Alligator Weed, or Alternanthera philoxeroides, is a sprawling semi-aquatic plant with opposite leaves and hollow stems that enable it to float or grow along saturated soils. Native to South America, it has spread to wetlands, drainage canals, rivers, and agricultural edges across numerous continents. Alternanthera philoxeroides grows aggressively through stem fragmentation and creeping runners, forming thick mats that disrupt water flow and reduce biodiversity.

It can survive on land or in standing water, making it unusually versatile. A defining trait of Alternanthera philoxeroides is its ability to thrive in nutrient-rich, disturbed environments, where it often outcompetes native species. Its dense growth can impede navigation, alter ecosystem function, and affect fisheries. Despite its simple foliage, Alternanthera philoxeroides is one of the most impactful wetland invaders.