Giant salvinia is a problem along the coastal plain of North Carolina (and other southern states). It spreads via vegetative reproduction, i.e., small stem fragments will disperse (often on boats) and form large dense mats. This plant is a problem in many parts of the world and degrades freshwater aquatic habitats by blocking light and depleting oxygen.
Young giant salvinia plants have leaves (0.5 - 1.5") which lay flat on the water surface, but as the plants grow the leaves become packed in tighter and overlap. As the mats get denser, stems of leaves will emerge upright. The emergent leaves are green, oblong, and are covered in arching hairs. The submerged leaves are brown and stringy and look like roots.
Fact Sheet: Giant Salvinia
Photo credit: Scott Robinson, Georgia Department of Natural Resources, Bugwood.org