Brazilian watermilfoil, parrot’s feather, parrot-feather, parrotfeather, parrot feather watermilfoil, Enydria aquatica (Vell.), Myriophyllum brasiliense (Camb.), Myriophyllum proserpinacoides (Gillies ex Hook. and Arn.)

The submersed shoots, similar to those of Eurasian watermilfoil (M. spicatum), are comprised of whorls of four to six filamentous, pectinate leaves, 1.5 to 3.5 cm long, arising from each node (Mason 1957, Washington State Department of Ecology 2011). Submersed leaves are reddish orange. When the submersed shoots reach the water surface, plant growth changes and begins to creep along the water surface with extensive branching from nodes followed by vertical growth of emergent stems (Moreira et al. 1999).

Small, white flowers occur in the leaf axils on the emergent shoots and are approximately 1/16 inch long (Washington State Department of Ecology 2011). Parrot feather lacks structures for storage, dispersal, and perennation (e.g., tubers, turions, and winter buds), and therefore stolons serve all these functions (Sytsma and Anderson 1993).

Nonindigenous Occurrences: Myriophyllum aquaticum has been introduced into France (1880), North America (1890) South Africa (1919), Japan (1920), Southeast Asia (Java), New Zealand (1929), Japan (1920), South Africa (1918), United Kingdom (1960), Australia (1960s), and England (1970s) (Washington State Department of Ecology 2011). It can also be found in Portugal (Moreira et al. 1999) and is established in Germany (1988) but still considered rare (Hussner et al. 2010). It is currently undergoing range expansion and excessive spreading in its native South America (Fernandez et al. 1993) and in Southern Africa, Southeast Asia (Anderson 1993), and Portugal (Teles and Pinto da Silva 1975).

In North America, the first record of parrot feather was in New Jersey (1890) on the east coast, and by 1944 it had reached Washington. The earliest specimen recorded in the United States was collected April 20, 1890, from Haddonfield, New Jersey (Nelson and Couch 1985). A Missouri collection in 1897, clearly introduced as an ornamental which escaped from aquaria and water garden cultivation, (Couch and Nelson 1985b), was probably a separate introduction rather than originating from localities on the east coast. Myriophyllum aquaticum was first reported in the southern New England region (southeastern New York) in 1929 (Couch and Nelson 1985b). By 1940, it was well established in southeastern New York and on Long Island (Couch and Nelson 1985b, Muenscher 1944, Ogden 1974). Couch and Nelson reported a single population of parrot feather in western Washington in 1944. Washington's parrot feather infestations are found in coastal lakes and streams and in the southwest Washington portion of the Columbia River. Parrot feather is found throughout the drainage system in the Longview/Kelso area, infests many of the drainage ditches in Wahkiakum County, and was discovered growing in the Chehalis River in 1994. Recently parrot feather was discovered in some backwater ponds along the Yakima River and also in Asotin County (Washington State Department of Ecology 2011). An herbarium specimen was collected from Skamokowa, Wahkiakum County in 1983 (Washington State Department of Ecology 2011).

It is now distributed throughout nearly every southern state, and in regions including Alabama, Arkansas, Arizona, California, Connecticut, District of Columbia, Delaware, Florida, Georgia, Hawaii (Nelson and Couch 1985), Idaho, Kansas, Kentucky, Louisiana, Maryland, Missouri, Mississippi, Montana, North Carolina, New York, New Mexico, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, and British Columbia.

A specimen of M. aquaticum (originally misidentified as Proserpinaca sp.) was collected in southern Connecticut (West Lake, Guilford, New Haven Co.) in 1946. However, the authors failed to detect the species in a 1993 survey of the lake (Les and Mehrhoff 1999).

Parrot feather is a dioecious species, however only pistillate (female) plants are found outside of South America. Staminate (male) plants are rare even in native populations of South America (Orchard 1981). For this reason, seed production is not known to occur (Aiken 1981) and reproduction is exclusively vegetative in North America (Orchard 1981). Reproduction occurs by fragmentation of emergent and/or submersed shoots, roots, rhizomes, or attached plant fragments (IFAS 2010, Les and Mehrhoff 1999, Mabulu 2005).

Parrot feather has an annual growth pattern, forming shoots in spring from overwintering rhizomes as water temperature increases. Rhizomes provide support for adventitious roots and buoyancy for emergent summer growth. Flowers usually appear in spring, or in fall for some plants. The plant usually dies back to its rhizomes in the autumn (Mabulu 2005).

Potential pathways of introduction: Natural dispersal through waterbody connections; Hitch-hiking or fouling of recreational gear, boat structures, fauna, or other objects entering the Great Lakes from surrounding region; Unauthorized intentional release from live trade; Accidental introduction to Great Lakes by escapees

With established nonindigenous populations in states adjacent to the Great Lakes, parrot feather has potential to be introduced to the Great Lakes from nearby water bodies. The closest parrot feather population to the Great Lakes has been recorded from Meserve Lake, Indiana, which drains though the Pigeon River into the St. Joseph River, a tributary of Lake Michigan (Wersal 2011). Fragments of this plant are capable of transport by river currents and could also become attached to or entangled with recreational boats (e.g., propellers, trailer tires) or fishing gear. Its rhizomes are very tough and can be transported long distances on boat trailers, surviving for up to a year when kept moist and cool (Washington State Department of Ecology 2003, in Mabulu 2005).

Parrot feather has been an ornamental favorite in hanging baskets, fountains, and aquaria for more than a century due to its blue-green color, feather-like leaves, oxygenating properties, and cascading pattern of growth (Les 2002, Les and Mehrhoff 1999). Often sold under incorrect names, introductions of this species are usually attributed to the water garden and aquarium trades (Davis 1996, IFAS 2010, Les 2002, Les and Mehroff 1999). It has escaped cultivation through mechanical fragmentation and unintentional plantings, readily taking root. In a Great Lakes regional study, this aquatic plant was found in 25% of the stores surveyed in Michigan and Ontario, near Lake Erie, between 2002 and 2003 (Rixon et al. 2005). Moreover, water garden plants are often left outside to overwinter, which can lead to unintentional escape during spring flooding. The locations of Ontario water gardens indentified by 2006 survey respondents suggests that many of these gardens are within the coastal regions of four of the five Great Lakes, though if these were also flood-prone areas was not determined (Marson et al. 2009b).

Parrot feather is of growing interest for environmental remediation of soil and water contaminated with chlorinated solvents, trinitrotoluene (TNT), and other nitrogenated explosive/aromatic compounds, but this is currently a technology in limited, experimental use (Medina et al. 2000, Nwoko 2010).

Among the Great Lakes states and provinces, M. aquaticum is prohibited in Illinois, Michigan, and Wisconsin and regulated in Minnesota. Furthermore, it is listed as a noxious weed by nine non-Great Lakes states (Alabama, Connecticut, Idaho, Maine, Massachusetts, Maryland, New Hampshire, Vermont, and Washington) (IISG 2008, GLPNS 2008, WIDNR 2011). Without more stringent laws regulating sale and disposal throughout the entire region, introduction could occur through disposal of aquarium fragments, unintentional escape from culture, or intentional unauthorized planting to support live trade.

Myriophyllum aquaticum is a hardy species with broad environmental tolerances (see Ecology above). It occurs as a floating plant in the deep water of nutrient-enriched lakes like the Great Lakes (Washington State Department of Ecology 2011). It is known to tolerate freezing temperatures in California’s Bay area winters (Aiken 1999). However, this plant can be killed by extended periods of frost (WNDR 2011) and so may benefit from warmer winters predicted to result from climate change.

Parrot feather grows vigorously and quickly following invasion in new habitats, forming dense canopies that occupy large amounts of space and block sunlight and oxygen exchange. As a result, this species outcompetes and replaces native flora that might be of more value to fish and wildlife (Stiers et al 2010, WNDR 2011).

Reproduction and dispersal of M. aquaticum in North America occurs by vegetative fragmentation, which is an effective method for short-range, but not long-range, dispersal (Les and Mehrhoff 1999). Although parrot feather’s natural dispersal potential is limited, this species is widespread outside its native range (Moody and Les 2010). Myriophyllum aquaticum has expanded its range mainly in the southern United States and may be relatively innocuous in the northeast due to a smaller number of occurrences (Hoyer et al. 1996). Nonetheless, this species has survived in southern New England and caused serious local infestations (WIDNR 2011). The rapid spread of M. aquaticum is correlated with its widespread cultivation and the transport of fragments by waterfowl or vehicles. When transport agents are not present, the threat of its escape and establishment depends more on the number of localities where it is grown. Unfortunately, M. aquaticum remains widely available from sources of cultivated water plants and dealers occasionally plant it intentionally to propagate a local supply (Aiken 1981, Les and Mehrhoff 1999).

Nonindigenous M. aquaticum specimens collected from geographically diverse locations in North America have been found to have identical ITS genotypes and are all female. Seed production has not been recorded (Moody and Les 2010).

Dense infestations of parrot feather can rapidly overtake small ponds and sloughs, changing their physical and chemical properties, including impeding water flow, which can result in increased flood duration and intensity. The spread of aquatic nonindigenous plants into a waterbody can also lead to increased rates of evapotranspiration and water loss. One mesocosm experiment found that colonization by M. aquaticum was correlated to an increase in water loss of about 1.5 to 2 times that experienced by an open water surface (Rosa et al. 2009).
Myriophyllum aquaticum can dramatically alter ecosystems by shading out algae, pondweeds, and coontail on which waterfowl feed (Ferreira and Moreira 1994, Washington State Department of Ecology 2011). Floating mats of M. aquaticum have been measured at up to 26 kg of fresh weight in Europe and are capable of reducing the oxygen content of the water below to <1 mg O2L-1, which can be detrimental to fish (Fonseca 1984 cited in Moreira et al. 1999, Hussner 2008 in Hussner 2009). In Germany, the infestation of these mats created anoxic, shaded conditions in shallow waters, and appeared to be correlated with a decline in native macrophyte diversity (Hussner 2008 in Hussner 2009).

In Chinese laboratory experiments, parrot feather outcompeted native species with respect to relative growth rate, with the most significant results on high-nutrient sediment (Xie et al. 2010). A separate mesocosm study by Wersal and Madsen (2011) found that the yield (biomass) of M. aquaticum was positively related to tissue nitrogen content, suggesting that high levels of nitrogen contribute to nuisance levels of growth. However, an inverse relationship existed between M. aquaticum yield and tissue phosphorus content. Wersal and Madsen (2011) proposed that high levels of phosphorus favored the growth of algae (superior competitors in phosphorus uptake) causing shading in the water column and suppressing the growth of M. aquaticum (Wersal and Madsen 2011).

Stiers et al. (2011) compared Belgian lake sites that were heavily invaded (90-100% cover), semi-invaded (~25% cover), and uninvaded by M. aquaticum and found that native species richness was 57% lower in heavily invaded sites relative to uninvaded sites. Parrot feather cover was also negatively correlated with invertebrate species richness and abundance. The authors observed lowered levels of dissolved oxygen at some sites, as well as a dense mat of decomposed plant litter and sediments at the bottom of heavily-invaded sites; they hypothesized that this condition created unsuitable habitat for invertebrate colonization (Stiers et al. 2011). Plant species that are rare (Utricularia vulgaris) and vulnerable (Hydrocharis morsus-ranae) IUCN Red List species in Belgium were absent in heavily invaded sites but present in semi-invaded sites (Steirs et al. 2011). Furthermore, mayflies (Caenis spp.) were present in uninvaded sites, but were not reported in invaded sites (Steirs et al. 2011).

Myriophyllum aquaticum can also alter the cycling of heavy metals in aquatic systems. Cardwell et al. (2002) found that M. aquaticum accumulated the highest overall levels of metals (zinc, cadmium, copper, and lead) in its tissues of all 15 aquatic plants that underwent testing. While this suggests that M. aquaticum could be used as an important indicator species (see below), the consumption of M. aquaticum by grazers could increase the bioaccumulation of heavy metals in the food web.

Myriophyllum aquaticum has the potential for moderate socio-economic impact if introduced to the Great Lakes.

Parrot feather infestations have been reported in both natural and man-made water bodies, including lakes, ponds, canals, drainage and irrigation ditches, and lagoons. Plants and floating mats of vegetation are sometimes uprooted, choking waterways, inhibiting navigation, and potentially blocking pumps or drainage (Engineer Research and Development Center 2007, Sheppard et al. 2006). Dense growth can also diminish the recreational value and seriously affect the perceived aesthetic qualities of infested waterways (Banfield 2008, Washington State Department of Ecology 2011).

Myriophylum aquaticum monocultures provide prime mosquito habitat; higher parrot feather density has been correlated with higher mosquito egg and larval abundance (Orr and Resh 1992), which may lead to increased prevalence of mosquito-born diseases.

Myriophyllum spp. have invaded rice paddies could adversely affect wild rice (Zizania palustris) found in the upper Great Lakes (Quayyum et al. 1999). One account by South African farmers also reported that tobacco crops gained a red tint (reducing the sale value of the crop) when irrigated with water from an area colonized by M. aquaticum roots (Cilliers 1999).

Myriophyllum aquaticum has the potential for moderate potential benefits if introduced to the Great Lakes.

Assessment protocols have been developed using M. aquaticum as a primary indicator species of sediment toxicity in potentially polluted areas (Feiler et al. 2004, Knauer et al. 2008). It is an important species in the aquarium trade and can be found in shops in both the American and Canadian Great Lakes regions (Marson et al. 2009a, Rixon et al. 2005). It is reportedly sold as an “oxygenating plant” in Europe (Sheppard et al. 2006).

Parrot feather may provide cover for some aquatic organisms (Washington State Department of Ecology 2011). Parker et al. (2007) found that beavers (Castor canadensis) in Georgia fed on M. aquaticum to the extent that invasive populations were reduced, although no strong preference for this plant species over others was documented. Myriophyllum aquaticum could be used for nitrogen and phosphorus remediation (e.g., in a constructed wetland remediating nutrient runoff), but Polomski et al. (2009) found that other invasive macrophytes (Eichhornia crassipes and Pistia stratiotes) had equal or greater uptake efficiency levels relative to M. aquaticum. Parrot feather can also aid in environmental remediation of soil and water contaminated with chlorinated solvents, trinitrotoluene (TNT), and other nitrogenated explosive/aromatic compounds (Medina et al. 2000, Nwoko 2010).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control
Preface: Although parrot feather is not considered a widespread nuisance, once it becomes established in an area it is very difficult to control. Several methods, including chemical, mechanical, and biological control, have been evaluated with mixed results. Chemical and mechanical methods can provide short to medium term control of parrot feather. Herbicides have been used most often for control of parrot feather and results have been dependent upon herbicide choice. Mechanical methods are much less documented; however, their use may facilitate regrowth and further spread of parrot feather. Biological control has been evaluated; however, there are no viable options available in the United States. The most effective method to avoid infestations is likely to prevent unintentional release from water gardens.

Chemical
Parrot feather’s waxy cuticle on stems and leaves can only be penetrated with a wetting agent, making chemical control challenging—the weight of spraying may cause the plants to sink in the water, which can wash the herbicide off before it can take effect. Nevertheless, the most successful herbicides currently used for parrot feather control include those that can be applied to foliage, such as 2,4-D, triclopyr, diquat, carfentrazone, imazapyr, and imazamox. The use of 2,4-D and triclopyr as a foliar applications have resulted in consistent control of parrot feather (Hofstra 2006, Moreira et al. 1999). Glyphosate is generally not recommended as this herbicide only kills emergent shoots and plants often regrow in greater densities (Moreira et al. 1999). Diquat is a contact herbicide that will kill the vegetation it comes in contact with, but significant regrowth is common (Westerdahl and Getsinger 1988). Carfentrazone-ethyl will not control parrot feather as a foliar application (Richardson et al. 2008). The use of imazapyr and imazamox have been evaluated on small infestations with excellent to fair results, respectively (Wersal and Madsen 2007).

Subsurface herbicide applications do not result in increased control relative to foliar applications (Wersal and Madsen 2010). Carfentrazone-ethyl will not control parrot feather and is not recommended as a stand-alone treatment (Glomski et al. 2006, Gray et al. 2007). However, when carfentrazone-ethyl was combined with 2,4-D it resulted in excellent control of small parrot feather populations (Gray et al. 2007).

Multiple applications are often necessary to completely control parrot feather. The effectiveness of herbicide applications will be site specific and depend upon the environmental conditions at the time of application.

Physical

Cutting plants will only increase spread, as parrot feather reproduces vegetatively. Hand pulling and harvesting may offer temporary control, however this approach is very labor intensive as dense mats are heavy and difficult to haul out of the water (Guillarmod 1977). Raking and chaining (long chains of sharp blades pulled by tractors) may not be feasible due to the rapid biomass production of parrot feather; moreover, dense mats may damage equipment. Sebbatini et al. (1998) reported that parrot feather was tolerant to mechanical disturbance (raking and chaining) and the repeated application of mechanical techniques favored parrot feather dominance in canals. Care must be taken to remove all plant parts (emergent shoots, submersed shoots, and roots), as well as fragments created by the removal, or re-growth will occur.

Water drawdown may be a viable option for parrot feather control, but the effectiveness of this approach has yet to be determined. To be successful, a drawdown would have to be sustained long enough to completely dry the soil, as parrot feather can and will survive in moist soil. Dredging is generally very expensive and not feasible for most management situations.

Biological
Currently, the grass carp (Ctenopharyngodon idella) and a leaf feeding beetle (Lysathia spp.) have been evaluated for control of parrot feather infestations. Grass carp are not recommended for parrot feather control as fish generally avoid eating this plant due to its high tannin content (Catarino et al. 1997, WSDE 2003 in Mabulu 2005, Pine and Anderson 1991). The leaf-feeding beetle showed some promise in South Africa by significantly reducing emergent shoot biomass (Cilliers 1999, Mabulu pers. comm. 2004 in Mabulu 2005); however, this agent is not approved for use in the United States. Existing evidence supports that beaver (Castor canadensis) provides some control of M. aquaticum in the Gumby Swampland (Georgia); when beavers were excluded at certain sites, M. aquaticum abundance increased nearly 8-fold and accounted for up to 95% of the increased vegetative growth in the exclusions (Parker et al. 2007).

Cultural Control & Prevention of Spread
Parrot feather is a common component of aquatic landscaping because of its aesthetic appearance and ease of cultivation (Sutton 1985). Aiken (1981) reported observations of aquarium plant providers in the San Francisco Bay area placing of parrot feather plants into local waterways to have a convenient source of saleable material. The ease of cultivation and attractiveness as a pond plant has aided in its escape and subsequent colonization of natural areas.

Cultural prevention approaches are the best way to avoid parrot feather infestations, as this plant is almost exclusively spread by human means (e.g., propeller or fishing gear entanglement, ornamental release) (Guillarmod 1977). This species is also likely to be resilient to water level fluctuations resulting from climate change (Huessner et al. 2009).

Ultimately, to prevent the future introduction and spread of parrot feather into new areas it must be prohibited from sale by the water garden and aquaculture industries.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

Anderson, L.W.J. 1993. Aquatic weed problems and management in the western United States and Canada. Chapter 19a In: A.H. Pieterse and K.J. Murphy, eds. Aquatic Weeds, 2nd Edition. Oxford Scientific Press, Oxford, U.K. pp. 371-391.

Banfield, S. 2008. Aquatic Vegetation Management Plan 2008-2012: Meserve Lake, Steuben County. Available http://www.aquaticenhancement.com/AES%20documents/Meserve%202008%20final3.pdf. Accessed 13 September 2011.

Cardwell, A.J., D.W. Hawker, and M. Greenway. 2002. Metal accumulation in aquatic macrophytes from southeast Queensland, Australia. Chemosphere 48: 653-663.

Catarino, L.F., M.T. Ferreira, and I.S. Moreira. 1997. Preferences of grass carp for macrophytes in Iberian drainage channels. Journal of Aquatic Plant Management 36:79-83.

Cilliers, C.J. 1999. Lysathia n.sp. (Coleoptera: Chrysomelidae), a host-specific beetle for control of the aquatic weed Myriohphyllum aquaticum (Haloragaceae) in South Africa. Hydrobiologia 415:271-276.

Couch, R., and E. Nelson. 1986. Myriophyllum spicatum. Pages 8-18 in Proceedings, First International Symposium on Watermilfoil (Myriophyllum spicatum) and Related Haloragaceae Species. The Aquatic Plant Management Society, Vicksburg, Mississippi.

Engineer Research and Development Center. 2007. Myriophyllum aquaticum (Vell.) Verdc. (Parrotfeather). In PMIS, Noxious and Nuisance Plant Management Information Systems. U.S. Army Engineer Research and Development Center, Environmental Laboratory, Vicksburg, MS. Available http://el.erdc.usace.army.mil/pmis/pmishelp.htm.

Feiler, U., I. Kirchesch, and P. Heininger. 2004. A new plant-based bioassay for aquatic sediments. Journal of Soils and Sediments 4(4): 261-266.

Fernández, O.A., D.L. Sutton, V.H. Lallana, M.R. Sabbatini, J.H. Irigoyen. 1993. Aquatic weed problems and management in South and Central America. In: A.H. Pieterse, K.J. Murphy, eds. Aquatic Weeds, 2nd Edition. Oxford University Press, Oxford, U.K. pp. 406-425.

Global Invasive Species Database. IUCN-World Conservation Union—Invasive Species Specialist Group. 27 January 2011. http://www.issg.org/database/

Glomski, L.M., A. G. Poovey, and K.D. Getsinger. 2006. Effect of carfentrazone-ethyl on three aquatic macrophytes. Journal of Aquatic Plant Management 44:67-69.

Godfrey, R.K., and J.W. Wooten. 1979. Aquatic and Wetland Plants of Southeastern United States: Dicotyledons.  University of Georgia Press, Athens, GA.

Gray, C.J., J.D. Madsen, R.M. Wersal, and K.D. Getsinger. 2007. Eurasian watermilfoil and parrotfeather control using carfentrazone-ethyl. Journal of Aquatic Plant Management 45:43-46.

Great Lakes Panel on Aquatic Nuisance Species (GLPANS). 2008. Prohibited species in the Great Lakes Region. Report November 2008.

Guillarmod, A.J. 1977. Myriophyllum, an increasing water weed menace for South Africa. South African Journal of Science 73:89-90.

Hofstra, D.E., P.D. Champion, and T.M. Dugdale. 2006. Herbicide trials for the control of parrotsfeather. Journal of Aquatic Plant Management 44:13-18.

Hussner, A., C. Meyer, and J. Busch. 2009. The influence of water level and nutrient availability on growth and root system development of Myriophyllum aquaticum. Weed Research 49: 73-80.

Hussner, A., K. Van de Weyer, E.M. Gross, and S. Hilt. 2010. Comments on increasing number and abundance of non-indigenous aquatic macrophyte species in Germany. Weed Research 50: 519-526.

Illinois-Indiana Sea Grant (IISG). 2011. Sea Grant Database of Aquatic Species Regulations. Available http://www.iiseagrant.org/speciesregs/index1.asp?commonName=parrot%27s+feather. Accessed 25 October 2011.

Knauer, K., S. Mohr, and U. Feiler. 2008. Comparing growth development of Myriophyllum spp. in laboratory and field experiments for ecotoxicological testing. Environmental Science and Pollution Research 15: 322-331.

Les, D.H., and L.J. Mehrhoff. 1999. Introduction of nonindigenous aquatic vascular plants in southern New England: a historical perspective. Biological Invasions 1(2): 281-300.

Mabulu, L.Y. 2005. Myriophyllum aquaticum (aquatic plant). Global Invasive Species Database. Available http://www.issg.org/database/species/ecology.asp?si=401&fr=1&sts=sss&lang=EN.

Marson, D., B. Cudmore, D.A.R. Drake, and N.E. Mandrak. 2009a. Summary of a survey of aquarium owners in Canada. Canadian Manuscript Report of Fisheries and Aquatic Sciences 2905: iv + 20 pp.

Marson, D., B. Cudmore, D.A.R. Drake, and N.E. Mandrak. 2009b. Summary of a survey of water garden owners in Canada. Canadian Manuscript Report of Fisheries and Aquatic Sciences. 2906: v + 23 pp.

Mason, H.L. 1957. A Flora of the Marshes of California. Universtiy of California Press, Berkeley, CA.

Medina, V.F., S.L. Larson, A.E. Bergstedt, and S.C. McCutcheon. 2000. Phyto-removal of trinitrotoluene from water with batch kinetic studies. Water Resources 34(10): 2713-2722.

Moreira, I., T. Ferreira, A. Monteiro, L. Catarino, and T. Vasconcelos. 1999. Aquatic weeds and their management in Portugal: insights and the international context. Hydrobiologia 415: 229-234.

Moreira, I., A. Monteira, and T. Ferreira. 1999. Biology and control of parrotfeather (Myriophyllum aquaticum) in Portugal. Ecology, Environment and Conservation 5:171-179.

Muenscher, W.G. 1944. Aquatic plants of the United States. Comstock Publishing Company, Ithaca, New York.

Nelson, E.N. and R.W. Couch. 1985. History of the introduction and distribution of Myriophyllum aquaticum in North America. Proceeding, 1st International Symposium on watermilfoil (Myriophyllum spicatum) and Related Haloragaceae Species. 23-24 July 1985. Vancouver, B.C. pp. 19-26.

Nwoko, C.O. 2010. Trends in phytoremediation of toxic elemental and organic pollutants. African Journal of Biotechnology 9(37): 6010-6016.
Ogden, J. 1974. The reproductive strategy of higher plants. II. The reproductive strategy of Tussilago farfara L. J. Ecol. 62: 291-324.

Orchard, A.E. 1981. A revision of South American Myriophyllum (Haloragaceae), and its repercussions on some Australian and North American species. Brunonia 4:27-65.

Orr, B.K., and V.H. Resh. 1989. Experimental test of the influence of aquatic macrophyte cover on the survival of Anopheles larvae.  Journal of the American Mosquito Control Association 5:579-585.

Orr, B.K., and V.H. Resh. 1992. Influence of Myriophyllum aquaticum cover on Anopheles mosquito abundance, oviposition, and larval microhabitat. Oecologia 90: 474-482.

Parrot Feather. Center for Aquatic and Invasive Plants, University of Florida, IFAS. 2010. http://plants.ifas.ufl.edu/node/276.

Pine, R.T., and W.J. Anderson. 1991. Plant preference of triploid grass carp. Journal of Aquatic Plant Management 29:80-82.

PLANTS Profile: Myriophyllum aquaticum. http://plants.usda.gov/java/nameSearch. Accessed 2 January 2011.

Polomski, R.F., M.D. Taylor, D.G. Bielenberg, W.C. Bridges, S.J. Klaine, and T. Whitwell. 2009. Nitrogen and phosphorus remediation by three floating aquatic macrophytes in greenhouse-based laboratory-scale subsurface constructed wetlands. Water, Air, & Soil Pollution 197: 223-232.

Quayyum, H.A., A.U. Mallik, and P.F. Lee. 1999. Allelopathic potential of aquatic plants associated with wild rice (Zizania palustris): I. Bioassay with plant and lake sediment samples. Journal of Chemical Ecology 25(1): 209-220.

Queensland Herbarium. 2002. Invasive naturalized plants in Southeast Queensland, ranked list. [Extracted from Batianoff, G.N. and D.W. Butler. 2002. Assessment of Invasive naturalized plants in south-east Queensland. Appendix. Plant Protection Quarterly 17:27-34.] Queensland Environmental Protection Agency, Queensland Herbarium, Queensland, Australia. Available http://www.derm.qld.gov.au/register/p00727aa.pdf.

Richardson, R.J., R.L. Roten, A.M. West, S.L. True, and A.P. Gardner. 2008. Response of selected aquatic invasive weeds to flumioxazin and carfentrazone-ethyl. Journal of Aquatic Plant Management 46:154-158.

Rixon, C.A.M., I.C. Duggan, N.M.N. Bergeron, A. Ricciardi, and H.J. MacIsaac. 2005. Invasion risks posed by the aquarium trade and live fish markets on the Laurentian Great Lakes. Biodiversity and Conservation 14: 1365-1381.

Rosa, C.S., R.D. Antunes, R.A. Pitelli, and R.L.C.M. Pitelli. 2009. Comparative evaluation of water losses by evapotranspiration in mesocosms colonized by different aquatic weeds. Planta Daninha 27(3): 441-445.

Sebbatini, M.R., K.J. Murphy, and J.H. Irigoyen. 1998. Vegetation-environment relationships in irrigation channel systems of southern Argentina. Aquatic Botany 60:119-133.

Sheppard, A.W., R.H. Shaw, and R. Sforza. 2006. Top 20 environmental weeds for classical biological control in Europe: a review of opportunities, regulations, and other barriers to adoption. Weed Research 46: 93-117.

Sutton, D.L. 1985. Biology and ecology of Myriophyllum aquaticum. Proceeding, 1st International Symposium on watermilfoil (Myriophyllum spicatum) and Related Haloragaceae Species. 23-24 July 1985. Vancouver, B.C. pp. 59-71.

Sytsma, M.D., and L.W.J. Anderson. 1993. Biomass, nitrogen, and phosphorus allocation in parrotfeather (Myriophyllum aquaticum). Journal of Aquatic Plant Management 31:244-248.

Teles, A.N., and A.R. Pinto da Silva. 1975. A “pinheirinha” (Myriophyllum aquaticum (Vell.) Verde), uma agressiva infestante aquática. Agronomia lusitania 36: 307-323.

U.S. EPA (United States Environmental Protection Agency). 2008. Predicting future introductions of nonindigenous species to the Great Lakes. National Center for Environmental Assessment, Washington, DC; EPA/600/R-08/066F. Available from the National Technical Information Service, Springfield, VA, and http://www.epa.gov/ncea.

Washington State Department of Ecology. 2011. Non-native invasive freshwater plants: Parrotfeather (Myriophyllum aquaticum), Technical Information. Washington State Department of Ecology, Olympia, WA. Available http://www.ecy.wa.gov/programs/wq/plants/weeds/aqua003.html.Wersal, R.M., B.R. McMillan, and J.D. Madsen. 2005. Food habits of dabbling ducks during fall migration in a prairie pothole system, Heron Lake, Minnesota. Canadian Field Naturalist 119:546-550.

Wersal, R.M., and J.D. Madsen. 2007. Comparison of Imazapyr and Imazamox for Control of Parrotfeather (Myriophyllum aquaticum (Vell.) Verdc.). Journal of Aquatic Plant Management 45:132-136.

Wersal, R.M., and J.D. Madsen. 2010. Comparison of subsurface and foliar herbicide applications for control of parrotfeather (Myriophyllum aquaticum). Invasive Plant Science and Management 3:262-267.

Wersal, R.M., and J.D. Madsen. 2011. Influences of water column nutrient loading on growth characteristics of the invasive aquatic macrophyte Myriophyllum aquaticum (Vell.) Verdc. Hydrobiologia 665: 93-105.

Westerdahl, H.E. and K.D. Getsinger. 1988. Aquatic Plant Identification and Herbicide Use Guide; Vol II: Aquatic Plants and Susceptibility to Herbicides. Technical Report A-88-9, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.

Wisconsin Department of Natural Resources (WIDNR). 2011. Aquatic Invasive Species Literature Review. Available http://dnr.wi.gov/invasives/classification/pdfs/Myriophyllum%20aquaticum.pdf. Accessed 25 October 2011.

Xie, D., D. Yu, L. Yu, and C. Liu. 2010. Asexual propagations of introduced exotic macrophytes
Elodea nuttallii, Myriophyllum aquaticum, and M. propinquum are improved by nutrient-rich sediments in China. Hydrobiologia 655: 37-47.