Duckweed can choke out a pond, and the fish that live in it

Thursday, July 31, 2014

Fish need a steady supply of oxygen in the water to survive. Without it, pond owners may find their pond surfaces littered with dead fish.

Many ponds and lakes throughout the region can be seen with duckweed growing on the surface. While this plant does not typically cause too many problems, if left uncontrolled, it can cover the whole pond and choke its oxygen supply.

Marley Beem, Oklahoma State University Cooperative Extension aquaculture specialist, said you should never let any plant entirely cover the surface of a pond.

“When you do that, you’re preventing light from getting into the water column, preventing photosynthesis, preventing oxygen production and that leads to fish kills,” he said. “In our typical pond, the wind action is usually enough to keep it blown over to the side.”

However, duckweed can become a serious problem for ponds located down in a hollow or completely surrounded by trees, protected from the wind.

“It spreads and reproduces rapidly and covers the entire surface of the pond,” Beem said.

Being able to identify the plant and taking some early steps to prevent its spread are essential to the overall health of the pond.

“Get after it early,” Beem said. “Once it becomes thick and covers the entire pond to a depth of several inches, you are facing some major labor in trying to thin that out.”

Aquatic-safe herbicide treatments are usually a safe bet in management of pond plants. However, herbicides used on land can be toxic to aquatic life, so caution should be taken when putting it in the water.

For help with identification, or herbicide recommendations, contact your local county Extension educator.