I came accross this recently, sounds like a safe natural alternative.
Over the past eight years, EnviroScience, Inc. has been working with Middlebury College (VT) to make MiddFoil® available to the general public. MiddFoil uses an aquatic beetle native to Canada and the northern US to combat the spread of Eurasian watermilfoil, an invasive aquatic weed that has spread like wildfire across North America.
Based on eight years of intensive field application and more than 12 years of university research, MiddFoil has proven itself to be the only long-term, environmentally-friendly alternative to herbicides and mechanical harvesting for large scale infestations.
http://www.enviroscienceinc.com/cgi-...section&id=253