Plants are natural water filters Aquatics help keep ponds healthy

Trident Technical College horticulture students launch floating wetlands in a campus retaining pond.

We had goldfish.

They lived in a 60-gallon pond next to our back deck where they swam among lily pads and nibbled on algae. It was a good life for a fish. When the rains came last month, their home expanded across the backyard when the nearby creek flooded. Suddenly, their world was limitless. They’re probably happier now. Or not.

Ponds can be a nightmare to manage in the backyard. If they’re not properly designed, the water can turn into pea-green soup with thick mats of slime. If a pump isn’t installed correctly, stagnant water can give rise to hordes of mosquitoes.

While there are many elements that go into pond maintenance, plants are one of the most important. They are natural filters. Naturalized swimming pools are more commonly found in Europe. These designs utilize the filtering power of plants by pumping water through naturalized bogs. Excess nutrients are one of the major pollutants of waterways, in particular phosphorus. Nutrient pollution might be due to poor fertilization practices or runoff after a heavy rain.

Turf is an excellent filter, intercepting nutrients and capturing particulates before they percolate into groundwater or run into a waterway. Impermeable surfaces such as roads and parking lots, however, contribute an excess of materials.

In backyard ponds, pollutants can come from too much fish food and the subsequent fish waste. Aquatic plants, those plants that thrive at or below the water’s surface, remove nutrients through their roots.

Floating wetlands are sometimes used to beautify retaining ponds as well as take advantage of the filtering power of plants ( These buoyant mats support potted aquatic plants. The dangling roots absorb free nutrients in addition to supporting healthy populations of microbes.

Shorelines also are important transitions between water and land. Too often turf is mowed and trimmed right to the water’s edge. Waves undercut the roots and, over time, erodes the shore. Wetland plants are excellent means of protecting the exposed perimeter in addition to supporting wildlife and reducing runoff from entering water. A great example of shoreline management is at the pond behind the Goose Creek municipal center.

In backyard ponds, potted plants can be placed in the water and occasionally removed for maintenance. Iris, thalia, cyperus and lilies are just a few examples of maintenance-free aquatic plants that will not only thrive but protect goldfish and koi as well as maintain water pH and nutrient balance.

In natural filtering systems, tubs are filled with nutrient-hungry aquatic plants separate from the pond. Water is pumped through the tub before spilling back into the pond, sort of a small version of naturalized swimming pools.

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One creative means of managing fish is to take advantage of the excess nutrients for profit. Aquaponics are closed systems that pump water from fish hatcheries into holding tanks of aquatic plants and then back to the hatchery. Fish waste provides nutrients for plants. In return, the plants filter the water coming from the fish tank. Plants and fish can be harvested, sold and sometimes eaten.

While aquaponics can be constructed in numerous ways, it typically orients the fish tank below the container of plants. Water is pumped up to the plants and will return via gravity.

At the Trident Technical College horticulture program, we recently built a demonstration aquaponics system that resembles bunk beds. Instead of mattresses, the 12-inch deep shelves are covered with rubber pond liner. The fish are in the lower bunk and a small pump sends the water to the top bunk where potted plants are kept. The water returns through a 2-inch pipe set in the middle of the top bunk.

The water level can be altered in the top bunk by raising or lowering the pipe. This allows us to adjust for growing aquatic plants that prefer complete submersion or bog plants that may do better sitting in a few inches of water. Other than topping off the lower bunk once a week, the plants do not have to be watered.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, e-mail him at tony.

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