November 27, 2021

Ponds 101: Tips for managing aquatic weeds

RICHMOND, Ind. — The recipe for aquatic weeds is a mixture of warm water, sunlight and nutrients. In order to disrupt weeds, one of those ingredients must be disrupted.

But first, it’s a good idea to remember that some weeds are good, said Jonathan Ferris, Purdue Extension educator in Wayne County, during a webinar.

“We need aquatic weeds in a pond,” he said. “They provide valuable escape cover for young fish.”

They also contribute to the natural aesthetics of the pond, produce oxygen and stabilize the shoreline.

What conditions lead to weed problems? The No. 1 culprit is usually shallow water.

“Most ponds in Indiana are too small or too shallow,” Ferris said. “Ideally we’d like to see a pond be 1 acre in size and average 8 feet in depth.”

Weed ID

When identifying aquatic plants, there are three basic categories: algae, submersed plants and free floating.

Algae, also called moss or scum, is very common on farm ponds. Microscopic algae form the base of the food chain in the pond.

Some common submersed plants include pondweeds, naiads and coontail. Common free-floating plants include duckweed and watermeal.

“They can get really out of hand, quickly,” Ferris said of floating plants. “They are not easy to manage. They’re easy to identify because they’re not attached to anything. They float on the surface of the pond.

“Watermeal is the world’s smallest flowering plant. Both of these require high nutrient levels to survive. In a well-managed pond that has nutrients under control, you’re probably not going to have a lot of duckweed or watermeal.”

Plants of special concern include Eurasian Watermilfoil, Hydrilla and Egeria. If you suspect you have these plants, you can report them at:

Management Techniques

“The first thing I do not recommend is to throw a chemical at it,” Ferris said. “There are a lot of other things we can try before we try chemicals, like preventative control, mechanical control, biological control and habitat alteration.”

If you can keep the weeds out to begin with, you’re ahead of the game.

“We try to discourage things like geese and waterfowl,” he said, noting birds can carry in weeds and weed seeds.

He advised pond owners to wash off boats and trailers before leaving a site. Other preventative control measures include cleaning out live wells and avoiding non-native plants.

Mechanical control involves pulling or raking the plants. Biological controls include using grass carp to consume plants.

“Just know their limitations,” Ferris said of the fish. “If you want to try grass carp, the recommendation is 15 to 30 per surface acre.”

Habitat alteration can include adding dyes to cool the water, or dredging the pond to start with a clean slate.

Keep livestock out of the ponds, Ferris said.

“With chemicals, I want to stress that you need to positively ID the weeds and follow the label,” he said. “Observe water use restrictions. Apply at the right time — not mid-summer. And have realistic expectations of control.”

Erica Quinlan

Erica Quinlan

Field Editor