Alternanthera philoxeroides
Alternanthera philoxeroides
(alligatorweed)
Plants
Exotic
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Alternanthera philoxeroides (Mart.) Griseb.

Common name: alligatorweed

Synonyms and Other Names: Pigweed, alligator weed, alligator grass, red legs, Achyranthes philoxeroides (Mart.) Standl., Alternanthera paludosa Bunbury, Alternanthera philoxerina Suess., Bucholzia philoxeroides Mart., Telanthera philoxeroides (Mart.) Moq.

Taxonomy: available through www.itis.govITIS logo

Identification: Alternanthera philoxeroides is a perennial with prostrate, sprawling, floating hollow stems, often in a dense tangled mass, rooted in shallow water or growing from the shoreline, occasionally free-floating (Long and Lakela 1971; Godfrey and Wooten 1981). The hollow stems provide considerable buoyancy of the mat (Buckingham 1996). Roots form at stem nodes. Morphology and habit of A. philoxeroides are similar to many aquatic primose (e.g., Ludwigia palustris) and hygrophila (e.g., Hygrophila costata) species.

While recognized as a major pest in aquatic environments where it has been introduced, A. philoxeroides may also grow terrestrially in moist cultivated soils (Zeiger 1967). When growing as a terrestrial, stems are smaller in diameter, more lignified, with shorter internodes. Additionally, there is variability with stems and leaves of the two recognized biotypes of A. philoxeroides. The narrow-stemmed alligatorweed (NSA) biotype have relatively slender stems and longer internodes when compared to stems of the broad-stemmed alligatorweed (BSA) biotype, which have broader stems and longer internodes (Kay and Haller 1982). There is a line of hairs on each side of the stem internodes, originating from the leaf axils and extending to the base of the next distal node (Stratford Kay, pers. comm.).

The leaves of NSA are smaller and more blunt-tipped than BSA leaves, which are larger, longer, and have an acute leaf tip (Kay and Haller 1982). In general, leaves are bright green, arranged in opposite pairs (90 degree angles), entire, elliptic-linear to ovate, 5-11 cm long and 1-2 cm wide. A leaf mid-vein is prominent on both sides of the leaf (Godfrey and Wooten 1981).

Flowers perfect (with both male and female reproductive structures), on a terminal spike (5-6 cm long), white to greenish-white, 8-10 mm in diameter, with a clover-like shape (Long and Lakela 1971; Godfrey and Wooten 1981). Vogt et al. (1992) noted wild seed production in Arkansas, Lousiana, and Mississippi populations, though none germinated.

Size: Floating stems up to 15 meters long (Zeiger 1967).

Native Range: The Parana River region of South America (Brazil, Paraguay, Argentina, and Uruguay) (Vogt et al. 1979; Julien et al. 1995).

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Interactive maps: Point Distribution Maps

Nonindigenous Occurrences:
  • Alabama - All major drainages (Schardt and Schmitz 1991; Missouri Botanical Garden 2007; University of Alabama Biodiversity and Systematics 2007)
  • Arkansas - Boeuf-Tensas, Upper Ouachita (Madsen et al. 2010), Lower Arkansas-Fourche La Fave, Lower Ouachita, and Lower White (Smith 1988) drainages
  • California - Los Angeles, San Gabriel, San Luis Rey-Escondido, Santa Ana, Tulare-Buena Vista Lakes (Regents of the University of California 2015), Suisun Bay, Tulare Lake Bed, Upper Kaweah, Upper Tule, and Ventura (Calflora 2015) drainages
  • Florida - All drainages (Reimus and Robertson 1997; Anderson 2009; University of Connecticut 2011; Center for Invasive Species and Ecosystem Health 2015)
  • Georgia - Altamaha, Savannah (Newman and Thomaston 1979), Apalachicola, Ogeechee (University of Florida Herbarium 2016), St. Marys-Satilla (Carter 1999), and Suwannee (Southeast Exotic Pest Plant Council 2008) drainages
  • Kentucky - Kentucky Lake in Kentucky Lake drainage, and Knox County in Upper Cumberland drainage (Chester 1988)
  • Louisiana - All major drainages (Lynch et al. 1950; Valentine 1976; Montz 1979; Craft and Kleinpeter 1989; White 1993; Thomas and Allen 1996; Louisiana State University Herbarium 2010; Madsen 2010)
  • Maryland - C&O Canal near MacArthur Blvd in Middle Potomac-Catoctin drainage (Center for Invasive Species and Ecosystem Health 2015)
  • Mississippi - Lower Mississippi-Greenville (Madsen et al. 2010), Black Warrior-Tombigbee (Kight 1988), Middle Tennessee-Elk (Madsen 2010), Mobile Bay-Tombigbee (Mississippi River Basin ANS Regional Panel 2006), Pascagoula (University of Florida Herbarium 2016), Pearl, and Yazoo (Aurand 1982) drainages
  • North Carolina - Albemarle-Chowan, Pamlico (Radford et al. 1968), Cape Fear (University of Florida Herbarium 2016), Lower Pee Dee, Neuse, Onslow Bay, and Roanoke (North Carolina Division of Water Resources 1996) drainages
  • Oklahoma - Chouteau Wildlife Management Area in Lower Verdigris drainage (Hoagland and McCarty 1998)
  • Puerto Rico - Mayaguez in Culebrinas-Guanajibo drainage, and Lago Loiza and Rio de la Plata in Eastern Puerto Rico drainage (Zeiger 1976)
  • South Carolina - All major drainages (Radford et al. 1968; Hooker and Westbury 1991; University of Florida Herbarium 2016)
  • Tennessee - Hatchie-Obion, Lower Cumberland (Chester et al. 1997), Lower Tennessee (Wofford 1977), Middle Tennessee-Hiwassee (Aurand 1982), and Upper Tennessee (Woffard and Dennis 1976) drainages
  • Texas - Big Cypress-Sulphur, Galveston Bay-Sabine Lake, Guadalupe, Lower Brazos, Lower Trinity, Middle Brazos-Bosque, Neches, San Jacinto (Helton and Hartmann 1996), Little (New York Botanical Garden 2015), Middle Colorado-Llano, San Bernard Coastal, Upper Trinity (Texas Invasive Plant and Pest Council 2015), Sabine (University of Florida Herbarium 2016), and Southwestern Texas Coastal (Missouri Botanical Garden 2007) drainages
  • Virginia - Albemarle-Chowan, James, Lower Chesapeake (Ware 1998), and Potomac (Chester 1988) drainages


Ecology: Like dense, floating mats of Eichhornia crassipes, A. philoxeroides can form dense floating mats and, with the subsequent build-up of organic detritus in the mat, can create an environment that supports the growth of emergent aquatic and terrestrial species, including woody species such as Salix spp. and Cephalanthus occidentalis. These floating islands (also referred to as tussocks, sudds, and flotants), accelerate succession and create concern for quality aquatic habitat, navigation and infrastructure (Russell 1942; Penfound and Earle 1948).

Plants produce viable seed in its native range, but observed seeds were all non-viable in its adventive range (Vogt et al. 1992). Reproduction is thought to be entirely through vegetative means. Stem fragments produce roots at stem nodes that can float to new locations, rooting in the subsurface soils producing new colonies. Along with stem nodes, vegetative reproduction can also occur from thick root mass and underground stem fragments (Sainty et al. 1998).

Alligatorweed may grow in waters deeper than 2.5 meters (Stratford Kay, pers. comm.), but must remain rooted in hydrosoil for optimum growth (Sculthorpe 1967). Plants can uproot, drift, and establish in new locations, but they cannot compete if unrooted for long periods of time (Sculthorpe 1967). The mat can extend up to 15 meters from where it is rooted in the soil (Zeiger 1967), but mats were observed much further from banks in Mississippi and North Carolina populations where they rooted into stumps and small trees (Stratford Kay, pers. comm.).

The introduced alligatorweed flea beetle (Agasicles hygrophila Selman and Vogt), along with other introduced insects, have provided exceptional biological control for this species. However, the northern spread of A. philoxeroides is beyond the range of the introduced insect's ability to overwinter. The flea beetle produces the most damage to A. philoxeroides plants and can only survive winters where the mean January temperature is 11°C or warmer, which includes Florida and coastal areas of the southeastern U.S. (Buckingham 1996). However, the introduced alligatorweed stem borer (Vogtia malloi) has produced more damage to A. philoxeroides in the interior regions of alligatorweed’s adventive range than has the flea beetle in the southern and coastal regions (Vogt et al. 1992).

Means of Introduction: Alternanthera philoxeroides was first recorded in the US in 1897 near Mobile, Alabama. The plant was present in New Orleans in 1898 (Zeiger 1967; Coulson 1977). Plants are believed to have been contaminants in ship ballast water (Zeiger 1967).

Status: Established in all previously mentioned areas.

Impact of Introduction: After its introduction into the US in the late 1800’s, A. philoxeroides quickly spread throughout the Southeast creating problems similar to those described for Eichhornia crassipes (Penfound and Earle 1948; Zeiger 1967). Following the development of the herbicide 2,4-D in the 1940’s, aggressive herbicide spraying initiated against E. crassipes allowed for A. philoxeroides, which was more resistant to the herbicide, to replace the niche formerly occupied by E. crassipes. By 1963, as estimated 65,700 hectares of waters throughout the Southeast were infested (Buckingham 1996). As a result, in 1959, the US Army Corps of Engineers (USACE), under the “Expanded Project for Aquatic Plant Control” authorized by Public Law 85-500, 85th Congress, requested the Agricultural Research Service, USDA, to begin surveys for A. philoxeroides natural enemies in South America (Zeiger 1967). In 1964, the USDA began releasing imported insects from South America as a biocontrol for this pest. Extensive testing and quarantine procedures were completed prior to release (Zeiger 1967). By all accounts, the insects that were approved and subsequently released against A. philoxeroides have been successful in managing this pest plant, although the effectiveness of alligatorweed thrips (Amynothrips andersoni), which are flightless and rarely seen on wild populations, is questionable (Stratford Kay, pers. comm.).

Remarks: The Aquatic Plant Control Program staff with the USACE Jacksonville District, upon request, annually coordinates the shipment of flea beetles (Agasicles hygrophila) collected in the St. Johns River in Florida to areas of the country where the flea beetles do not overwinter and alligatorweed persists.

State, local and federal agencies interested in receiving flea beetles can contact the USACE to request these insects. For further information on the alligatorweed flea beetle collection and shipments, contact Invasive Species Management Branch, USACE, P.O. Box 4970, Jacksonville, Florida 32207, (904) 232-1067 or email: alligatorweed@usace.army.mil

References: (click for full references)

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Author: Thayer, D.D., and I.A. Pfingsten

Revision Date: 5/16/2016

Peer Review Date: 4/4/2016

Citation Information:
Thayer, D.D., and I.A. Pfingsten, 2018, Alternanthera philoxeroides (Mart.) Griseb.: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=227, Revision Date: 5/16/2016, Peer Review Date: 4/4/2016, Access Date: 1/16/2018

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Citation information: U.S. Geological Survey. [2018]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [1/16/2018].

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