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Taxonomic name: Alternanthera philoxeroides (Mart.) Griseb. Synonyms: Achyranthes philoxeroides (Mart.) Standl., Alternanthera paludosa Bunbury, Alternanthera philoxerina Suess., Alternanthera philoxeroides (Mart.) Griseb. forma angustifolia Suess., Alternanthera philoxeroides (Mart.) Griseb. var. acutifolia (Mart. ex Moq.) Hicken, Alternanthera philoxeroides (Mart.) Griseb. var. lancifolia Chodat, Alternanthera philoxeroides (Mart.) Griseb. var. luxurians Suess., Alternanthera philoxeroides (Mart.) Griseb. var. obtusifolia (Mart. ex Moq.) Hicken, Bucholzia philoxeroides Mart., Telanthera philoxeroides (Mart.) Moq., Telanthera philoxeroides (Mart.) Moq. var. acutifolia Mart. ex Moq., Telanthera philoxeroides (Mart.) Moq. var. obtusifolia Mart. ex Moq. Common names: alligator weed, alligatorweed, pig weed, xi han lian zi cao (Chinese) Organism type: aquatic plant, herb Alternanthera philoxeroides, commonly known as alligator weed, is a perennial stoloniferous herb that can be found in many parts of the world, infesting rivers, lakes, ponds and irrigation canals, as well as many terrestrial habitats. The aquatic form of the plant has the potential to become a serious threat to waterways, agriculture and the environment. The terrestrial form of Alternanthera philoxeroides grows into a dense mat with a massive underground rhizomatous root system. The canopy can smother most other herbaceous plant species. It has proven to be extremely expensive to attempt controlling Alternanthera philoxeroides. Description The Commonwealth of Australia (2000) states that, "Alternanthera philoxeroides is a summer growing perennial herb. It has small white papery flower heads 8-10cm in diameter, that appear generally from November to March. A. philoxeroides does not produce viable seed. The leaves are shiny, spear-shaped, opposite, sessile, entire and about 2-7cm long and about 1-2cm wide. One of the main identifying features of alligator weed is that the stems are hollow when mature. This weed produces masses of creeping and layering stems, up to 10m long. Over water, roots are adventitious. On land adventitious roots and thickened taproots occur. Over water stems grow to 60cm high and have large, hollow internodes. On land stems are shorter and internodes smaller and much less hollow. Frost and ice kill exposed stems and leaves; however, protected stems survive to support the next season's growth. Mats may extend 15 metres over the water surface and become so robust they can support the weight of a man. Reproduction is asexual with maximum growth in mid-summer. The plant responds to high levels of nutrients and withstands 10 percent sea-strength salinity or up to 30 percent salinity in flowing brackish water." Similar Species Alternanthera caracasana, Alternanthera denticulata, Alternanthera sessilis, Ludwigia adscenciens, Persicaria decipiens, Polygonum amphibium, Tradescantia fluminensis More Occurs in: estuarine habitats, lakes, riparian zones, water courses, wetlands Habitat description Alternanthera philoxeroides is a perennial stoloniferous herb found in both aquatic to terrestrial habitats. Rui-Yan and Ren (2004) have found A. philoxeroides infesting rivers, lakes, ponds, and irrigation canals, as well as many terrestrial habitats.Sainty et al. (1998) state that, "A. philoxeroides grows on a wide range of substrata, from sand to heavy clay. When floating on water, it may be rooted in the bank or substrate, or free floating." General impacts Gunasekera (1999) states that, "Alternanthera philoxeroides is considered to be one of the worst aquatic weeds in the world. The aquatic form of the plant has the potential to become a serious threat to waterways, agriculture and the environment. The terrestrial form grows into a dense mat with a massive underground rhizomatous root system. The canopy can smother most other herbaceous plant species." Buckingham (1996) states that, "Floating stems grow across the surface of the waterway forming a dense interwoven mat. This mat clogs the waterway and out competes native plants along the shore." The Commonwealth of Australia (2000) states that, "Alligator weed disrupts the aquatic environments by blanketing the surface of the water impeding penetration of light, gaseous exchange (sometimes leading to anaerobic conditions) with adverse affects on flora and fauna. Mats impede flow and lodge against structures thereby promoting sedimentation and contributing to flooding. They prevent access to and use of water, promote health problems by providing habitats for mosquitoes and degrade natural aesthetics." Control of this species has proven to be an expensive and complicated ordeal wherever it has established. Geographical range Native range: South America (Coventry et al. 2002). Known introduced range: Asia, Australasia-Pacific, Europe, North America (Coventry et al. 2002; Julien et al. 1995; USDA-NRCS, 2004) Introduction pathways to new locations Ship ballast water: It was probably introduced into Australia at Carrington (Newcastle docks area) in NSW when ship's ballast was dumped. It was first recorded there in 1946 (Commonwealth of Australia, 2000). Transportation of habitat material: The movement of contaminated plant mulch was identified as one reason for the recent spread (Coventry et al. 2002). Local dispersal methods Horticulture (local): Gunasekera and Bonila (2001) states that, "In all Australian states and territories, the weed is cultivated as a green leafy vegetable by the local Sri Lankan community, in the mistaken belief that it is another plant sessile joy weed (Alternanthera sessilis), very popular in Sri Lanka." Management information Preventative measures:: The Australian Department of the Environment and Heritage (2003) state that, "Alternanthera philoxeroides has rarely, if ever, been successfully eradicated once it has infested a water body, despite numerous costly attempts. For this reason, the highest priority for the management of alligator weed is an effective system of early detection and eradication before infestations become established." A Risk assessment of Alternanthera philoxeroides for Australia was prepared by Pacific Island Ecosystems at Risk (PIER) using the Australian risk assessment system (Pheloung, 1995). The result is a score of 12 and a recommendation of: reject the plant for import (Australia) or species likely to be of high risk (Pacific). Mechanical: Coventry et al. (2002) report on the use of a weed harvester, which was used to remove 120 tons of alligator weed. The authors state that, "A further 18kgs of fragments broken of by the machine were removed by hand using small boats. This is an effective way of removing biomass from accessible areas but other methods are required to deal with re-growth and inaccessible areas." Caution must be taken to ensure that all fragments are collected. Mechanical harvesting can exacerbate the invasive potential of the species of all fragments are not gathered. Chemical: Gunasekera and Bonila (2001) tested four chemicals, Dichlobenil, Glyphosate, Metsulfuron methyl and Metsulfuron methyl with Glyphosate on A. hiloxeroides. The authors found that each chemical provides its own measure of control against A. philoxeroides. Coventry et al. (2002) offer an in-depth review of 12 combinations of different herbicidal treatments that have been studied. Please refer to Coventry et al. 2002 in the section labeled Herbicides Biological: Gagne et al. (2004) have found a new species of cecidomyiid, Clinodiplosis alternantherae that is injurious to A. philoxeroides. C. alternantherae forms galls on branch tips and is a likely candidate to aid in the control of the invasive, alien A. philoxeroides. Barreto and Torres (1999) identified two fungal species, Nimbya alternantherae and Cercospora alternantherae which have pathogenic effects on A. philoxeroides. The authors conlcude that, "N. alternantherae was shown to be very damaging and appears to have biocontrol potential." Buckingham (1996) states that from 1964 to 1971 three South American species were released in North America for biological control of A. philoxeroides. The author states that, "The flea beetle Agasicles hygophila strips the leaves from the stems and the moth Vogtia malloi bores inside the stems. Heavy damage by either species kills the stems thereby causing the mat to break up, clearing the waterway. The thrip Amynothrips andersoni feeds on the young apical leaves. Heavily damaged plants are often stunted." Coventry et al. (2002) recommend two insects for introduction to Australia for biological control of A. philoxeroides; the thrips Amynothrips andersoni, and the flea beetle Disonycha argentinensis. Tan et al. (2002) have identified a disease caused by Fusarium sp that occurs occaisonally on natural populations of A. philoxeroides. Mass production of the fungal inoculum should be possible, but a system for mass production needs to be tested and established. The fungus controlled A. philoxeroides effectively under laboratory and greenhouse conditions, as well as in the field. Therefore, it is considered a good biocontrol agent candidate for managing A. philoxeroides in some terrestrial and aquatic crops." Reproduction Sainty et al. (1998) states that, "Alternanthera philoxeroides does not produce viable seed under field conditions (Center and Balciunas, 1975; Sainty, 1973; Julien, 1995). Consequently reproduction is entirely vegetative and relies on the production of nodes. Each node has two axillary buds. Stem nodes, portions of thicker roots, and underground stems are all capable of growth. Dispersal is by fragmentation." Reviewed by: Dr. Lalith Gunasekera Research Scientist, CRC Australian Weed Management Department of Primary Industries Frankston Centre Victoria Australia Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG) with support from the Terrestrial and Freshwater Biodiversity Information System (TFBIS) Programme (Copyright statement) To contribute information, please contact Shyama Pagad. Last Modified: Tuesday, 11 April 2006