Hydrilla (Hydrilla verticillata):

Hydrilla is a submersed plant with long stems branching at the surface to form dense mats. These dense mats shade and displace native species, affect aquatic animal habitat and impede recreation and navigation. Originally from India and Korea, hydrilla is now common throughout the southern US and is especially problematic in Florida. It is also found in Virginia, Maryland, Delaware, Connecticut, Massachusetts, New Jersey, Maine, California and Washington. Hydrilla is controlled using herbicides and biological agents (insects and grass carp). Mechanical means are not recommended because hydrilla spreads readily through fragmentation.