By Carolyn Weise
A bog is the term for any water retentive garden, wet, spongy ground, small marsh or swamp.
In pond and water gardens today we loosely use the term bog as a filtration device for our ponds. It is a man-made shallow pond, ¾ filled with soil or sand, topped with rocks and planted with water loving plants. The water is circulated through this bog from the filter and back into the pond. In certain pond construction it is simply used as an in-pond device to make the pond more natural looking, and as a glorified planting area (rather than using potted plants the plants are planted directly into the media and allowed to spread freely within this area).
Some water gardens have filters for the bogs consisting of French drains beneath the rocks, like undergravel filters in a fish tank, to prevent buildup of anaerobic bacteria. In these conditions, the plants are used to filter nitrites from the pond and convert them to nitrates. Others have built “out-of-pond” vegetable bogs to filter the nitrites by having water flow from the filter, through a container of plants and return to the pond. These will be built out of various waterproof containers and may not be permanent fixtures.