Plants are a great help in keeping water clear and clean, and one of the best ways to optimize their abilities is in a Bog. Bogs are shallow planted gravel beds where water from the pond feeds colonies of beneficial bacteria growing right where plant roots can pull out the nitrates they produce. Removal of the nitrates by bog plantings effectively starves out both floating and string algae, at the “cost” of weeding out the bog plants once a year.
How to Build a Bog Filter for a Pond?
When you build a bog filter in your pond, you may either have a passive bog or an active bog. A passive bog just sits at the edge of the pond where water can seep into the gravel, but the most effective way to set up this all-natural filter is in an active bog, where a pump delivers a constant small flow of water through a perforated pipe right into the base of the gravel bed.
Plants in the gravel have no soil to sustain growth, so they are forced to extract their nutrients directly from the water. Sized properly, at about 10-30% of the total surface area of the pond, a well-planted bog can effectively remove all the nitrates, starving out algae and cleaning and clearing the water column. Whether passive or active, Bogs are easy to build into your new pond, or even to retrofit into existing water features, even Koi ponds!.
Active and Passive Bogs
Installation of perimeter bogs is simple when building a new pond; just widen the flat rock shelf you’re putting your coping or edge stones on as far back as you have excess liner, and make sure the liner is secured higher than water level at the outer edge of the bog.
The 8” depth of the RSVP Rock Shelf is a perfect depth for a shallow gravel bed that will be easy to clean if ever necessary. The size of the bog or bogs should be at least 10% of the total surface area of the pond, up to 30% if you plan on a lot of fish, and will be easiest to maintain if they run along the edge of the pond without being very wide, maybe 1-3 feet.
A Passive Bog simply allows the pond water to flow into the gravel bed unassisted. Water will seep through the coping rocks into the gravel. To really supercharge nitrate removal, consider installing an Active Bog.
The ‘active’ part refers to running water forced through the gravel, which brings nutrients constantly and directly into the root systems of whatever is planted in the Bog. Typically, this is accomplished by placing a perforated pipe under the gravel and feeding 500-1000 gph into the base of the gravel bed. Not only will the nitrates be removed, fine particles will also be trapped, polishing the water as well or better than mechanical filters, and there are no pads to clean. Simply compost the excess plant material at the end of the season.