The main weed control problem in ponds is algae, that greenish, slimy stuff in many forms.
“The key to controlling algae is to start in early April while the water is cooler and the algae is growing slowly,” says Mike Plumer, University of Illinois Extension natural resources educator. “The safest herbicides are copper-containing products like copper sulfate or copper chelates. Just make sure you read and follow all label directions.”
Applied in April, these products will provide good control with little danger to the fish, and there are few restrictions on using the pond. The water is still good for drinking, fishing and swimming.
Another product that can be used is a dye. Dye products are known by various labels but are normally a dark blue color that, when added to the pond, change the water to a bright blue. A dye product does two things: reduces the light penetrating the water and reduces the plants' ability to grow. The results are the same, less algae and submerged plant growth.
Plumer says that weed control needs to start in April before the water temperature gets above 60 degrees. The warmer the temperature, the faster the plant kill. If this weed control is done later with warmer temperatures and more weeds, the decomposing weeds in the pond may remove too much oxygen from the water, which may cause a fish kill if aeration isn't provided. That's why it is important to start early in the season while there are few weeds and the water is cool.
It is not necessary to control all the weeds in a pond. Having some vegetation in a pond is beneficial because it provides cover for the young fish, adds oxygen to the water and is a food source for fish and other organisms that fish feed on. But when there is too much vegetation, it can interfere with other uses of the pond. The ideal situation is to control the problem yet leave some vegetation for the fish.