A pond liner is a flexible waterproof membrane that covers the floor and the sides of a pond excavation. There are several different kinds to choose from. Polyethylene is useful when sandwiched between two layers of soil, while PVC, composition and rubber liners are the types to use for a clean pond lining when the aquatic plants are to be grown in containers.

Most ponds can be constructed from most pond liners, but in some circumstances one will offer particular advantages over another. Where no part of the liner is exposed, then polyethylene is perfectly adequate. It need not be a specially manufactured pond liner, any heavy gauge polyethylene will be sufficient. That commonly used by builders is ideal.

Composition liners and those manufactured from rubber are necessary where exposure is inevitable, especially in formal features where there is limited planting and the water is the main attraction. Not all pond owners are plant enthusiasts and often water is a design element which is introduced for its reflective qualities. Certainly in formal circumstances where a fitted liner is desired it is more practical to use a material like rubber as it can be manufactured neatly welded to the shape desired and provide accurate wrinkle-free corners and curves.

For most gardeners though, it is a case of digging a hole to the required shape and then putting in the liner. However, it is important to never buy a liner and then dig a hole. Always finish the excavation first and then calculate the size of liner required to fit. Irrespective of the pond's shape, profile or size, calculating the liner size follows a simple formula. Take the length and then add to it twice the maximum depth. Then make the same calculation for the width. This will provide a liner that will suit the excavation and have sufficient overlap for the edges with a minimum of wastage.

It is necessary to mark out the shape of the proposed pond before digging commences. This not only permits accurate excavation, but gives an impression of how the finished pond will fit into the garden landscape. It is quite likely that original ideas will change somewhat when the proposal is seen in reality on the ground. Use stakes and string to create the outline of a formal pond, or a length of rope or hosepipe to give some guidance for an informal pond shape.

When creating an outline for an informal lined pond, take into account that while it is perfectly possible to line almost any shape or excavation, there is a limit to what can be achieved without significant folds and creases appearing in the liner. Whatever the final shape of the pond, marking out should be from a fixed point. When a building or path is used as a base-line this is straightforward, but if the pond outline has been created by eye, then create a point at one end using a stake knocked firmly into the soil as a marker.

The installation of a pond liner is the same for all materials except polythene. This needs molding completely to the shape of the excavation before water is added as it has little flexibility. Other materials used for pond lining are very flexible and when carefully installed mold to the contours of the excavation with the weight of added water. When digging ensure that the walls of the excavation remain as solid as possible. Replacing or filling with soil and attempting to compact it once disturbed is very difficult. Ideally the excavation should be carved out of the soil in rather the same manner as cutting cheese, that which remains behind being firm and solid.

Dig the whole of the pond down to the marginal shelves. Mark out the deep area of the excavation and dig down again. This helps to ensure that the hole is accurately produced with a solid base and walls that will remain firmly in place when the liner is laid and water added. During the digging process, make sure that the pond is level from end to end and side to side using a series of wooden stakes and a board with a spirit level. If the edges are not level, then when the liner and water are introduced there will be parts of the pond that will flood and other areas with large expanses of bare wall.

Irrespective of how tough the pond liner is believed to be, it will benefit from some form of cushioning in the hole. Make sure that there are no sticks or sharp stones in the excavation that may puncture the liner when the weight of water is added, and then add a generous layer of fine sand. The kind that is used for bricklaying is ideal. If damp, it can be smoothed around the excavation and will even adhere to the sides. Fleece also provides a useful protection, as do thick wads of old wet newspapers or pieces of discarded carpet.

Spread the liner across the pond, weighing down the edges with bricks or rocks, then run water from a hosepipe into the center so that the liner molds to the shape of the hole. As the liner molds to the contours straighten out any wrinkles and where necessary make bold folds to create corners. Spread the liner across the pond, weighing down the edges with bricks or rocks, then run water from a hosepipe into the center so that the liner molds to the shape of the hole. Once installed, finish the edge with a neat arrangement of tiles, paving or stone.