Pond primer: Controlling algae takes patience, timing


SALEM, Ohio – Pond scum. Seaweed. Slime.

“That green floaty stuff on my pond.”

“Call it what you want, but it’s that green, slimy stuff floating on the water or sliming up the bottom of the pond, also known as algae, and hardly anybody wants it,” said Ernie Oelker, agriculture and natural resources agent with Ohio State University Extension in Columbiana County.

Growth of wanted and unwanted plants in ponds is related to construction and greatly affects fish health, Oelker said, during an aquatic weed control presentation in April. It also affects overall aesthetic appeal and pond use.

Key factors. Pond construction, temperature and oxygen content are key factors in weed control, according to Oelker.

The easiest way to head off problems before they start is to build a pond with proper side slopes.

A recommended bottom slope ratio of 3-to-1 from the edge minimizes the amount of shallow water and creates a natural mechanical control. By those calculations, for every 3 feet of movement toward the center of the pond, the depth should increase approximately 1 foot.

When constructing a pond, one should aim to have as much water at least 4 foot deep as possible and a depth of at least 10 feet in the center.

“What a lot of people don’t realize is that algae grows in the bottom and then floats to the top. If the light can’t reach the bottom, you’re cutting down on the ability of that algae to grow,” Oelker said.

Oelker also recommends building a pond with sides that are as steep as possible while keeping safety in mind. Beach areas for swimming entry in the pond should have a flatter slope.

Warm enough. In planning and executing treatment, water temperature plays a huge role in the effectiveness of most chemicals.

“Temperature is one of the biggest keys. You’ve got to wait until things warm up enough that the vegetation is actively growing if you want to accomplish anything,” Oelker said.

Some chemical labels also discourage application when water exceeds a temperature range because of increased risk of oxygen depletion due to decomposition of dead vegetation, and a fish kill could result.

The amount of oxygen water molecules can hold is an important consideration when determining when to treat and with what method.

“The capacity to hold oxygen goes down as water warms, so that’s something to keep in mind. If you’ve got fish in there, you’ll have less cushion for them during treatment,” he said.

In general, fish require 4 parts per million of oxygen, a level that can be endangered with treatment from chemicals like copper sulfate.

Because cooler water holds more oxygen and causes things to break down more slowly, oxygen is pulled from the water at a slower pace.

“If you’re not ready to treat by May 15 or so, hold off,” Oelker said.

That cut-off date also helps to protect fish and eggs during spawning as well as fry soon after they hatch.

Natural helper. The first step in treating unwanted plant growth is checking plankton levels, Oelker said. Plankton help decrease plant growth in deeper water by absorbing sunlight before it reaches the plant.

A simple test to determine plankton levels can be done by lowering an aluminum can lid or shiny object nailed onto a stick into the water.

“When you get to about 18 inches, that thing should disappear. If it’s still visible at 3 feet, there’s not enough plankton,” he said.

“But if you’ve got healthy fish and the weeds aren’t too bad, it’s not broke, so don’t fix it,” he said.

Levels of treatment. “Every pond needs vegetation, something that’s important in the oxygen cycle of water,” Oelker said.

A pond owner’s comfort level with the amount of submerged or emerged plants also affects the pond’s use, and is the determining factor in selecting treatment.

“It’s perfectly OK to choose not to control the weeds. It all depends on what you want to do with your pond,” he said.

Treatment options. Biological and mechanical treatments are easiest to manage plant growth. The addition of chemicals to the water is also an option for many landowners.

Mechanical factors include proper pond construction and location. Pond vegetation can also be controlled by raking, cutting, or mowing – a treatment that should begin in spring as soon as leaves begin to appear.

Weeds and algae pulled from a pond “contribute nicely to compost piles,” Oelker said.

A popular biological control is the use of inert dyes, which help block light penetration. The dyes are commercially available in products like Aquashade and are not effective in water less than 2 feet deep, or if weeds are on or above the water surface.

A drawback of use of inert dyes is the necessity to get and maintain a correct concentration of the dye.

Another biological control includes stocking triploid white amur, also known as grass carp, in the pond. The species, which eat vegetation, may grow up to 60 pounds and live up to 15 years. The fish prefer submerged and emerged plants and will feed on floating plants, such as filamentous algae, only when other supplies have been depleted.

No quick fix. Oelker once received a phone call from a pond owner wanting to purchase amurs to control the plant growth in her pond.

“It was Friday, and she wanted to buy them that day. I didn’t understand why until she told me she was hosting a family reunion that same weekend,” he said.

The owner wanted to immediately get rid of the algae floating on the pond so her family could enjoy swimming and boating.

“You’ve got to realize that there’s no quick fix. Treatment takes time,” Oelker said.

For more information on weed control contact your county SWCD or extension office.

(You can contact Andrea Myers at 1-800-837-3419, ext. 22, or by e-mail at amyers@farmanddairy.com.)

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