Alligatorweed forms dense mats along the shoreline of lakes, ponds, streams, ditches, and wetlands, with the mat extending out into open water. The dense growth suppresses native plant species, reduces the quality of habitat for waterfowl, wildlife, and fish, and will reduce oxygen levels in the water under the mat. The dense growths interfere with navigation and recreational use, and will exacerbate flooding.
Alligatorweed is not listed as a federal noxious weed, although the related sessile joyweed (Alternanthera sessilis
) was listed out of concern that it too would become a widespread weed. It is listed as a noxious weed in AR and SC, and a Class C weed in AL. Alligatorweed is widely recognized as an invasive weed by state Exotic Pest Plant Councils (EPPC).
Leaves are Elliptical, About 4? in Length, and Opposite, While Leafbases form a Sheath Around the Node
Alligatorweed is an emergent herbaceous perennial plant, forming dense stands up to 3 ft tall. The stems vary in color, are approximately ¼” thick, and often hollow, particular in the floating mat stage. The stem nodes are ½” thick, and hollow. Stems will root from the nodes, and in standing water the stems will float on the surface, forming a dense mat, with upright branches. Leaves are opposite, without a petiole, and the orientation of the leaves will shift 90˚ from one node to the next. Leaves are entire, elliptical in shape, and approximately 4” in long. The leaf base forms a sheath around the stem at the node.
Alligatorweed Inflorescence is a Roundish Cluster of White Flowers Formed by White or Clear Sepals
An inflorescence may be formed, one per node, with at least one node separating flowering nodes. The flower stalk is ½ - 3” long, and the flower spike is a collection of numerous flowers forming a roundish cluster. The flower itself has no petals, but has five white or colorless sepals, which are ¼” long. One seed is produced per flower. Flowering will occur throughout the growing season.
Stems that break off can root at the nodes and form a new colony. Seeds are fertile, and may be transported by animals or water, or germinate from the seedbank if the plants are removed.
Plants are spread naturally by animals or water. Stems may also be transported on boats, boat trailers, and other equipment.
Alligatorweed can grow on damp soil into shallow standing water. Once a floating mat is formed, water depth is no longer a limiting factor. While alligatorweed can invade into upland sites, it prefers saturated soil to shallow water habitats.
Alligatorweed is found throughout the southeastern United States and California.
Alligatorweed is found throughout the MidSouth states of AL, AR, LA, MS and TN, though its distribution is poorly documented.
Biological control of alligatorweed has been a phenomenal success, with certain caveats. The alligatorweed flea beetle (Agasicles hygrophila
Selman and Vogt), particularly combined with the alligatorweed moth (Vogtia malloi
Pastrana), has completely controlled the aquatic nuisance-forming floating mat in southern regions where the insects survive the winter, approximately 100 miles from the Gulf Coast in LA, MS, and AL. Formerly a significant nuisance in FL, no herbicides are used to control alligatorweed floating mats in FL due to the insect. However, these insects do not control alligatorweed growing on terrestrial or moist soil sites, and control of the aquatic mats does not occur in northern portions of its range. Some success in northern portions of alligatorweed range has been achieved with augmentative releases of the insects.
Chemical Control Options for Alligatorweed
||1.28 fl oz./gal
||1.9 lbs ae/acre
||3 lbs ae/acre
||Systemic with slow results
||0.25 - 1 lbs ae/acre
(1 - 4 pints/acre)
||Systemic with slow results
||1 % solution
(1.25 fl oz/ gallon)
|2 - 6 lbs ae/acre
(3 to 8 quarts/acre)
In northern portions of the alligatorweed range, herbicides are the best management option. Good control has been achieved with both 2,4-D and triclopyr in both spot and broadcast applications, with selectivity that allows survival of native vegetation. Both glyphosate and imazapyr have been used with excellent results in spot and broadcast applications, with slightly higher efficacy with imazapyr. Application in mid-summer provides longer-term control than applications in spring. All formulations of aquatic herbicides listed should be applied to the foliage, and a nonionic surfactant included in the spray mixture. Use only products labeled for aquatic use in sites with standing water.
Mechanical control with mowers, choppers, and harvesting equipment may provide short-term control, but the increased risk of spreading alligatorweed from stem fragments would suggest that these are not good long-term control options. Hand removal of individual plants has been an effective technique.
Drawdown will increase the abundance of alligatorweed, through stimulation of seed germination.
Allen, S.L., G.R. Hepp, and J.H. Miller. 2007. Use of herbicides to control alligatorweed and restore native plants in managed marshes. Wetlands 27(3):739-748.
Aulbach-Smith, C. A. and S. J. de Kozlowski. 1996. Aquatic and Wetland Plants of South Carolina. Second Edition. South Carolina Department of Natural Resources, Columbia, South Carolina. 123pp.
Buckingham, G.R., D. Boucias, and R.F. Theriot. 1983. Reintroduction of the alligatorweed flea beetle (Agasicles hygrophila
Selman and Vogt) into the United States from Argentina. Journal of Aquatic Plant Management 21:101-102.
Sainty, G., G. McCorkelle, and M. Julien. 1998. Control and spread of alligator weed Alternanthera philoxeroides
(Mart.) Griseb., in Australia: Lessons for other regions. Wetlands Ecology and Management 5:195-201.
Aquatic Plant Information System, U.S. Army Engineer Research and Development Center
Center for Aquatic and Invasive Plants, University of Florida, IFAS
Geosystems Research Institute, Mississippi State University
Plants Database, U.S. Department of Agriculture Natural Resources Conservation Service
Dr. John D. Madsen
Ryan M. Wersal, Geosystems Research Institute, Mississippi State University
Mississippi State University
Geosystems Research Institute
Mississippi State, MS 39762-9652