For any pond, the longer spent on planning, the better the result. The shape, siting and finishing of any pond are important, no less for clay.

Existing vegetation

Don't site your clay pond near trees because the roots will make digging difficult, and future growth will probably break through the clay. As a rough guide, tree roots extend as far outwards as the branches, with poplar and willow twice as far.

Also beware of other nearby vegetation that could cause problems later. A lawn leading up to the pond looks lovely, but if your lawn contains couch crass then you'll be in trouble. Couch grass has very invasive roots (rhizomes) - you'll spot them once you begin digging. If you have this plant, the best solution is to spray off the future pond's surroundings with a translocated weedkiller such as Roundup, which will kill all the greenery that it touches.

Fish and pond plants

Plan the pond depth to ensure any fish will survive winters in your area.

Also plan for plants. You can't plant into the clay, so plan shelves at the correct depth for baskets of marginal plants. Also check on the depths that deeper water plants (such as water lilies) need to thrive.

Finishing off the pond edge

The top edge of the clay lining needs protection from drying out, and from feet or rain eroding it away. So you need the edging materials to hand on the day you puddle your pond so you can protect the edge immediately.

You could use slabs, pebbles, a layer of topsoil or turves (of seed-grown grass, not meadow grass which could contain couch).

Ease of topping up

Hot and/or windy weather both cause evaporation, so be sure that you can easily top up the pond from a nearby water source (for example by hosepipe from a tap or rainwater butt).