Filter the Pond with a Bog Garden
A bog is an area that consistently has
a water level several inches below the “soil” surface. In nature, bog soils are
usually very high in organic materials and low in pH. Traditionally, using
semi-permeable liners, ornamental bog gardens have imitated these conditions in
order to grow plants that have adapted to these areas, such as pitcher plants.
For filtration purposes, bog filter gardens
are constructed using waterproof pond liner and pea gravel. They may utilize a
wide range of marginal pond plants, bog plants, and even many terrestrial
These bog filter gardens have two
primary functions: beauty and filtration. The bog filter provides a
perfect background for the pond, showcasing plants while allowing more pond
surface area to remain open. It also provides a natural looking transition from
land to pond, and enhances the landscaping around the pond. Bog gardens are the
ultimate pond filter for water purity, clarity, and low maintenance. Unlike man
made filters, bogs can completely process organic waste, including solid
waste, and even process some inorganic waste, such as minerals. Man made
filters merely mechanically trap debris and some biologically detoxify waste.
Waste by-products still build up in the water and create various problems,
including a build-up of nitrates and phosphates that promote algae growth. In a
bog filter, gravel traps organic debris until it can be broken down by bacteria
and used by plants. By-products are processed and used as food by microbes and
with bog filters usually experience no algae blooms, even when brand new, and
those that do quickly balance themselves. Bog filters are also extremely
economical to build and maintain, and are compatible with other filtration
systems if you desire them. Once you’ve used a bog filter, you won’t ever want
a pond without one.
Typical Bog Filter Construction:
1) The size of bog filter required
varies according to the organic load it is expected to process, often a
function of how much fish food is used. For most purposes, a bog filter whose
surface area is 10% of the surface area of the main pond will suffice, with up
to 20% or more for heavy loads. Intense sunlight and high temperatures
stimulate algae growth and so require higher performing bog filters, while
lower temperatures and light levels reduce demand. Shape is more of an
aesthetic function, dependent on each pond’s landscaping. Bog filters are often
placed opposite from where people usually view the pond--they provide a
beautiful background for the pond, and can wrap around several sides if desired.
2) Excavate an area next to the main
pond to a depth of 12-14". Shallower bog filters provide less filtering area
and heat up quickly, while deeper ones tend to create too many anaerobic
low-flow zones (a certain amount of anaerobic activity is normal and even
beneficial, but too much can produce toxic quantities of by-products). Build a
retaining wall of concrete or concrete blocks to separate this area from the
main pond. The top of the wall should be 1-2" below the pond surface when
filled. Remember to make the wall for the bog filter level, and to allow extra
liner for the sides of the retaining wall. Use felt protection mat and/or scrap
liner to protect the liner from rough or sharp edges. Fill in any large cracks
deeper than 2" below pond surface which would “leak out” gravel, using
mortar or black expanding foam. An alternate filter placement is to build the bog
as an extra-wide shelf, place the liner, and build the retaining wall on top of
the liner, using rock if desired. Another style is to retrofit one end or
corner of the pond into a bog filter.
3) Place liner in pond and bog filter,
following vendor’s liner installation instructions. Hide all liner after
4) Install the pump in the pond as far
away from the bog filter as possible, using a pump which circulates pond volume
at least once every two hours for pond under 4,000 gallons, and at least once
every four hours for ponds over 4,000 gallons (more is better). Use over-sized
pipe or tubing to maximize flow to the bog -- 2” diameter is standard. Screen
or pre-filter the pump enough to prevent clogging, but do not over-filter; let
the bog filter trap and eat the detritus. Some people split and valve the
output from one pump to operate both a waterfall and a bog filter, while others
use two pumps. The waterfall can even empty into one end of a wrap-around bog
filter, with water falling into the pond at the other end of the bog.
5) Use PVC pipe to build a distribution
manifold the length of the bog. Drill 1/4" holes about 3" apart, in a
line about halfway between the side and bottom of the pipe, or cut slots into
the pipe (we prefer slots). Use an elbow at the end of all manifold lines, add
a short piece of pipe to bring it to the bog surface, and glue a male adapter
with a threaded cap on it for cleanout. Center the pipe along the bottom of the
bog. Bogs wider than 3’ benefit from multiple lines set 12-18” apart.
6) 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, through the gravel, and flow back into the pond over the retaining wall
through cracks between rocks.
7) Fill bog filter with 3/8” diameter
rounded (pea) gravel, mounding the gravel so it is 2” to 3" above water
level for most of the bog. Having gravel above the water surface is important
because water will follow the path of least resistance; if allowed to go over
the gravel, water will bypass most of the gravel, bacteria and plant roots
doing the filtering. Using smaller or crushed gravel or sand restricts water
flow and clogs too easily. Using larger diameter gravel or rock is less
hospitable for plant roots, and provides insufficient surface area for
colonizing bacteria. It’s all right to mix larger pieces of river rock into the
top layer of gravel for a more natural look. Incorporate some of the rock that
is around the pond and landscape in the bog filter as well, to visually tie the
landscaping together. Feel free to use some large pieces of wood or other
decoration in the bog. While all this decoration is likely to be covered by
plant material soon, it looks more natural in the meantime, and the filter will
be exposed from time to time after harvesting or during winter dormancy.
8) Plant the bog filter, using any
moisture loving plants. Your choices will be dictated by climate to a certain
extent, unless you are willing to sacrifice or repot and bring in tender
species. Different plants obviously thrive in different climates.
We emphasize using two plant
categories for greatest efficiency:
plants with persistent (year round) fibrous roots such as various rushes,
Thalia, Irises including Iris versicolor, and cannas--their fibrous roots
provide excellent colonization sites for beneficial microbes in both
summer and winter. Gunnera and Thalia make great focal points. Other tall plants
include cattails, pickerel, Louisiana
iris, lizard’s tail, ribbon grass, horsetail, hostas, and daylilies.
“ground cover” plants around the bog edge and among the tall plants. Excellent
fast growing ground covers include parrot’s feather, creeping Jenny,
variegated or green water celery (edible!), and aquatic mint (edible!). Shade
loving plants such as aquatic forget-me-not and watercress can find homes
in a bog filter. These fast growers provide the bulk of the nutrient
absorption, and soften the transition between pond and land; in fact, they
will probably grow out into the soil bordering the bog. Use only a few
varieties to minimize an untidy look.
Remember, bog filter plants have their
roots in the water but their crowns above water, plus the water is fresh and
highly oxygenated. This means you can use plants that are not usually
considered aquatic, such as impatiens. Most ferns are also moisture-loving. Now
may be the time to experiment with hydroponics tomatoes!
When planting the bog filter, do not
strip the soil away from the plant’s root ball. Instead, gently remove the
plant from the pot, and plant the entire root ball in the gravel, soil and all
(but definitely without the pot!). This small amount of soil will not harm the
pond or filter, and will help the plant quickly adjust to its new home.
To achieve a full effect, use up to
one plant per square foot of bog filter surface; or use as few as one plant per
three square feet and let time do the rest. Fewer plants means reduced
filtration until they grow and fill in.
9) For faster cycling of nutrients,
inoculate the pond with a pond bacterial culture. This can help with a new
pond, or if the pond accumulates organic debris quickly. While occasional
bacterial inoculation may be useful with heavy organic loads (overcrowding and
overfeeding fish ponds is the #1culprit), it is not normally needed.
10) Little maintenance is required, other
than harvesting excess plant material and some trimming of dead foliage in the
fall. Trim and divide plants as needed. When plants are overcrowded, they can’t
do their job--their growth slows, and they are not pulling as many nutrients
from the water. Groundcover plants grow especially quickly. Pulling plants out
of gravel is quick and easy, as is trimming. Don’t feel bad about harvesting
them, as they make great compost, and your garden is a better place for the
nutrients than your pond. Watercress is enjoyed by people and koi. With the
right balance, sediment will build up very slowly, if at all. Every 5-10 years
or so, a partial cleaning of the gravel may be required in areas where large
amounts of inorganic debris are deposited, such as dry, windy climates where
wind blows a lot of dust in.
There are many variations to this
basic design. Existing ponds may be retrofitted with a bog filter by making the
bog slightly higher than the main pond, and connecting bog and pond with a
streambed or piping (they don’t even need to be close to one another!). Or make
a bog in the corner of an existing pond, create a bog island, convert a
waterfall filter, use oak barrels or ceramic pottery bog filters…you are
limited only by your imagination.
Bog filters work incredibly well in
promoting crystal clear water, and they exert a high degree of control over
algae. Use and enjoy their beauty and simplicity.