Alligatorweed (Alternanthera philoxeroides)

Native to South America, Alligatorweed was introduced inadvertently to Southeast states in the late 1800s. The white clover-like flowers bloom throughout the summer. Alligatorweed typically grows in floating mats along the water's edge. Although it will also grow emersed and even terrestrially. The leaves are opposite, lance-shaped, about 1-2 inches long, and have a distinct midrib. The midrib is a vein or crease that runs from the stem (alligatorweed has no petioles) to the tip of the leaf. The stems are hollow when growing aquatically, providing buoyancy.


There are several threats that Alligatorweed infestations impose. Flooding and erosion; floating mats get hung-up around dams, structures, fallen trees and bottlenecks.  It can grow across an entire waterway and become a navigation hazard.  Infestations depreciate aesthetic value, impede recreational use, and pose a health problem by harboring mosquitoes.


Alligatorweed is found in 12 states and Puerto Rico.  It reproduces primarily by fragmentation.  Boaters must take caution when transporting their trailers and boats from waters infested with Alligatorweed, and be sure they are not inadvertently spreading this nuisance plant.  Alligatorweed tends to be common around boat ramps because it is often introduced at these sites by human activity.


In North Carolina, Alligatorweed is managed with the use of herbicides.  Even with annual herbicide applications, Alligatorweed can be persistent. Its ability to grow both aquatically and terrestrially complicates the management of infestations. There are two insects that have been used as biological control agents, a beetle and a moth. The release of Alligatorweed flea beetles in Florida has been a huge success and has greatly reduced the need for herbicide applications there. Flea beetles have also been released in North Carolina but their ability to control Alligatorweed is limited because the insect doesn't over-winter well (temperatures get too low). The best use of flea beetles occurs in the Coastal Plain when a combination of mild winters and multiple releases coincide.  The other biological control used for Alligatorweed management is the stem borer moth.   This insect is not very effective in North Carolina because it does not survive through our winter temperatures.