Synonym(s): Achyranthes philoxeroides
Family: Amaranthaceae (Amaranth Family)
Duration and Habit: Perennial Herb
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Alligatorweed is an emergent or rooted floating perennial invasive that often forms very dense stands along the shore. The stems growing in the water are hollow and can be single or branched. Its leaves are opposite and non-succulent, elliptical or lance-shaped, up to 3/4 inch wide and 5 inches long with a prominent midrib. Soft, whitish hairs are found in the leaf axis. The white flowers occur in short, headlike spikes. The flowers resemble those of white clover. A single seed develops within the fruit.
Native Lookalikes: Water willow (Justicia americana) is the native species most likely to be confused with alligatorweed. Its flower is more orchid-like (bilaterally symmetric) and purple-and-white with purple bee-guides, while alligatorweed's flower is more radially symmetric and white with papery sepals that look like petals. Alligatorweed's inflorescence is reminiscent of a clover's. Water willow grows taller than alligatorweed, about 1.5 to 3 ft tall compared to about 1.5 ft. While the leaves look similar, alligatorweed's can be up to 2 inches shorter than the minimum 3-inch length of water willow leaves.
Water smartweed (Polygonum amphibium) is another aquatic perennial that might be confused with alligatorweed. It is distinguished by having alternate leaves and pink flowers.
Water willow (Justicia americana)
Note orchid-like shape of individual flowers of water willow (right), and the color difference compared to alligatorweed (left).
Ecological Threat: Alligatorweed forms thick mats that crowd out native aquatic vegetation, retard water flow, lower dissolved oxygen levels, and increase sedimentation. Flooding may result from impeded drainage. Can restrict water flow for irrigation. Inhibits fishing, and other water recreation.
Biology & Spread: 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.
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.
History: Alligatorweed is native to South America and was first introduced into the United States around 1900.
U.S. 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.
U.S. Nativity: Introduced to U.S.
Native Origin: So. Amer. (Germplasm Resources Information Network); NatureServe Explorer
U.S. Present: AL, AR, CA, FL, GA, KY, LA, MS, NC, PR, SC, TN, TX, VA
List All Observations of Alternanthera philoxeroides reported by Citizen Scientists
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 irrigation ditches will reduce soil erosion and prevent alligatorweed 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 sever 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.
Encycloweedia, California Department of Food and Agriculture
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