Alternanthera philoxeroides | Alligator weed
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].
Invasive species - risk of Medium Impact
Introduction pathways - 1
Release in Nature
Introduction pathways subclass - 1
Release in nature for use
Introduction pathways - 2
Introduction pathways subclass - 2
Contaminant nursery material, Seed contaminant
NAPRA Ireland risk assessed
Dark-green, perennial, herbaceous, aquatic or terrestrial plant. Leaves are glossy, spear-shaped, opposite and entire, approximately 2-7cm long (GISD, 2015) and have a distinctive central vein (Australian Government, 2003). Stems are hollow aiding in flotation and can reach up to 10m long (EPPO, 2016; GISD, 2015). Roots are adventitious over water but on land the plant produces a deep rhizomatous root system (up to 2m deep) allowing it to survive long dry periods (Government of South Australia, 2010). The small white ball-like flowers are held above the water surface on stalks.
Establishes a thick mat in shallow waters, either fresh or brackish (salinity 10-30% that of sea water) (GISD, 2015) or in semi-aquatic or terrestrial habitats. Mats can be up to 1m thick and are substantial enough to support a man's weight (GISD, 2015). They shade out other herbaceous plants and reduce oxygen levels by preventing oxygen transfer between air and water (Government of South Australia, 2010; GISD, 2015). Decaying plants further reduce water quality for native flora and fauna by depleting oxygen (Government of South Australia, 2010). Large colonies interfere with water flow causing increased sedimentation and provide breeding grounds for snails and mosquitoes (GISD, 2016). Exposed stems are killed by frost/ice but protected parts of the plant can survive winters as cold as -12°C (Q-bank, 2017). This species is also able to tolerate low light conditions (Weber, 2003 in EPPO, 2015). Presence has a significant impact on agriculture and the European and Mediterranean Plant Protection Organisation (2016) reports reductions in production of sweet potato of 63%, lettuce 47%, rice 45%, wheat 36%, and maize 19%. Navigation, angling, canoeing and swimming are all negatively impacted (Caffrey, 1993) and natural aesthetics become degraded (CABI, 2015). Will be eaten by grazing horses and cattle causing photosensitivity, skin lesions, liver damage and death (van Oosterhout, 2007).
Fresh or brackish waters. Terrestrial zones.
Reproduction is vegetative outside its native range. Stem nodes, roots and underground stems can all produce a new plant (Q-bank, 2017). Viable seed production unlikely in Europe (Q-bank, 2017).
Pathway and vector description
No consensus on how Alternanthera philoxeroides reached Europe, possibly as a contaminant of a similar ornamental species (EPPO, 2015). Has been found in ship ballast water and as a contaminant of bird seed in Europe (EPPO, 2015). The Government of South Australia (2010) noted deliberate planting and disposal of plants when it has been mistaken for Alternanthera sessilis, which is used as a leaf vegetable in Sri Lankan cuisine. Once established fragments can be transported by boats and other recreational/commercial equipment. Since the stems are hollow fragments travel readily by water movement especially during flooding (Government of South Australia, 2010). Waterfowl and other animals such as cattle and horses also contribute to its spread (Australian Government, 2003).
Mechanism of impact
Competition, Disease transmission, Poisoning/Toxicity, Bio-fouling
The best weapon against this invasive species is preventing entry. There is an EU wide ban on the sale, growing and keeping of this plant (European Commission, 2017). Protocols for the early detection and elimination of the species before it can become established are critical (van Oosterhout, 2007).
Control is difficult. In terrestrial settings there is up to ten times more biomass underground (EPPO, 2015; van Oosterhout, 2007). The plant needs to be removed in its entirety to avoid regrowth (van Oosterhout, 2007). There was a report in Australia of regrowth 10 years after eradication was thought to be complete (van Oosterhout, 2007).
Hand digging down to a depth of 1m to remove the plant and the surrounding material is the most effective method of eradication for small infestations (Australian Government, 2003). The soil must then be hand sieved to ensure every part of the plant is removed (van Oosterhout, 2007). Van Oosterhout (2007) recommends using floating booms to catch any fragments that may float downstream. Larger stands can be shallow scraped using diggers first and then completed using hand digging followed by hand sieving (van Oosterhout, 2007).
Control but not elimination can be achieved by mechanical cutting but may spread the plant to new areas (van Oosterhout, 2007).
Glyphosate application may be a viable option in some situations but repeat applications will be required due to the plant's rhizomatous reserves (Government of South Australia, 2010). Gunasekera (2006) notes that Glyphosate causes the plant to heavily fragment and may contribute to its spread. Metsulfuron-methyl is the most effective chemical for Alternanthera philoxeroides suppression both on land and in water (Gunasekera, 2006; van Oosterhout, 2007) but it is not licensed for aquatic use in Ireland and an off label permit would have to be sought.
Van Oosterhout (2007) suggests the use of steam weeders effective to a depth of 24 inches for suppression prior to eradication.
Shallow fresh or brackish water including rivers, streams, ponds, lakes, canals, marshes, ditches and reservoirs. Also terrestrial settings such as riverbanks and agricultural lands. Can survive in semi-aquatic systems such as floodplains.
Alternanthera philoxeroides can be confused with A. sessilis. However, A. sessilis is an annual and its flowers are not held on a stalk (CABI, 2007).
Introduced to North and Central America, the Carribbean, Tropical Asia, Oceania and Europe. In Europe it is established in both France and Italy (Q-bank, 2017; EPPO, 2015; DAISIE, 2017).
Parana River Basin – Argentina, Brazil, Paraguay (EPPO, 2016).
Records submitted to Data Centre in 2022
The following map is interactive. If you would prefer to view it full screen then click here.
How can you help
Report any sightings to the National Biodiversity Data Centre. Do not purchase or plant in your garden. Dispose of any garden plants or weeds responsibly.
Australian Government (2003) Alligator weed (Alternanthera philoxeroides) weed management guide. https://www.environment.gov.au/biodiversity/invasive/weeds/publications/guidelines/wons/pubs/a-philoxeroides.pdf Site accessed 22 August 2017.
CABI (2015) Invasive Species Compendium. Datasheet report for Alternanthera philoxeroides (alligator weed). Wallingford, UK: CAB International. http://www.cabi.org/isc/datasheetreport?dsid=4403 Site accessed 22 August 2017.
DAISIE. (2017). Delivering Invasive Alien Species for Europe. European Invasive Alien Species Gateway. http://www.europe-aliens.org/speciesFactsheet.do?speciesId=6584 Site accessed 22 August 2017.
European and Mediterranean Plant Protection Organization (EPPO). (2016). Datasheets on pests recommended for regulation. Alternanthera philoxeroides (Mart.) Griseb. Bulletin, 46 (1). pp 8-13. https://gd.eppo.int/search?k=alternanthera+philoxeroides Site accessed 22 August 2017.
European and Mediterranean Plant Protection Organization (EPPO). (2015). Pest risk analysis for Alternanthera philoxeroides. EPPO, Paris. http://www.eppo.int/QUARANTINE/Pest_Risk_Analysis/PRA_intro.htm Site accessed 22 August 2017.
European Commission. (2017). Invasive Alien Species of Union Concern. European Union, Luxembourg.
Global Invasive Species Database (GISD). (2015). Species profile Alternanthera philoxeroides. http://www.iucngisd.org/gisd/speciesname/Alternanthera+philoxeroides Site accessed 22 August 2017.
Government of South Australia. (2010). Alligator weed (Alternanthera philoxeroides). http://www.pir.sa.gov.au/__data/assets/pdf_file/0007/137293/alligator_weed_policy.pdf Site accessed 22 August 2017.
Gunasekera, L; Ainsworth, N; Bonilla, J. (2006). Alligator weed eradication in Victoria – a developing success story. Fifteenth Australian Weeds Conference. Papers and Proceedings, Adelaide, South Australia, pp. 784-786. http://www.cabi.org/isc/FullTextPDF/2008/20083096027.pdf Site accessed 22 August 2017.
Q-bank. (2017). Alternanthera philoxeroides (Mart.) Griseb. Alligator weed. Factsheet. http://www.q-bank.eu/Plants/Factsheets/Alternanthera_philoxeroides_EN.pdf Site accessed 22 August 2017.
Van Oosterhout, Elissa. (2007). Alligator weed control manual: Eradication and suppression of alligator weed (Alternanthera philoxeroides) in Australia. NSW Department of Primary Industries. http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0015/210444/alligator-weed-control-manual.pdf Site accessed 22 August 2017.
Weber, E. (2003) Invasive plant species of the World. A reference guide to Environmental weeds. CABI Publishing, 548p.
Its ability to colonise both fresh and brackish water as well as terrestrial locations make this a highly successful invasive species but its spread is currently limited by colder northern latitudes (EPPO, 2015). The European and Mediterranean Plant Protection Organisation (2015) warns that the countries most at risk are within the Mediterranean area. However, climate change may put Ireland at risk of future invasion (EPPO, 2015).