ALLIGATORWEED (alternanthera philoxeroides) was first reported in the U.S. in 1897. It originated in South America and was probably introduced in the U.S. through ballast water. It is now also considered an invasive weed in Australia, China, New Zealand and Thailand.
Alligatorweed has long elliptical leaves that branch opposite on hollow stems that stand 4 feet high. It has a whitish papery ball-shaped flower that closely resembles clover. The flowers are located on spikelets that rise from the leaf base. Alligatorweed spreads by seed and fragmentation.
When Alligatorweed invades waterways it can reduce flow, prevent light penetration and deplete oxygen due to weed decay. This oxygen depletion can reduce both waterfowl population and fish activity. It also makes a perfect habitat for breeding mosquitoes that can potentially carry disease like West Nile or Encephalitis.
Mechanical control has not been shown to be effective because Alligatorweed can easily root from fragmentation.
2 great options for Alligatorweed control are biological & chemical control.
Biological control can be obtained with Alligatorweed Flea Beetles, Alligatorweed Thrips and Alligatorweed Stem Borer.
Chemical control can be achieved with 2,4-D or glyphosate. 2,4-D is best applied early spring at the highest labeled rate as new weeds begin to appear. Glyphosate is best applied mid-season when growth is up and in full bloom. Repeat treatment may be needed to kill the entire root system.
There is a chance of oxygen depletion after chemical treatment caused by weed decomposition. This oxygen depletion can adversely affect fish. If your pond is heavily infested, treat in sections. Let each section of weeds decompose before treating another area.