GARDENER'S READING ROOM
Ponds and Fountains

Garden Bog Filters

One of the easiest and most attractive ways to filter your fishpond is with a bog filter. The advantages of bogs are many. They allow a more natural transition from land to water. They solve the problem of fish grazing plants. Most importantly, they can completely process both organic and inorganic waste. Bog filters can support a broad spectrum of plants and also create more habitat for creatures such as frogs, salamanders, and birds. A bog filter is truly nature’s filter.

The size of your bog filter will depend on the organic load you expect it to process. Generally a bog filter will be 10% of the surface area of your main pond; for heavy loads, go up to 20%.

Excavate an area that size next to your main pond to a depth of at least 12”. Build a concrete (or concrete block) retaining wall separating this area from the main wall. The top of the wall should be 1-2” below the pond surface when filled. Make the liner for the bog filter level; be sure to allow extra liner for the side of the retaining wall. Use felt protection mat or scrap liner to protect the liner from rough or sharp edges. Place liner in pond and bog filter according to vendor’s instructions.

(Alternatively, build the bog filter as an extra-wide 12” deep shelf, place the liner on it, and build the separating wall on TOP of the liner. Fill in any large cracks that would leak gravel with mortar or black expanding foam. Do not fill in cracks that are shallower than 2” below the pond surface.)

Install the pump in the main pond as far from the bog filter as possible. For ponds under 4,000 gallons use a pump which will circulate the pond volume at least once every two hours; for ponds over 4,000 gallons, circulate once every four hours. Screen or pre-filter the pump to prevent clogging. A full size filter is unnecessary.

Use oversized PVC pipe to build a distribution manifold the length of the bog; the pipe connecting to the pump should also be oversized. (For example, if the pump has a 1” outlet, use 2” pipe). To make the manifold, drill ¼” holes every 3” about halfway between the side and bottom of the pipe. Cap the far end of the pipe. Lay it on the bottom of the bog as far from the main pond as possible. (If the bog is wider than 3’, multiple lines are desirable.)

Place decorative (usually flat) rock across the top of the retaining wall, allowing small cracks between and under rocks for water flow. Water will be pumped from the main pond into the bog filter, filter through the gravel, and flow back into the pond over the retaining wall through the cracks.

Fill the bog filter with pea gravel, mounding the pea gravel so it is 3” ABOVE the water level for most of the bog. Mix larger river rock into the top layer of gravel for a more natural look. Incorporate rock from around the pond and landscape in the bog filter to visually tie the landscaping together, or use logs and other wood pieces if that’s appropriate.

Finally, plant up your new bog with marginals and other moisture-loving plants. Plant directly into the pea gravel. Bacteria that will break down any wastes in the water will colonize the pea gravel, the plants will use the nutrients to grow, and your new bog filter will return naturally clean water to your pond.

Instructions on creating the bog taken from an article by Jim Purcell, Jim’s Water Gardening, Eugene, OR. Used by permission.

By Charlie Shull, W.C.N.
Sky Nursery

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