Species Description: Alligator weed, Alternanthera philoxeroides, is a non-woody perennial aquatic/shoreline plant found in Florida but non-native to the U.S.
Leaves and stems vary greatly in size and shape. Fleshy, succulent stems can grow horizontally and float on the surface of the water, forming rafts, or form matted clumps which grow onto banks. The horizontal stems (called stolons) may reach a length of 10 m. The leaves are opposite in pairs or whorls, with a distinctive midrib, and range in size from 5-10 cm (Environment Waikato, undated; Virginia Cooperative Extension, undated).
Fibrous roots arising at the stem nodes may hang free in water or penetrate into the sediment/soil. Flowers, which appear from December to April, are thin and clover-like in shape. The white flowers grow on stalks and are approximately 1.25-7.6 cm in length and 13 mm in diameter (Virginia Cooperative Extension, undated).
Alligatorweed occurs in a range of habitats ranging from dry terrestrial to aquatic. To facilitate buoyancy, plants growing in aquatic habitats tend to have stems that are hollow and larger than those of plants growing on land (Julien et al. 1992).
Potentially Misidentified Species: A number of non-native Alternanthera species occur in the United States, including sessile joyweed Alternanthera sessilis which is similar in appearance to A. philoxeroides and co-occurs with it in the IRL region of Florida. Mis-identification of these two species has had negative ecological ramifications elsewhere. A. philoxeroides was accidentally spread in Australia by members of the Sri Lankan immigrant community because it was mistaken for its congener A. sessilis which is the Indian herb mukunawanna (Hosking, et al 1996).
Several species of aquatic primrose (genus Ludwigia) and spiderwort (genus Tradescantia) that occur in Florida also have appearances and/or growth habits somewhat similar to those of A. philoxeroides.
Regional Occurrence: The native range of Alternanthera philoxeroides is reportedly the Parana River region of South America (Maddox 1968, Vogt et al. 1979). It can now be found elsewhere in South America, and also on the continents of North America, Australia, and Asia, and on a number of adjacent islands.
In the United States, A. philoxeroides occurs throughout the southeast from Virginia south to Florida and west to Texas. It has been reported as far north as Illinois, and now also occurs in California (Virginia Cooperative Extension, undated; USDA).
IRL Distribution: Distribution records indicate Alternanthera philoxeroides occurs in the northern IRL counties of Volusia and Brevard, and also at the far southern end of the IRL region in Palm Beach County (Wunderlin and Hansen 2004).
Age, Size, Lifespan: Alternanthera philoxeroides Alligator weed is a summer perennial that typically grows to around 1-2 m and whose horizontal stolons can reach 10 m.
Abundance: Alternanthera philoxeroides was substantially more abundant in the southeastern U.S. four decades ago than it is now. In 1963, over 65,000 ha of land in 8 southern states were overrun with invasive alligatorweed. Within two decades, however, the infested land in those states was brought down to only about 1% of that total, primarily through successful biological control (Coulson 1977). During this same period, however, the amount of infested land in Texas and Louisiana increased (Cofrancesco 1988). Biocontrol of A. philoxeroides is further discussed below.
Reproduction: Reproduction in Alternanthera philoxeroides is predominantly through vegetative means; individuals rarely produce seeds, and when they do the seeds are typically non-viable. Vegetative growth occurs at the apical stem buds and axillary stem and root buds and the plant is spread through fragmentation (Julien et al. 1992, Virginia Cooperative Extension, undated).
Temperature: Alligatorweed thrives in subtropical to cool climates; optimum growth temperature of alligatorweed has been experimentally determined to occur between approximately 15 and 20°C. The plant is susceptible to winter die-back of exposed portions at higher latitudes (Julien et al. 1992, 1995).
Salinity: Ensbey (2004) reports that Alternanthera philoxeroides can withstand salinities of approximately 10% full strength seawater in static aquatic environments and up to 30% full strength seawater in flowing brackish water.
Hydrology: The ability of this plant to grow in aquatic as well as terrestrial habitats allows it to persist in regions with pronounced wet and dry seasons, including Florida.
Trophic Mode: Autotrophic (photosynthetic).
Associated Species: Closely associated with Alternanthera philoxeroides is the alligatorweed flea beetle, Agasicles hygrophila, an insect not native to the U.S. but intentionally introduced here in the mid-1960s as potential biological control agent of A. philoxeroides (Buckingham 2002). A. hygrophila has the distinction of being the first biocontrol insect released in the U.S. in order to combat an invasive plant. Overall, management impacts on alligatorweed have been excellent as indicated by the dramatic decrease in the amount of infested aquatic habitat since the the insect was first released. However, it is not considered to be effective against plants occurring in terrestrial habitats (UF/IFAS CAIP undated).
Additional alligatorweed-associated insects purposely introduced to the U.S. as potential biocontrol agents are the alligatorweed thrips, Amynothrips andersonii, the alligatorweed stem borer moth, Arcola (=Vogtia) malloi and another species of moth, Arcola malloi. Several additional insects are being studied to determine their suitability as A. philoxeroides control agents, primarily in Australia (Buckingham 2002).
Invasion History: Alternanthera philoxeroides is native to South America. It was first reported in the U.S. in 1897 near Mobile, AL. The introduction of this aquatic /terrestrial plant is interesting in that it is believed to have been the result of accidental release from ship ballast (Carley and Brown 2006). Ballast release has also been implicated as the vector for introduction to Australia.
Alligatorweed now occurs as an invasive exotic in subtropical to temperate regions of the Americas, Asia, Australia, New Zealand and a number of Pacific island nations. In the U.S. A. philoxeroides occurs throughout the southeast as far north as Illinois, and west to Texas, and coastal California.
The ability of A. philoxeroides to persist in terrestrial, semi-aquatic, and aquatic environments, the ability to rapidly take root along waterway banks, and the ability to propagate via vegetative fragmentation and waterborne dispersal of vegetative propagules all contribute to its success as an invasive species.
Potential to Compete With Natives: Dense mats of aquatic alligator weed displace native aquatic and shoreline vegetation and also alter aquatic systems by decreasing water flow, increasing sedimentation, shading submersed plants, reducing oxygen levels, and filling in and choking off formerly open water column habitat (Quimby and Kay 1976, Holm et al. 1997, Carley and Brown 2006).
Possible Economic Consequences of Invasion: Alternanthera philoxeroides has adverse economic impacts as an invasive species in several ways. It is capable of choking waterways and thereby impacting boating and sportfishing activities. Aquatic alligatorweed mats provide breeding habitat for mosquitoes. The terrestrial form of the plant can also invade agricultural and pasture lands, and drainage and irrigation may be impacted as well (Coulson 1977, Julien and Bourne 1988, Julien and Broadbent 1980).
Mechanical removal of A. philoxeroides mats is costly, and often results in the dispersal of large numbers of vegetative fragments that can exacerbate the infestation (GBEP/HARC 2006). Although biocontrol by means of the aligatorweed flea beetle (Agasicles hygrophila) and other control agents has greatly attenuated the threat of this plant, the cost associated with carefully studying, planning and managing the release of biocontrol agents is substantial.
Alligatorweed is a federal noxious weed and a prohibited or noxious plant in Arizona, California, Florida, and South Carolina (USDA/NRCS).
Buckingham G.R. 2002. Alligatorweed. In: Van Driesche R., Lyon S., Blossey B., Hoddle M., and R. Reardon (eds.). Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04. 413 p. Available online.
Carley M., and S. Brown. 2006. Invasive plants; Established and potential exotics, Gulf of Mexico Region. Gulf Coast Research laboratory, University of Southern Mississippi. 8 p. Available online.
Cofrancesco A.F., Jr. 1988. Alligatorweed survey of ten southern states. Miscellaneous Paper A-88-3. U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi, USA.
Coulson J.R. 1977. Biological control of alligatorweed, 1959-1972. A review and evaluation. USDA Technical Bulletin 1547. 98 p.
Ensbey R. 2004. Alligator Weed. Agfact P7.6.46, second edition. NSW Agriculture.
Environment Waikato. Undated. Alligator Weed fact sheet. Available online.
Galveston Bay Estuary Program and the Houston Advanced Research Center (GBEP/HARC). 2006. The Quiet Invasion: A Guide to Invasive Plants of the Galveston Bay Area. Available online.
Holm L., Doll J., Holm E., Pancho J., and J. Herberger. 1997. World Weeds: Natural Histories and Distribution. John Wiley and Sons, New York. 1160 p.
Hosking J.R. Sainty G.R., and S.W.L. Jacobs. 1996. Certainty and uncertainty in plant identification. Pp. 464-467 in: Shepherd R.C.H. (ed.). Proceedings of the Eleventh Australian Weeds Conference, 30 September-3 October, 1996, Melbourne, Australia.
Julien M.H., and A.S. Bourne. 1988. Alligator weed is spreading in Australia. Plant Protection Quarterly 3:91-96.
Julien M.H., and J.E. Broadbent. 1980. The biology of Australian weeds 3. Alternanthera philoxeroides (Mart.) Griseb. The Journal for the Australian Institute of Agricultural Science 46:150-155.
Julien M.H., Bourne, A.S. and V.H.K. Low. 1992. Growth of the weed Alternanthera philoxeroides (Martius) Grisebach, (alligator weed) in aquatic and terrestrial habitats in Australia. Aquatic Botany. 7: 102-108.
Julien M.H., Skarratt B., and G.F. Maywald. 1995. Potential geographical distribution of alligator weed and its biological control by Agasicles hygrophila Journal of Aquatic Plant Management 33:55-60.
Maddox D.M. 1968 Bionomics of an alligatorweed flea beetle, Agasicles sp., in Argentina. nnals of the Entomological Society of America 61:1299-1305.
Quimby P.C., Jr., and S.H. Kay. 1976. Alligatorweed and water quality in two oxbow lakes of the Yazoo River basin. Journal of the Mississippi Academy of Science 21 (supplement):13.
UF/IFAS Center for Aquatic and Invasive Plants. Undated. Biological control insects for aquatic and wetland weeds. University of Florida Aquatic and Wetland Plant Information Retrieval System. Available online.
Virginia Cooperative Extension. Undated. Virginia Tech Weed Identification Guide. Available online.
Vogt G.B., McGurie J.U., Jr., and A.D. Cushman. 1979. Probable evolution and morphological variation in South American Disonychine flea beetles (Coleoptera: Chrysomelidae) and their Amaranthaceous hosts. USDA Technical Bulletin 1593, 148 p.
Wunderlin R.P. and B.F. Hansen. 2004. Atlas of Florida Vascular Plants. Institute for Systematic Botany, University of South Florida, Tampa. Available online.