Bog Gardens, Marshes, Vegetative and Bog Filters
There is much confusion on the internet concerning bogs for your yard and most authors do little to try to explain the differences, resulting in many people building the wrong thing for their situation.
A bog garden is NOT a bog filter. A bog garden is just that, a garden for bog plants and will not act as a filter. It is typically built about 18 to 24” deep, filled with a peat / soil mix, may have drainage in the bottom and may or may not have running water across the top. Great for plants, beautiful but of limited utility for a pond.
A marsh filter is a bad compromise. In a marsh filter you have a liner or tray arrangement filled with gravel or planting medium in which water flows in one end at the surface and then across the top, winding through the plants before discharging at the other end through a pipe or spillway. This does do some filtering, but you get anaerobic bacterial growth in the gravel / planting medium which in turn generates hydrogen sulfide and methane, causing a smell anytime the gravel / medium is disturbed.
A vegetative filter (for lack of a better name) is what we will call a filter in which water runs through a bed of floating plants with no substrate, this might typically be a shallow tank or trough with water coming in at one end, flowing out at the other, through either spillways or pipes with the tank filled with something like water hyacinths. With sufficient flow and plants this makes a very effective filter during the growing season. Unfortunately, here in Hampton Roads, as all of the floating plants will either die back or just die at the start of cold weather, this filter is not a year round filter.
A true bog filter is the best pond filtration you can have, the least maintenance and the prettiest filter. A bog filter with a surface area equal to about 20% of the surface area of the pond and with a flow through rate of at least one and one-half times the pond volume is usually sufficient to filter the entire pond adequately. You should also be sure to have a flow rate of 100 gph per square foot of bog surface. The key element is your water flow. Water should enter at the bottom through a grid plate or a grid of pipes. You want aerobic bacterial growth in your bog filter rather than anaerobic, so you want to avoid dead spots in the gravel where there is no water flow to support aerobic growth. Towards this end you want your pipe grid to be on 12” to 18” centers with either slots on ¼” to ½” spacing about one third of the way through the pipe, or holes. We prefer the slots facing down, though not all bog installers agree on this point. You can also drill holes instead of slots. Holes should be about three eighths of an inch diameter and no more than an inch apart and on both sides of the pipe. Our recommendation is to then have 10” to 14” of gravel on top of the pipes. If your gravel bed is too shallow, plant roots get down in and plug up your pipes. Too much gravel makes your pump work too hard and adds un-necessary expense. Gravel should average nickel to quarter diameter size, usually referred to as river gravel or #57 gravel (NOT #57 stone). Pea gravel is a little too fine and sometimes plugs up and bigger gravel is harder to plant your plants in. The bog can be done as either a wet bog, that is, a bog with the top two inches or so being free flowing water, or a dry bog, that is a bog where the gravel comes up above the water level by an inch or so. Most people do a combination, with either an area in front of the spillway wet and the rest dry, or a wet meandering stream like area through the bog ending at the spillway and the rest being dry. We also like to put a clean out arm and cap at the end of each of the grid laterals. These clean-outs should be used once a year at least to clean the bog, simply by uncapping and sticking the hose from a pond or wet/dry shop vacuum in and sucking all of the built-up muck and snail shells out.
Plants in the bog should be planted bare root in the gravel for maximum benefit. We like to plant a mix of perennial marginals and evergreen marginals so that you still get some advantage from the plants in the winter. We will add an occasional annual for fun and appearance. We caution against putting some plants that may get too large and restrict flow and crowd everything else out, such as giant papyrus.
As a bog filter is in effect an aquaponics set-up (hydroponics with fish) you can grow almost anything that will grow with wet roots. We have seen success with tomatoes, peppers, and okra. We regularly grow watercress. The flood and drain regimen used for many aquaponic / hydroponic set-ups should not be tried with a bog filter, nor is it necessary for any but a very few leafy vegetable plants. Rhubarb can also be grown in a bog. Rhubarb makes excellent pies. I really like rhubarb pies. Hint, hint.