Green water in your garden pond?

The key to clarifying water in backyard ponds is a healthy balance between good design, water circulation, plants and fish. (GoodSeed Farm photo)

From Steve Böhme

With the arrival of spring, those of you prepare them with water features. Summer is the most difficult time of the year for pond and water play enthusiasts. In order to keep the pond water clear and clean with 90 degrees heat and 16 hours of sunlight, some preparations have to be made. This is because water temperature is key to controlling pond algae. There are many tricks you can use to keep pond water cool and you should use as many as possible.
One way is with aquatic plants. Experienced water gardeners stock up with annual pond plants at the beginning of the year. “Floaters” like water lettuce and water hyacinth. These plants multiply quickly on the pond surface and form a living “umbrella” that casts shadows on the water. Perennial water lilies are also very helpful in preventing the sun from warming the pond water. By August, the vegetation should cover most of the surface of your pond.
Ponds that are exposed to direct sun all day are the most difficult to manage. You can use plantings along the pond’s banks, especially on the southwest side, to shade the water during the hottest part of the day. Overhanging shrubs and ornamental grasses work well, as do trees. There are many perennials that thrive on the banks of a pond or in shallow water. These plants are known as the “aquatic fringes,” and many of them are perennials that come back every year. They not only ensure a natural appearance, but also create a balanced environment around ponds in which algae can be combated.
The rapid movement of the water is another important aspect of controlling the “green water” in your pond. Waterfalls and fountains mix with fresh oxygen to keep the water from stagnating. Fast flowing water prevents “hot spots” and constantly mixes cool water under the surface. To avoid “hot spots”, the water should circulate completely from one end of the pond to the other twice an hour. Make sure your pump is big enough. A 1000-gallon pond should have at least a 2000-gallon-per-hour pump (or larger if the pump “lifts” water up a hill or to a waterfall).
Well-designed water features are completely lined with rocks. Black plastic pond liners only have a third of the surface area required for algae-eating bacteria to colonize. Black liners absorb the sun’s rays and act as solar water heaters. Hiding the liner with stone reduces the sun’s heat. Stones and pebbles equalize the night and day temperatures, cool the water during the day and warm it up at night. Gaps between stones give fish a place to hide from predators and act as natural filters by harboring beneficial bacteria.
A healthy pond environment provides enough food for goldfish and snails to help keep the pond clean. Fish waste, in turn, helps feed pond plants. Overfeeding of fish is a major cause of algal bloom in water features. We fill our pond with bait goldfish and never add fish food. The fish thrive by eating mosquito and frog eggs, tadpoles, algae, and insects, and by naturally scrubbing the pond.
Every water feature has “problems” with thread algae, especially in the summer months. We recommend fish-safe, plant-safe algae control products such as AlgaeFix and barley straw and use them in our own pond. However, they are only one ingredient in a successful “recipe” for clean, clear water. The key to clear water is a healthy balance between good design, water circulation, plants and fish. These elements create a healthy environment similar to natural stream beds, where plants, insects and animals live harmoniously in clean, clear water.
Steve Böhme is a landscape architect / installer who specializes in landscape remodeling. “Let’s Grow” appears weekly; Column archives can be found on the Garden Advice page at For more information, visit or call GoodSeed Farm Landscapes at (937) 587-7021.

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