Marginal plants--short-stemmed aquatic plants which grow around the edges of water bodies or in shallow bogs--can provide superb natural water filtration while adding to the beauty of your pond or water garden. Marginal plants add oxygen and remove nitrates and even some heavy metals from your pond water. Marginal plants grown on a shallow shelf or slope at the edges of your pond will help filter the water, but circulating your pond water through a special filtration bog will maximize the positive effects of marginal water plants in your aquatic garden.
Determine a location for your marginal plant filtration bog at an elevation between the upper surface of your pond and the exit point of your waterfall pump. Elevate the waterfall pump if necessary to create enough of a gradient to locate your filtration bog between the waterfall and the pond, or extend the course of the stream from the waterfall to the pond by placing the filtration bog to one side or the other, creating a meandering effect.
Dig a hole with the shovel one-fifth the volume of your main pond at the chosen filtration bog location. Shape the hole so that it is approximately twice as wide as it is deep, with a gentle slope from the bottom to grade level.
Place pond liner material in the newly dug hole, smoothing it over the bottom and extending it to create an ample overlap with the pond lining material for the waterfall stream above the filtration bog, and the stream entering the pond below the filtration bog. (If you are constructing the filtration bog at the same time as a new pond installation, use a single piece of pond liner material to line the entire watercourse whenever possible.)
Cover the edges of the pond liner material with the dirt dug out of the hole. Place a piece of slate or stone, about 2" in height, across the exit course of the filtration bog, between the bog and the main pond, to deter planting material from washing out of the bog. Place 2 to 3 inches of gravel, clay soil, or aquatic planting medium in the bottom of the filtration bog.
Insert the roots of the marginal plants into the gravel, clay soil or aquatic planting medium. Secure the plants with a pair of fist-sized rocks on each side of the stem until the roots grow and take hold in the growing medium.
Turn on the water pump, allowing the pond water to flow through your waterfall pump, down the watercourse into the filtration bog, then out of the bog into your main pond.