Parrot feather (Myriophyllum aquaticum):

Often mistaken for its cousin the Eurasian watermilfoil, parrot feather has long stems and feather-like leaves. Its emergent stems and leaves are bright green and can grow up to a foot above the water surface. Parrot feather can clog waterways and provide habitat for mosquito larvae. A native of South America's Amazon River, parrot feather is now found in 26 southern and coastal US states, including Hawaii. Mechanical control is not used because parrot feather spreads by fragmentation. Biological agents are still being researched. Therefore, herbicides are currently the only form of control.