Bog Filtration: Form, Meet Function

Bog Filtration Form, Meet Function

Mother Nature’s miracles bewilder and astonish—blooming tulips, glowing fireflies and booming thunder. Bog filtration seems almost magical, too. But learning the scientific process behind it makes it perfectly rational, —and recreatable.

Pond owners who have been cursed with a familiar green hue would likely agree—filtration is one of the most challenging elements of maintaining a pond. Keeping algae in balance has caused many a furrowed brow. Clean and clear water is a sign of a homeostatic ecosystem—plants, fish and water living in harmony and balance—but how to get there? Building an adjacent bog filter is a beautiful alternative to mechanical pond filtration.

Bog filtration is a natural method of ridding pond water of excess nutrients which catalyze algae growth. Pond water flows through a system involving a pump, PVC pipe, plants and gravel. The filter’s organic appearance bolsters pond aesthetics, leaving onlookers unaware of the scientific process occurring just out of view.

First, determine the size needed; typically 10%-15% of the surface area should be bog, for koi ponds there should be 25%-30%. The bog should spill into your pond, so account for an elevation differential and ensure the bog remains level and 12” deep throughout. If you are using a liner, protect it with underlayment.

Next, plumb the bog with perforated PVC pipe, ensuring the pipe covers the bed adequately and evenly and is capped on each end. Next, fill the bed with pre-washed pea gravel.  

Turn on the pump to check for water movement about 1-2 inches above the gravel bed.

Plants are the workhorses of the bog filter; as a rule of thumb, plant one for each square foot of water surface. Situate taller plants toward the back of the filter and shorter plants closer to the front of the bog. Choose plants such as dwarf papyrus, bog lily, dwarf horsetail, creeping Jenny, ruby creeper, variegated water celery, and Japanese iris. Incorporating a variety of textures, forms, colors and heights (at maturity) will ensure your bog filter’s beauty rivals its effectiveness. Avoid invasive plant species such as cattails and pennywort if you want to avoid regularly pruning and thinning.

Plant root balls directly into gravel, soil and all; pots hamper the filtration process. Plant roots filter the water by extracting nutrients; so, the more roots there are, the more filtered the water will be. Next, add pea gravel until it (barely) exceeds the water level; avoid any standing water.  

Building a bog filter will ensure your water stays clean and clear and balance is maintained in your pond’s ecosystem. Its form is as impressive as its function, punctuating your landscape and pondscape with effortless beauty.