Myriophyllum aquaticum has white flowers unlike M. heterophyllum, which has reddish flowers with characteristic bracts. M. Aquaticum emergent leaves are blue or bright green, deeply dissected, with a feathery appearance (Wellendorf, 2008). M. heterophyllum emergent leaves are bright green, lanceolate, stiff, and toothed (EPPO, 2016).
Native to Eastern North America, Mexico and Canada (EPPO, 2016).
Introduced in China, Guatemala and Europe – Austria, Belgium, France, Germany, Hungary, the Netherlands, Spain and Switzerland (EPPO, 2016). It is not yet regarded as invasive in Europe.
There is no consensus on which parts of North-eastern USA it is native.
Records submitted to Data Centre in 2022
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How can you help
Report any sightings to the National Biodiversity Data Centre. Dispose of aquarium plants responsibly. Do not purchase or plant in your garden.
Brunel, S; Schrader, G; Brunder, G; Fried, G. (2010). Emerging invasive alien plants for the Mediterranean Basin. Bulletin OOEP/EPPO. Bulletin 40 (2), pp. 219-238. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2338.2010.02378.x/pdf Site accessed 16 August 2017.
Caffrey, JM et al. (2010). A novel approach to aquatic weed control and habitat restoration using biodegradable jute matting. Aquatic Invasions, 5 (2), pp. 123-129. http://www.aquaticinvasions.net/2010/AI_2010_5_2_Caffrey_etal.pdf Site accessed 4 August 2017.
EPPO. (2016). Data sheets on pests recommended for regulation. Myriophyllum heterophyllum. Bulletin, 46 (1), pp. 20-24. https://www.eppo.int/QUARANTINE/data_sheets/plants/MYPHE_ds.pdf Site accessed 15 August 2017.
EPPO. (2015). Pest risk analysis for Myriophyllum heterophyllum. EPPO, Paris. http://www.eppo.int/QUARANTINE/Pest_Risk_Analysis/PRA_intro.htm Site accessed 15 August 2017.
European Commission. (2017). Invasive Alien Species of Union Concern. European Union, Luxembourg.
Global Invasive Species Database (GISD). (2015). Species profile Myriophyllum heterophyllum. http://www.iucngisd.org/gisd/speciesname/Myriophyllum+heterophyllum Site accessed 15 August 2017.
Lafontaine, R-M; Beudels-Jamar, RC; Delsinne, T; Robert, H. (2013). Risk analysis of the Variable Watermilfoil Myriophyllum heterophyllum Michaux. - Risk analysis report of non-native organisms in Belgium from the Royal Belgian Institute of Natural Sciences for the Federal Public Service Health, Food chain safety and Environment. p. 33
https://www.researchgate.net/publication/266617477 Site accessed 16 August 2017.
Wellendorf, N. (2008). Submersed aquatic plant ID. Florida Department of Aquatic Plants. Standards and Assessment Section. FAB Workshop. https://www.dep.state.fl.us/water/bioassess/docs/plants/submersed_plants.pdf Site accessed 16 August 2017.
CABI Datasheet (http://www.cabi.org/isc/datasheet/34940)
Global Invasive Species Database (http://www.iucngisd.org/gisd/speciesname/Myriophyllum+heterophyllum)
Q-bank Factsheet (http://www.q-bank.eu/Plants/Factsheets/Myriophyllum_heterophyllum_EN.pdf)
Not yet regarded as invasive in Europe, which may be due to genetic differences between European and North American populations. There is a risk of hybridization with native Microphyllum species (Lafontaine et al., 2013) as well as the risk of the import of one of the more aggressive North American hybrids (Lafontaine, et al., 2013).