Green water in your garden pond?


With the arrival of spring, those of you with water features are getting them ready. Summer is the most challenging time of year for pond and water feature enthusiasts.

Keeping pond water clear and clean in 90-degree heat with 16 hours of sunlight takes some advance preparation. This is because water temperature is the key to controlling pond algae.

There are many tricks for keeping pond water cool, and you should use as many as you can.

One way is with water plants. Experienced water gardeners store stock up on annual pond plants early in the year; “floaters” like water lettuce and water hyacinth.

These plants multiply rapidly on the pond surface, forming a living “umbrella” that casts shade on the water. Perennial water lilies are also very helpful in keeping sun from warming pond water.

By August, vegetation should cover most of your pond’s surface.

Ponds in direct sun all day are the hardest to manage. You can use plantings along pond banks, particularly on the southwest side, to shade the water during the hottest part of the day.

Overhanging shrubs and ornamental grasses work well, and so do trees. There are lots of perennials that thrive along pond banks or in shallow water. These plants are called “aquatic marginals,” and many of them are perennials that will come back each year.

In addition to making a natural appearance, they help create a balanced environment around ponds that helps control algae.

Keeping water rapidly moving is another important aspect to controlling “green water” in your pond. Waterfalls and fountains mix in fresh oxygen to keep water from becoming stagnant. Rapidly moving water prevents “hot spots”, mixing cool subsurface water constantly.

To prevent “hot spots”, water should circulate completely from one end of the pond to the other, twice each hour. Make sure your pump is sized large 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 lined completely with rock. Black plastic pond liners have only one-third the surface area necessary for algae-eating bacteria to colonize. Black liners soak up the sun’s rays, acting as solar water heaters. Hiding the liner with rock reduces solar heating.

Rocks and pebbles equalize the night and day temperatures, cooling the water by day and warming it at night. Gaps between stones give fish a place to hide from predators, and act as natural filters by harboring beneficial bacteria.

A healthy living pond environment provides enough food for goldfish and snails, who help keep the pond clean. Fish wastes, in turn, help feed pond plants.

Overfeeding of fish is a major cause of algae bloom in water features. We stock our pond with bait goldfish, and never add fish food of any kind.

The fish thrive by eating mosquito and frog eggs, tadpoles, algae and insects, scrubbing the pond naturally.

Every water feature has “issues” with string algae, particularly during the summer months. We recommend fish-safe, plant-safe algae control products like AlgaeFix and barley straw, and use them in our own pond, but they are only one ingredient in a successful “recipe” for clean, clear water.

The key to clear water is a healthy balance of good design, water circulation, plants and fish.

These elements create a healthy environment similar to natural stream beds, where plants, insects and animals live in harmony in clean, clear water.

Steve Boehme is a landscape designer/installer specializing in landscape “makeovers”. “Let’s Grow” is published weekly; column archives are on the “Garden Advice” page at For more information is available at or call GoodSeed Farm Landscapes at (937) 587-7021.

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