The first step in construction of a swamp garden (or bog garden) is to level the whole site, including the space for paving that will surround the pool. Then mark out the area of the pool itself and excavate the soil 9 in. Deep, but leaving the sides sloping inwards about 30 degrees. Now mark out the 9-12 in. Wide areas to be left as shelves for marginal plants and excavate the remainder by another 9 in., still sloping the sides inwards as you work.
When all the soil has been dug out, smooth the sides and bottom of the hole, removing any sharp stones which might damage the plastic, and filling any holes with moist sand or sifted soil. The liner will mould itself to the soil, so make sure there are no serious irregularities. If the soil is very stony, cover it completely with a layer of sandâit will cling to the sides if damp.
In with the Liner
Spread your PVC liner out in the sun for a while to become warm and in-crease its flexibility, then drape it over the excavation so that it overlaps evenly at the sides and ends. Weight the edges of the sheet with bricks or stone slabs, spacing them out evenly. A continuous line of house bricks provides about the right weight. The sheet should sag gently at the centre of the pool but not be creased.
Run water into the sagging liner from a hose pipe. As the pool fills the liner will stretch and become moulded to the sides of the hole and the weights will be gradually pulled to the pool edge. When the pool is full, trim off the surplus plastic to leave a 6 in. Wide nap around the edge. Finally surround the pool with paving, setting it level with any surrounding turf to ease mowing, and with the inner edge projecting 2 in. Over the water.
If an informal setting is preferred, the flap of plastic can be buried beneath the soil an inch or two from the pool rim and disguised with overhanging plants. One part of the pool edge can be made fractionally lower than the rest to provide an overflow for surplus water after heavy rain. The soil there will be an ideal place for plants that need boggy conditions.
For formal pools of rectangular shape, prefabricated flexible plastic liners are available which solve the problem of making neat corners. They are supplied in a box complete with a set of patterns to mark out the exact size of the pool when excavating.