Ponds are just as likely to be infested with weeds as areas on land are. Weeds in ponds occur in the form of various types of algae or emergent, floating or submersed plants. The Missouri University Extension site lists prevention as the best way to keep weeds from invading ponds. However in case weeds do occur, you can control them with a number of recommended chemical products or aquatic herbicides. Aquatic herbicides are those chemicals that are reviewed by the Environmental Protection Agency as those that are compatible with the aquatic environment.
Fluridone is recommended by the Washington State Department of Ecology as an aquatic herbicide for the control of weeds in ponds. Fluridone is a systemic herbicide and is sold under different brand names. The slow-acting chemical effectively controls weeds such as Eurasian watermilfoil and is most potent in slow-moving waters where the chemical faces minimal dilution. Weeds treated with fluridone take from six to 12 weeks to decompose. Herbicides containing fluridone are available in the pellet or liquid form.
Glyphosate is an active ingredient in a number of pond weed-control products. Glyphosate is a broad spectrum, systemic herbicide that controls a wide variety of floating weeds including water lilies and purple loosestrife. Glyphosate herbicides are applied to the weed foliage and are not effective for submerged weeds like Eurasian milfoil. Though the chemical is a non-selective herbicide, you can use it as a selective herbicide by focusing the spray only on the weeds that need to be removed. It takes several weeks for weeds to die. Repeat applications to cover the plants that might have been missed earlier.
Diquat, sold under a different trade name, is a non-selective, fast-acting herbicide. Diquat is used as a liquid and destroys the vegetative parts of the targeted weeds without affecting the roots. Though diquat is an effective chemical, it is commonly used as a short-term, spot treatment control for a number of submerged weeds that lasts only through one season. Diquat is not highly effective on very dense algae blooms. The chemical is specifically recommended for the control of Brazilian elodea. The efficacy of diquat is reduced in waters with high levels of sediment.