There are various forms of algae that grow in a water garden, and when kept under control, some algae is desirable because it acts as a water purifier and a home to necessary microorganisms. A well-balanced water garden has a thin layer of algae on everything that is under water.
You can control algae several ways -- chemically; through filtration; using bacterial inoculation; or by establishing a balance in the pond between plant, scavengers, and fish. Many gardeners use a combination, such as creating a natural balance aided by filters and bacteria.
A natural balance is created by installing water garden plants, scavengers, and ornamental fish in proper quantities. Plants with floating leaves, such as lotus and water lilies, are most important because they reduce the penetration of sunlight into the pond, keeping the water at a cooler, more constant temperature.
Try to cover 50 to 75 percent of your water garden surface with plants with floating leaves. Submerged plants, such as Anacharis and Cabomba (oxygenating grasses), are functional and vital to establishing an ecological balance by absorbing nutrients from the water that otherwise would be fed on by algae. Scavengers, such as snails, mussels, and tadpoles, are essential because they consume organic wastes that settle to the bottom of the pond, they graze on the sides and surfaces of the pond, and they consume suspended algae in the water. Most people choose to add fish, such as goldfish and koi, for their ornamental and aesthetic value.
There are bacteria that help control algae by consuming the nitrogen in the water garden. Once such a bacteria is established, the water should be clear except for a thin layer of algae on submerged surfaces that will house the beneficial bacteria.
A chemical control (algicide) should be considered only a temporary measure to control algae that is already out of hand. The algicide should be applied according to label directions, with careful attention paid to impact on or the need for temporary removal of any fish in the pond.
Filtration is the most common way to keep algae out of the water garden. The filtering process removes suspended particles and helps to oxygenate the water. Biological filters use bacteria to convert waste into usable forms of nitrogen for processing by aquatic plants. Most biological filters have a mechanical and biological chamber for the most effective algae control.
Remember, algae control takes time -- several weeks may be necessary to achieve a natural balance in your water garden. Give your garden at least six to eight weeks to establish a balance before using a chemical means to control algae.
Adapted from fact sheets provided by Springdale Water Gardens, Greenville, VA 24440, by Kate Dobbs, Research Associate, Consumer Horticulture, Virginia Tech (previously on gardensimply.com)