Alligator weed is a perennial plant from South America that has become an invasive species in many countries, including the United States and Australia. It is a problematic weed because of its ability to thrive in both aquatic and terrestrial environments. Its aggressive growth allows it to outcompete native vegetation, posing a threat to local ecosystems and agricultural operations.
Identifying Alligator Weed
Recognizing alligator weed, or Alternanthera philoxeroides, involves observing its distinct stems, leaves, and flowers. The plant features hollow, succulent stems that can be pale green, pink, or reddish and often form dense, tangled mats. These stems can grow up to 10 meters long in water. Along the stems, shiny, spear-shaped leaves grow in opposite pairs and are typically 2 to 7 centimeters long with a smooth, waxy appearance.
The plant’s flowers are another identifier. They are small, white, and papery, resembling clover blossoms, and measure about 8 to 10 millimeters in diameter. These flowers sit atop stalks that emerge from the leaf axils. Its extensive root system can penetrate deep into the soil, while in water, the roots are fine and short.
The Impact of Alligator Weed
The presence of alligator weed causes disruption to both natural ecosystems and human activities. The plant forms dense, sprawling mats over water surfaces, which block sunlight from reaching submerged aquatic plants. This lack of light kills off native vegetation, which reduces the available food and shelter for fish and aquatic invertebrates. These thick mats can also lead to deoxygenated water conditions, further harming aquatic life.
Beyond its ecological damage, alligator weed has economic consequences. Its thick growth clogs drainage canals, irrigation ditches, and water pumps, which can impede water flow, increase the risk of flooding, and interfere with agricultural practices. For instance, it has been known to eliminate small crops and damage turf farms. The dense infestations also hinder recreational activities such as boating, swimming, and fishing by creating impenetrable barriers on the water.
Control and Management Strategies
Mechanical methods like cutting or raking can provide temporary relief, but their effectiveness is limited. The plant easily reproduces from small stem or root fragments left behind, meaning these methods can inadvertently spread the infestation. Careful removal of all plant matter is necessary to prevent regrowth.
Chemical control using herbicides can be an effective option. Products containing glyphosate, imazapyr, or triclopyr can manage the weed, though repeat applications are often necessary. When using herbicides near water, caution is paramount to avoid harming non-target species, and professional application is often recommended. For best results, herbicides should be applied when the plants are actively growing and temperatures are above 60°F.
Biological control offers a long-term solution by introducing the plant’s natural enemies. The alligator weed flea beetle (Agasicles hygrophila) has been used successfully to suppress aquatic infestations. The larvae and adult beetles feed on the leaves, creating holes and damaging the plant’s structure. Another agent, the alligator weed thrips (Amynothrips andersoni), also contributes to controlling the weed.