Alternanthera philoxeroides (Mart.) Griseb., Amaranthaceae

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Present on Pacific Islands?  no

Threat only at high elevations?  no

Risk assessment results:  Reject, score: 22 (Go to the risk assessment)

Common name(s): [more details]

English: alligator grass, alligator weed

Spanish: lagarto

Habit:  herb

Description:  "Perennial herb; stems creeping or floating, ascending towards apex, rooting at the lower nodes, branched, hollow, with a longitudinal hairy groove on 2 opposite sides.  Leaves subsessile or with petiole to 5 mm long, with a ring of white hairs between the 2 opposite leaf bases.  Lamina 3-13 x 1-3.5 cm, elliptic to oblanceolate or obovate, glabrous or slightly hairy near the attenuate base; apex obtuse or acute.  Inflorescences in upper axils, mostly 1-2 cm in diameter, capitate, white; pedicles to 9 cm long with 2 opposite longitudinal hairy grooves, occasionally heads shortly pedunculate and terminal.  Bracts 2.5-3.5 mm long, ovate-acuminate; bracteoles similar to bracts, somewhat smaller, persistent.  Tepals 5-7 mm long, oblong to ovate, acute or obtuse.  Fertile stamens 5; staminodes = stamens.  Style short, thick; stigma capitate."  (Webb et al, 1988; p. 101).

Habitat/ecology:  "The plant grows best in aquatic sites but may establish as a terrestrial species in wet and poor pastures and on irrigated lands.  The weed prefers level areas of shore or shallow water where it is protected from wave erosion.  As with many weeds it thrives in eutrophic conditions and thus often increases where urban and industrial development have polluted and degraded water quality.  Severe storms may strip the leaves from anchored mats or tear the tangled vegetation loose and move it to a new location.  Terrestrial pants may remain under water for several days without ill effects.  The plant prefers fresh water but is found along the inland tidal reaches of rivers that run to the sea.  In summary, alligatorweed probably grows in a wider range of water and soil conditions than any other major aquatic weed.  Its growth is equally startling whether free floating, loosely attached and forming a mat, or as an emersed plant in a wet or relatively dry field.  It loves fresh water of high fertility but can tolerate saline soils and waters"  (Holm et al., 1997; pp. 39-40).  "...can grow as either a trailing, terrestrial herb or as a floating aquatic (usually attached to the bank)" (Csurhes & Edwards, 1998; p. 137).  A serious problem in waterways in tropical and warm-temperate regions of the world.

In New Zealand, "drains, streams, swamps and similar wet places. Alligator weed forms dense floating mats in stagnant or slow moving water, and also grows in dune hollows behind beaches" (Webb et al., 1988; p. 101).

Propagation:  Primarily by stem fragments carried by water (Csurhes & Edwards, 1998; p. 137).

Native range:  South America

Presence:

Pacific Rim
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia Australia (continental)	New South Wales	introduced invasive cultivated	Csurhes, S./Edwards, R. (1998) (p. 137)
Australia Australia (continental)	Queensland	introduced cultivated	Csurhes, S./Edwards, R. (1998) (p. 137)
China China	China (People's Republic of)	introduced invasive	Li-ying, Li/Ren, Wang/Waterhouse, D. F. (1997) (pp. 172, 178)
Colombia Colombia	Colombia (Republic of)		Holm, Leroy/Pancho, Juan V./Herberger, James P./Plucknett, Donald L. (1979) (p. 16)
Honduras Honduras	Honduras (Republic of)		Holm, Leroy/Pancho, Juan V./Herberger, James P./Plucknett, Donald L. (1979) (p. 16)
Indonesia Indonesia	Indonesia (Republic of)	introduced invasive	Csurhes, S./Edwards, R. (1998) (p. 137)
Mexico Mexico	Mexico (United Mexican States)		Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (pp. 37, 44)
New Zealand New Zealand	New Zealand (country)	introduced invasive	Webb, C. J./Sykes, W. R./Garnock-Jones, P. J. (1988) (p. 101)
Perú Perú	Perú (Republic of)	native	U.S. Dept. Agr., Agr. Res. Serv. (2007)
Taiwan Taiwan	Taiwan Island	introduced invasive	Li-ying, Li/Ren, Wang/Waterhouse, D. F. (1997) (pp. 172, 178)
Thailand Thailand	Thailand (Kingdom of)	introduced invasive	Csurhes, S./Edwards, R. (1998) (p. 137)
United States (west coast) United States (west coast states)	USA (California)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2007)
Vietnam Vietnam	Vietnam (Socialist Republic of)		Waterhouse, D. F. (1993) (p. 63)

Also reported from
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
United States (continental except west coast) United States (other states)	USA (Florida)	introduced invasive	U.S. Dept. Agr., Agr. Res. Serv. (2007)

Comments:  Listed as a noxious weed in Australia and New Zealand.

Control: 

Physical: Attempts at physical control usually only serve to spread the weed.

Chemical: Resistant to most herbicides. "Dicamba, triclopyr, and bentazone are used to control this plant" (Weber, 2003; p. 40).

Biological: A biological control program has been initiated in New South Wales, Australia. A beetle (Agasicles hygrophila) is somewhat successful in controlling the aquatic form, but does not control the terrestrial form (Csurhes & Edwards, 1998; p. 137). Use of this agent has been successful in New Zealand and Thailand (Julien, 1992; p. 1-3).

Additional information:  Photos and additional information at University of Florida, Center for Aquatic and Invasive Plants.
Fact sheet from the Department of Natural Resources, Queensland, Australia. (PDF format).
Information on the Environment Bay of Plenty, New Zealand, web site.
Fact sheet from the Bay of Plenty Regional Council, New Zealand.

Additional online information about Alternanthera philoxeroides is available from the Hawaiian Ecosystems at Risk project (HEAR).

Taxonomic information about Alternanthera philoxeroides may be available from the Germplasm Resources Information Network (GRIN).

References:

Csurhes, S./Edwards, R. 1998. Potential environmental weeds in Australia: Candidate species for preventative control. Canberra, Australia. Biodiversity Group, Environment Australia. 208 pp.

Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James. 1997. World weeds: natural histories and distribution. John Wiley & Sons. 1129 pp.

Holm, Leroy/Pancho, Juan V./Herberger, James P./Plucknett, Donald L. 1979. A geographical atlas of world weeds. John Wiley & Sons, New York. 391 pp.

Julien, M. H. (ed.). 1992. Biological control of weeds: A world catalogue of agents and their target weeds (third edition). CAB International, Wallingford, UK. 186 pp.

Li-ying, Li/Ren, Wang/Waterhouse, D. F. 1997. The distribution and importance of arthropod pests and weeds of agriculture and forestry plantations in southern China. ACIAR, Canberra, Australia. 185 pp.

Owen, S. J. 1997. Ecological weeds on conservation land in New Zealand: A database. Working draft. Wellington, New Zealand. Department of Conservation.

Parsons, W. T./Cuthbertson, E. G. 1992. Noxious weeds of Australia. Inkata Press, Melbourne/Sydney. 692 pp.

U.S. Dept. Agr., Agr. Res. Serv. 2007. National Genetic Resources Program. Germplasm Resources Information Network (GRIN). Online searchable database.

Waterhouse, D. F. 1993. The major invertebrate pests and weeds of agriculture in Southeast Asia. The Australian Centre for International Agricultural Research, Canberra. 141 pp.

Webb, C. J./Sykes, W. R./Garnock-Jones, P. J. 1988. Flora of New Zealand, Volume IV: Naturalised pteridophytes, gymnosperms, dicotyledons. Botany Division, DSIR, Christchurch. 1365 pp.

Weber, Ewald. 2003. Invasive plants of the World. CABI Publishing, CAB International, Wallingford, UK. 548 pp.