Making a pond with a flexible liner
A flexible liner is best method of making a wildlife pond. It is easy to form it into most shapes (except sharp corners and tight inlets). A good quality butyl liner should last many years if your pond is prepared correctly.
First cut the grass with a lawnmower or strimmer if the ground is uneven. Not only will it make it easier to dig your pond if the grass is short, it will make it easier to mark out the shape. Get a thick rope or a garden hose. Lay it on the ground and move it into the outline of the shape that you want your pond to be. Remember that simple smooth flowing curves will be easier to construct and will reduce wastage of the liner. Tight curves will mean that there will be excess liner that has to be trimmed and discarded. Simple curves will also look more natural. Adjust it as much as you like until you are happy with the shape. Changes are simple at this stage. Try to imagine the finished pond; fully planted with tall flowering marginals. Think where people will view the pond from. If possible, get up high - an overlooking upstairs window or even some stepladders - does it look right from high up?
Don't be tempted just to make it as large as possible. Holes might be free but liner is not. Calculate the price of liner at this stage and stick to your budget. For every extra foot of width or length, you are adding a considerable cost. Check that liner you have chosen is available in the size you are planning - especially the width. Buytl liner is cut from a roll so length is not usually a problem but width is limited. Check it out at this stage and double-check your calculations.
Calculating pond liner size
Calculating the amount of liner you need for your pond is really part of the planning stage but you can't really know how much you need until you have marked out your desired shape. Be prepared to change your plans if your liner size is not available or just too expensive.
Here's how to calculate the size of the pond liner you need:
Length = 2x maximum depth + maximum pond length + 0.3m for overlap
Width = 2x maximum depth + maximum pond width + 0.3m for overlap
With a regular pond these dimensions are obvious enough but for an irregular wildlife pond they are not so clear. So to clarify: The maximum depth is the measurement from the bottom of the deepest part of the pond to the top of the highest bank. The maximum width/length is the width/length across the pond at its widest point. The overlap of 0.3m assumes simple flat banks. If you have high curved banks then increase this to 0.5m or more.
More easily said than done but this is simply a matter of putting your back into it. If you are used to digging or manual work and are fairly fit then a pond, say, 5m x 5m should only take a weekend to dig. If not it can be a real chore. If you don't think you are up to the job then consider paying or otherwise pursuding someone to do the job for you. For really big ponds a mini digger with a driver can be hired, usually for around £100-£150 per day, which should be enough to do the job. Just make sure you are around to supervise.
The type of soil is also a big factor in determining how long it will take to dig your pond. A good quality stainless steel spade will be a real asset especially in heavy clay soil.
Be aware if the pond site is especially waterlogged. This is great once the pond is in place as it will provide ideal conditions for a bog garden. However, it can make digging the pond extremely difficult and potentially hazardous. If your hole files with water as you dig, you may need to hire a pump. This can happen in heavy clay areas where the clay traps a layer of water. You might think this will allow you to make a pond without a liner but the water is likely to dry up quickly in hot weather which would be disasterous for your wildlife.
First dig around your outline, you can then remove your rope or hose. The top layer is the hardest as you will probably be cutting through turf and plant roots. If possible try to slice off slabs of turf with soll attached. Lay them out and keep them moist, these can be used to edge the pond and hold the liner down later. Don't stack them unless you intend to finished the pond in a day or two otherwise the grass will die without light available.
Once you've skimmed the turf off, dig down to a depth of around 1 foot (0.3m). This is the level of the outer shelf of the pond. This shelf will hold the pots for your marginal plants. Us a board and a spirit level to check that the soil is level.
Start to dig down parallet to the edge of the pond about 1 foot (0.3m) in from the edge. In effect you are leaving a shelf around the edge. Dig this second level down to about 18 inches to 2 feet (0.6m).
However well you prepare the hole for your pond, it's possible that shape stones, glass, tree roots etc can still be in the soil. Buytl liners are stong but it better to line the hole before installing the liner. Trying to repair a punctured or torn liner when the pond is complete is extremely difficult.
First line the hole with a layer of wet sand about 1 inch deep. Wet sand is easy to spread and mould into shape as we all know from our childhood. Next add a layer of pond lining felt or underlay. This is available from all good pond suppliers and is basically a thick felt. It is reasonably priced and well worth the investment. A budget alternative is to use newspapers. Don't be tempted to use old carpet as this is not biodegradable.
Install the liner
Drape the liner across the hole. Don't attempt to mould it into the hole yet. Weight the edges of the liner with stones or bricks and leave it for an hour or two so that the warmth (assuming you are doing this in the summer) makes the lining more flexible. If you have a water butt attach a hose and start to slowly pour water onto the liner. Don't use tap water. If you don't have a full water butt, wait for the next rainy day (that shouldn't be a long wait!). As the water pours in, adjust the stones/bricks one by own so that the weight of the water slightly stretches the liner but without pulling the stones in. The aim is to allow the water to mould the liner smoothly into the shape of the hole. If you are allowing the rain to do the work it may take a few weeks. Check regularly and move the stones are required. Unless there is a really torrential downpour it's unlikely that a single shower or day's rain will move the liner too much.
Trim the liner
When the water level is almost at the top of the lowest bank trim the excess liner to within about 1 foot (0.3m) of the water's edge. Pleat the overlap into neat folds to cater for the curves and lay it flat. Take the turf you cut at the start of the digging and lay it around the edge. Be sure that at least some of the turn is off the liner and on solid ground so that it can root. You may prefer to use stones or logs depending on the style of pond you are trying to create.