Alligator weed, Alternanthera philoxeroides, is a highly invasive, perennial, aquatic, and semi-aquatic plant originating in South America. It poses a significant threat to waterways and adjacent terrestrial habitats by forming dense, floating mats that choke out native vegetation. Due to its aggressive growth and deep, extensive root system, it is listed as a federally noxious weed. Eradication is challenging because of its capacity for vegetative reproduction, which requires a comprehensive, multi-year management strategy.
Identifying the Plant and Its Spread
Accurate identification is the first step in successful management, as alligator weed can be confused with some native species. The plant is characterized by hollow, green to reddish-purple stems, which can grow up to 3 feet long while floating or sprawling across the ground. Leaves grow in opposite pairs along the stem, are typically elliptical or lance-shaped, and are slightly fleshy. The plant produces small, white, clover-like flowers on short stalks emerging from the leaf axils, usually during the warmer months.
The plant’s resilience comes from its highly efficient method of vegetative reproduction, known as fragmentation. Every single node along the stem has the potential to sprout roots and form an entirely new, independent plant. Even a small, broken-off piece of the stem or root that drifts downstream can establish a new colony. This aggressive reproductive habit is why mechanical efforts to remove the plant can often be counterproductive, inadvertently scattering fragments and worsening the infestation.
Effective Chemical Control Methods
Chemical treatment using systemic herbicides is generally considered the most effective method for controlling large or established alligator weed infestations. Systemic herbicides, such as aquatic-approved formulations of glyphosate, triclopyr, imazapyr, or bispyribac, are absorbed by the foliage and translocated throughout the plant, including the extensive root and rhizome systems. Triclopyr and imazapyr have often shown excellent control ratings. However, glyphosate is less effective on plant parts that are fully submerged.
The application technique is important, requiring the use of an aquatic-approved surfactant to help the herbicide adhere to the waxy leaves and penetrate the plant tissue. Timing is another factor influencing efficacy, as treatments applied in late summer or early fall often yield better results than spring applications. This timing coincides with the period when the plant is actively transporting nutrients down to its root reserves, drawing the herbicide with it. Multiple applications spaced throughout the growing season and over several years are required to deplete the plant’s robust underground energy stores and achieve sustained control.
Safety and Compliance
A strict adherence to the product label is mandatory, particularly the warning to use only herbicides explicitly labeled for aquatic use. Using non-aquatic herbicides in or near water bodies is prohibited and can result in severe environmental contamination and harm to aquatic life. Furthermore, because these herbicides can cause rapid die-off of large mats, treatment should be staggered to prevent a sudden drop in dissolved oxygen levels, which can lead to fish kills.
Mechanical and Biological Removal
For very small, contained infestations, manual removal may be a viable option, but it requires meticulous effort to remove all root and stem material. Any mechanical action, such as hand-pulling or using aquatic weed cutters, must be followed by the immediate and complete removal of all plant fragments from the water and shoreline. These fragments must be properly disposed of away from any water source to ensure they cannot re-root and initiate a new infestation.
Biological control offers a targeted, long-term approach that avoids the risks of fragmentation and chemical use. The Alligator Weed Flea Beetle, Agasicles hygrophila, is a well-known agent introduced for this purpose in the United States and has provided good localized control in many regions. The adult beetles and their larvae feed specifically on the leaves and stems of alligator weed, significantly reducing the biomass of floating mats. Other biological agents, including the alligator weed thrips and stem borer, have also been utilized.
The effectiveness of the flea beetle, however, is often limited by climate and the form of the plant. The beetle thrives only in warmer climates and tends to prefer the aquatic form of the weed, having little impact on terrestrial infestations. Additionally, the plant’s tendency to fragment when under heavy insect attack can sometimes cause a localized spread of the infestation to downstream areas. Biological control is best utilized as a sustained suppression tool integrated with other management techniques, rather than a standalone eradication method.
Follow-Up Management and Regulatory Compliance
Successful eradication of alligator weed necessitates a commitment to sustained follow-up management for several years. Monitoring previously treated areas is essential to detect and treat any regrowth from surviving root fragments or newly established plant parts. Even after the above-ground growth appears eliminated, the plant’s deep root system can allow it to persist and re-emerge.
Long-term prevention requires addressing the underlying environmental conditions that favor the invasive plant’s growth. Controlling nutrient runoff, particularly phosphorus and nitrogen, into the water body can reduce the resources available for the weed to proliferate. Planting native aquatic and riparian vegetation helps to restore the ecosystem and provides competition that can suppress the re-establishment of alligator weed.
Due to its invasive nature, alligator weed is classified as a noxious weed under various state and federal regulations. This designation means there are often strict legal requirements regarding its management, transport, and disposal. Before initiating any control program, individuals must check with local and state regulatory agencies to ensure compliance with laws concerning the use of aquatic herbicides and the handling of plant material.