Alligatorweed [Alternanthera philoxeroides (C. Martius) Griseb][ALRPH][CDFA Rating: A] Photographs Map of Distribution

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GENERAL DESCRIPTION:Noxious herbaceous aquatic to terrestrial perennial, with horizontal to ascending stems 1 m long that root at the nodes. Aquatic form has hollow, floating, emergent and submerged stems. Terrestrial plants have solid stems. Typically, plants grow rooted in soil in shallow water and form dense, interwoven floating mats that extend over the surface of deeper water. Mats can become dense enough to support the weight of a person. Floating mats can break away and colonize new sites. Mats disrupt the natural ecology of a site by reducing light penetration and crowding out native species. Serious infestations can create anoxic, disease, and mosquito breeding conditions. Introduced from South America. The alligatorweed flea beetle (Agasicles hygrophila), stem borer moth (Vogtia malloi), and alligatorweed thrips (Amynothrips andersoni) have been released as biocontrol agents in the Southeastern U.S. These insects can effectively control infestations of alligatorweed. However, none are established in California at publication time.

SEEDLINGS:Seedlings seldom encountered because viable seed is rarely produced.

MATURE PLANT:Stems simple or branched and lacking hairs or with 2 opposing lengthwise rows of hairs. Leaves opposite, more or less equal at a node, sessile or with narrowly winged petioles (to 1 cm long) that clasp the stem. Leaf blades mostly 4-11 cm long, 1-3 cm wide, narrowly lanceolate (to obovate), with entire margins and a smooth waxy surface.

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ROOTS and UNDERGROUND STRUCTURES:Stolons root at the nodes. Floating plants have shorter, finer roots than plants rooted in soil. Stolon fragments with a node often develop into new plants.

FLOWERS:June-October. Pleasantly fragrant. Spikes head-like,12-18 mm in diameter on terminal or axillary stalks 4-9 cm long. Flowers and bracts pearly white, glabrous. Flowers lack petals. Sepals 5, separate 5-7 mm long. Stamens 5, opposite sepals, and alternate with 5 longer sterile stamens (staminodia). Ovary superior, with a single chamber containing 1 ovule.

FRUITS and SEEDS:Utricles membranous, do not open to release the single seed. Seeds smooth, disc-shaped to flattened wedge-shaped. Mature fruits seldom encountered.

POSTSENESCENCE CHARACTERISTICS:Dead stems fall over and contribute to formation of the mat. Mild frost kills leaves but not stems. Severe frosts kill emergent stems but not submerged or buried parts.

HABITAT:Shallow water or wet soils, ditches, marshes, edges of ponds and slow-moving watercourses. Tolerates saline conditions (to 10% salt by volume). Requires a warm summer growing season. Tolerates cold winters, but cannot survive prolonged freezing temperatures.

DISTRIBUTION:Uncommon. San Joaquin Valley (Tulare and Kings cos.) and Southwestern region (Los Angeles Co.); to Southeastern U.S.; Central America. To 200 m (660 ft).

PROPAGATION/PHENOLOGY:Reproduces vegetatively from stolons. Each node or fragment with a node is capable of producing a new plant. Plants are highly competitive and have rapid growth rates. Plants rarely grow in water deeper than 2 m. Seeds rarely develop, and those that do are seldom viable.

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MANAGEMENT FAVORING/ DISCOURAGING SURVIVAL:Plants grow best under high-nutrient (eutrophic) conditions. Mechanical removal without careful removal of all plant parts can facilitate spread. Stolon can regenerate from burial to 30 cm (~12 in) deep.

SIMILAR SPECIES:Mat chaff-flower [A. caracasana Kunth][ALRPE] is a prostrate terrestrial weed related to alligatorweed. Unlike alligatorweed, mat chaff-flower has sessile flower spikes, leaves mostly 1-3 cm long, and spine-tipped sepals. It grows on drier soils of waste places, roadsides, vacant lots, and fields in the Southwestern region, to 150 m (500 ft). Water smartweed [Polygonum amphibium L.] is an aquatic perennial with rhizomes that is more likely to be confused with alligatorweed. Water smartweed (Polygonum amphibium L.) is distinguished by having alternate leaves with fused, sheathing stipules (ocrea) and pink flowers.

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Prevention: A. philoxeroides generally will not establish in water deeper than 2 meters. Proper pond construction can minimize shallow edges and prevent establishment. Likewise, establishment of competitive grasses or other native species on the banks of ponds and irigation ditches will reduce soil erosion and prevent A. philoxeroides from gaining a terrestrial foothold.

Mechanical: Since this plant will regenerate from rootstocks and fragmented stems, removal of the dense floating mats will only provide temporary control. Care must be taken to prevent transport of detached stems downwater, where re-establishment can rapidly occur. Tillage of terrestrial plants may severe roots and shoots, which may increase the spread of the plant.

Biological: There have been three South American insect species released between 1964 and 1971 to control A. philoxeroides, with varying degrees of success. The alligatorweed flea beetle Agasicles hygrophilia may cause considerable damage to aquatic mats of A. philoxeroides. It feeds on the leaves and bores into stems, where it pupates before adulthood. Unfortunately, it will neither feed upon nor reproduce in terrestrial plants. Considerable success has been shown in the southeastern United Sates. However, repeated attempts at establishment in California during 1967-1969 met with little success and no further colonizations were attempted. The alligatorweed stem borer Vogtia malloi is a small moth which lays eggs on the apical leaves. The larvae bore into the stem tips and move down the stems. Infested stems rapidly wilt and droop. This damage can be easily distinguished from the flea beetle's characteristic leaf stripping of plants. The insects were initially released in Florida, Georgia, the Carolinas and Alabama from 1971 to 1973, and have since been reported in Arkansas, Louisianna, Mississippi, and Texas. A thrips species, Amynothrips andersoni, attacks and deforms apical leaves of both aquatic and terrestrial plants. Damage, however is relatively minor and scattered. Attempts to establish this species from 1967 to 1971 in Albany, California were unsuccessful. Most adults are wingless and dispersal is somewhat limited.

Chemical: The following herbicide treatments have demonstrated considerable success, although retreatment is necessary.
1) 2,4-D at 8 lb/A mixed with 8 oz of detergent applied in 50 gallons of water per surface acre.
2) Glyphosate (Rodeo) at 6 pints per acre + X-77 non ionic surfactant at 3 pints per acre applied in 50 gallons of water per surface acre.
3) Dicamba (Banvel 720) at 1 gallon + Rodeo at 1 quart + X-77 at 1 pint applied in 50 gallons of water over plants.
Read and follow all label directions before applying any herbicide to water. Misuse may cause extensive damage to other nontarget plants, both native and agricultural.

Buckingham, G. R. 1996 Biological control of Alligatorweed, Alternanthera philoxeroides, the world's first aquatic weed success story. Castanea. 61:232-243.
Clark, W. R. 1973. Alligatorweed. Annu.Proc.Calif.Weed Conf. 25: 49-50.
Coulson, Jack R. 1977. Biological control of alligatorweed, 1959-72. A Review and Evaluation. USDA Technical Bulletin No. 1547.
Goeden, R. D. and Ricker, D. W. 1971 Imported alligatorweed insect enemies precluded from establishment in California. J.Econ.Entomol. 64:329-330.
Hill, W. G. and Donley, R. G. 1973. Alligatorweed report: Los Angeles County. Annu.Proc.Calif.Weed Conf. 25: 43-48.
Julien, M. H. and Broadbent, J. E. 1980 The biology of Australian weeds. 3. Alternanthera philoxeroides (Mart.) Griseb. J.Aust.Inst.Agric.Sci. 46:150-155.
Kay, S. H. and Haller, W. T. 1982 Evidence for the existence for distinct alligatorweed biotypes. J.Aquatic Plant Manage. 20:41-
Rees, N. E., Quimby, Jr. P. C., Piper, G. L., Coombs, E. M., Turner, C. E., Spencer, N. R., and Knutson, L. V. 1996. Biological Control of Weeds in the West. Bozeman, MT: Western Society of Weed Science.
Sandberg, C. L. and Burkhalter, A. P. 1983. Alligatorweed control with glyphosate Alternanthera philoxeroides, in aquatic environments. Proc.South.Weed Sci.Soc. 36: 336-339.
Tucker, T. A., Langeland, K. A., and Corbin, F. T. 1994 Absorption and translocation of 14C-imazapyr and 14C-glyphosate in alligatorweed Alternanthera philoxeroides. Weed technol. 8:32-36.
Weldon, L. W. and Blackburn, R. D. 1969 Herbicidal treatment effect on carbohydrate levels of alligatorweed. Weed Sci. 17:66-69.

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