Making a wildlife pond

First issue Feb 1999

For a printable (PDF) version click here

If there is one feature that all wildlife gardens should have it is a pond. A healthy pond swarms with life, and serves as a nursery for spectacular insects like dragonflies and for two of the gardener's most important helpers – frogs and toads. A large pond will support more varied aquatic life than a small one, but even a water-filled butt on the patio will be enough to tempt frogs and many insects to set up home.

Situation

A shady pond is better than none, but if possible site the pond open to the south so that plenty of sunlight enters the water, and with shelter from the prevailing wind. It is also best if it is not overshaded by trees, which cut out light and drop their leaves into it. Plants should run right up to it at some point, to provide cover for creatures entering and leaving its shelter.

A pond looks more natural in a hollow, but this brings the problem that it may flood in winter and the risk that heavy rain will wash debris and soil into it. Perhaps the ideal situation is on a slight slope or to one side of a dip, so that flood water can drain past and away. Drainage problems can sometimes be solved by running an overflow channel to a soakaway pit filled with rubble, but this will not be effective if the whole area drains poorly. If you have no other option, it can be built on a steep slope by cutting into the slope and using the spoil to build up the lower end.

Unless you are lucky enough to have a stream running through your garden, you will probably fill your pond initially from the mains with a hose. When you need to top it up later on during hot weather, try to do this with rainwater if possible, e.g. by running a hose from a water butt (which must of course be within reach).

If you really have no space for a conventional pond, then a raised pond on patio or lawn is an alternative: see Raised ponds.

Materials

For small ponds, moulded or fibreglass pools are little trouble to install, especially if raised, but you are limited to the designs available.

Liners are the most flexible option. Butyl is best and will last a lifetime if treated properly, but is also relatively expensive. PVC is an alternative if money is short, with a life expectancy of up to 15 years. Unlike butyl, PVC deteriorates in UV light so should not be left exposed to sunlight. Construction demands care - in particular, line the hole with soft sand and/or old newspapers and/or underlay to avoid punctures - but no special skills: more details later.

Concrete demands a lot of work and is not for the faint-hearted. It is also prone to cracking, and when this happens the best solution may well be to repair it with a liner anyway, so why not start off with one?

Shape and size

You need a minimum of 4 sq m to create a balanced environment, although smaller pools will work well if managed carefully.

If you are using a liner, allow for marginal shelves (at least 25 cm wide) to hold containers of emergent plants. Another option is to build in planting pockets - a good compromise between planting baskets used in formal ponds and the natural pond with a soil base which is very difficult to manage.

Avoid fussy shapes: go for sweeping curves with no sharp bends. A fat figure of eight or a kidney shape is good, because you can then include two zones of differing depths. A suggested profile for a pond with planting pockets is shown right.

Draw a rough cross-section of the pond to check how much depth you will get for your width. You need at least 60 cm depth for frogs to hibernate successfully, and ideally 90 cm or more. On normal soil, slopes should be 20° max. i.e. roughly 1 unit down for every 3 units across, to avoid the risk of slippage. You can get away with steeper slopes on a stiff clay soil.

Edges

Assuming you use a liner, you have four main choices for how you handle the edge. Often you will want to use more than one method, such as a paved edge for access and a planted edge round the back for wildlife value.

Buried edge

Run the liner up about 10 cm above maximum water level and bury it in the soil

• simple and natural effect, but liner shows when water drops;
• danger of damaging the liner when mowing or cultivating near the pond.

Cobbled edge

Set large stones or cobbles into a bed of sand/cement laid on the liner, running out of the pool up a shallow slope

• stones can just be piled loose on the liner, but will need regular maintenance to stop them silting up and becoming overgrown by plants;
• makes access by wildlife easy, and conceals liner completely.

Planted edge

A planting pocket is built on to the liner, edged with stones or bricks

• soil level should have a gradient, ending a few inches above water level, so that plants requiring a range of damp soil conditions can be grown;
• important that liner at outside of pocket is high enough to prevent water loss.

Paved edge

Concrete or stone slabs are laid on a sand/cement bed over the edge of the liner (ideally on a foundation of hard core)

• more formal but very practical;
• if you pave all the way round, make sure, e.g. by stacking loose rocks next to the edge, that very small animals like froglets can easily climb in and out.

Raised ponds

The easiest way to build a raised pond is to use a plastic or fibreglass mould set on a layer of soft sand, and build up round it using rocks or bricks. Make sure that the whole of the base is supported so that it doesn't crack.

As an alternative, use a wooden half-barrel, lined if necessary, or a plastic water butt cut in two.

Make sure that small animals can get in and out, by giving them a 'ladder' of bricks and/or planters inside the pool and, outside, either by providing another ladder or by siting the pond against a raised bed or low wall.

Choose plants with care: most native plants are too vigorous. Small plants like bog arum Calla palustris or brooklime Veronica beccabunga will be entirely in scale, while flowering rush Butomus umbellatus will fit but dominate. Among the exotics, pygmy water lilies such as Nymphaea pygmaea 'Helvola' or 'Alba' are fine, and also the dwarf Japanese reedmace Typha minima.

Construction

1. Lay out a rope or a length of hose to the shape of the pool, including its planting pockets, checking that it is level all the way round. You can do this initially with a spirit level on a long straight piece of wood or with a transparent hose filled with water. You may need to do some preliminary digging at this stage, but it is essential to establish exactly where the water level will be.
2. It will help when you come to digging and installing the edging if you hammer pegs in at intervals just outside the final outline, with their tops say 10 cm above the planned water level, and level them using a spirit level. When you have finished, look at the outline from all possible angles, including from above if you can, such as from an upstairs window.
3. Mark all round with a spade just inside the outline. Take out the turf or topsoil first and stack it or barrow it away. Then excavate the subsoil and do the same with that. Finally, cut back further to accommodate your chosen edging. Check carefully all over the base for protruding roots or sharp stones that might puncture the liner, and remove them. At the same time, make a final check of dimensions and slope angles.
4. When you are completely happy, line the hole with protective material. On a smooth clay soil 15cm or so of soft (builder's) sand and/or a layer of old newspapers is enough, but on stony soils or if you want to be really safe, use a layer of polyester matting. It is also wise to line with PVC or plastic sacks the upper edge of the pool where it borders any rough ground. While butyl seems able to resist most threats, some vigorous plants, and notably couch grass, grow straight through it.
5. Drape the liner into the hole with an even overlap all round, weight the edge with stones or bricks, then start filling it from a hose. As it fills, move the stones to allow the liner to fit as neatly as possible into the hole. Some creasing is unavoidable but can be minimised by easing and stretching the liner.
6. When the pool is full, secure the edge by pushing large nails through it into the ground, and cut off any surplus, making sure that you leave enough to anchor the edges securely. Where it terminates in the soil, take it up and over a small firm mound of earth and anchor the end with more soil or with rocks buried in turn in soil. Make sure the liner is high enough all round to prevent any water loss into the surrounding soil.
7. Finally, install any edging, making sure that it is horizontal and in exactly the right position relative to the planned water level. If using cement, take care not to drop any into the water, and it is also worth proofing cement exposed to pool water to stop salts leaching out.

Plant life

If you want your pond to be as much like a natural pond as possible, then you should put a layer of soil on top of the liner. But if you do this, you must be prepared to clear the plants regularly, because aquatic plants spread very rapidly and otherwise will soon choke the pond.

If you want to minimise management work, a good practical compromise is to use planting containers in the pond, supplemented by planting pockets around the edge. You will lose some wildlife value by doing this, but many creatures, including frogs, toads, newts and some dragonflies, are quite happy in these conditions.

For planting, use rotted turves or, failing that, ordinary garden soil that has not been treated with any chemicals. Spread gravel on top of the soil, especially in planting containers, because this helps to keep it where you want it.

You need three different types of plant in any pool (see table below). Plants for poolside or bog garden are listed on the back page

Submerged and floating plants to serve as oxygenators

They are essential to a healthy pond because they supply the oxygen that pond animals need and use up dissolved nutrients.

Submerged plants with floating leaves

You should have enough to cover about half the surface of the pond, otherwise you will certainly have problems with excessive algal growth. Water lilies come in a wide range of colours and sizes and are valuable for this purpose as well as being decorative, but native species are too vigorous for any but large ponds. The table lists other native plants that are suitable for smaller ponds.

Marginal plants, growing in shallow water at the edge of the pond.

They contribute shade and cover for animals and make up much of pond's visual appeal. They are also used by creatures like dragonfly nymphs to crawl up out of the water to pupate.

The native plants listed above are not too vigorous for the average garden pond. Others, such as reedmace and flag iris (and that are available from some pool specialists and garden centres), are so invasive that they are a menace in all but very large ponds.

Expect your pond to take some time to settle down after planting. It is normal for some algae to appear early in the year and then disperse, but if this problem persists, a bale of barley straw placed in the water often cures it.

Plant at any time in the growing season, i.e. April to September.

Animal life

It is worth acquiring water snails from a pond supplier because they help to keep the pond clean but otherwise, unless your garden is very remote from any other pond or waterway, you can safely leave nature to supply all the residents you need. Alternatively, seed it with a bucket of water from a friend's established pond, but beware of bringing in invasive plants like duckweed at the same time.

Do not introduce ornamental fish if you want frogs and dragonflies, because they eat their eggs and larvae.

We also recommend against collecting frogspawn because of the risk of spreading Red Leg Disease.

Maintenance

Keep an eye on the water level throughout the summer and top up if necessary, using rain water if possible. Do not be too fussy about this though, because it is quite normal for pond water levels to fall in summer and many plants and animals are adapted to it. Many marginal plants, for example, grow as happily in wet soil as they do in a few cm of water

Oxygenating and floating plants spread vigorously, and it is difficult to prevent them from rooting into the soil that accumulates on the base of the pool. It is best to thin regularly throughout the summer rather than going in for wholesale clearance which may seriously disturb the balance of the pond.

When leaves fall in the autumn, try to keep them out of the pool if you can and rake out those that do get in. At the same time, cut back some of the emergent plants above the water level. This helps to keep nutrient levels down, so that plants do not grow too vigorously. Make sure you never clear more than about a third of the vegetation so that you leave enough eggs and larvae behind for next year. Always pile the material on the bank for a while to let mobile creatures escape back to safety before you barrow it away for composting.