Before laying the rubber liner, I double-check to make sure the tops of the stakes are still level and pump out water that has seeped in. Using the tops of the stakes as my reference points, I measure down 12 centimetres to determine a uniform depth for a shelf for the edging rocks, which I want to be seven centimetres below the top of the pond. Using an old breadknife, I then cut a 15-centimetre-wide shelf all the way around.
Next, I dig out the bog area directly beside the pond, making sure its edges are as high or higher than the pond’s, and create a 30-centimetre-wide dike between the two to enable overflow to spill over the dike and into the bog. I measure and mark its perimeter with more stakes, then use the spade to dig a hole 60 centimetres deep, with a five-centimetre-deep shelf around the edge.
Laying the foam carpet underlay
Before putting in the lining that will cover the pond, dike and bog, I remove any sharp stones, then protect the pond and bog with a one-centimetre-thick layer of used foam carpet underpadding. (Fabric pond underpadding, available at most garden centres, will also work, as will cardboard or even a one-centimetre layer of sand.)
Laying the liner
I lay the folded rubber liner over the pond, carefully positioning its centre in the bottom of the hole, taking care not to stretch the liner as I unfold it and press it against the sides of the pond. To make the liner as flat as possible, I pleat any excess, then overlap the edges of the pond and bog by at least 30 centimetres.
Securing the fabric
To protect the rubber liner, I cover it with a piece of grey fabric pond liner, also pleating it and overlapping the pond edges. I then place thin, flat rocks on the plant shelves and several large rocks on the bottom of the pond to prevent the fabric from floating. Next, I fill the pond two-thirds full of water.
A row of five-centimetre-thick flat rocks is then set along the 15-centimetre-wide edging shelf I’ve created, making sure each rock overlaps the pond edge by 2.5 centimetres. Using scissors, I cut off excess liner and padding, as well as the foam underpadding just at the outer edge of the rocks, and fold an eight-centimetre overhang of liner and fabric over and onto the top of the edging rocks all the way around the pond and bog. A second row of rocks, each at least 30 centimetres wide and two to five centimetres thick, is then placed directly on top of the folded-over liner and fabric about two centimetres back from the water-facing edge of the lower layer of rocks. A second layer of rocks is also added to the dike between the pond and bog.
Edging the pond with two layers of rock is more costly but worth it. It’s difficult to hide the liner at the water’s edge with a single row, and an exposed liner not only looks artificial, but can be damaged by sunlight. A double row, with the liner folded under the top layer, allows the pond’s water level to come to the top of the first rock layer. A word of caution: If using limestone, be aware it can make the water very alkaline, which is harmful to plants and fish. Monitor your pond’s pH level with a pond water test kit—a pH of 6 to 7.5 is desirable.