Torpedo grass (Panicum repens):

Torpedo grass is a perennial rooted grass that forms large dense colonies along shorelines. Torpedo grass has rigid stems that can grow to 2 1/2 feet tall and it spreads rapidly by sending out numerous creeping rhizomes into the water. If left alone it can quickly cover an entire lake or pond with a dense floating mat of grass. Torpedo grass is a native of Australia. Mechanical and chemical control have been used with some success, but eradication is difficult unless every rhizome root is removed. Dense stands of maidencane (Panicum hemitomon) may help slow the spread of torpedo grass through direct competition.