A pond is the best thing you can do for British wildlife

If you have room for only one wildlife feature, make it a pond. Nothing else pulls its weight so quickly. Within a single season a well-sited pond will bring in common frogs, and within two or three you can reasonably expect smooth newts and perhaps palmate newts, along with dragonflies, damselflies, pond skaters and a cloud of insects that will feed the birds overhead. Frogs and newts are in decline across much of the countryside, largely because small ponds have disappeared from gardens, farms and village edges. A modest hollow in a suburban back garden genuinely matters.

It does not need to be large. A pond of two metres by three, with a deep centre and a shallow shelf, will do more good than a big, badly designed one. What follows is how to build and plant it properly.

Choose your spot with care

Frogs and newts both prefer water that is not in full sun all day. A site that gets sun for part of the morning and is shaded by midday or late afternoon will stay cooler, lose less water to evaporation and grow less algae. Dappled shade from a deciduous shrub or a low wall is ideal.

Try to avoid sitting the pond directly under trees and large shrubs. Falling leaves will fill it every autumn, and roots — particularly willow, poplar and leylandii — can find their way through a liner. Keep a metre or so of clear ground around the edge too, so you have somewhere to kneel and so that frogs and hedgehogs have a dry approach.

• Good: north or east facing edge of a lawn, near a log pile or long grass.
• Less good: the sunniest spot in the garden, or under a mature birch.
• Avoid: anywhere water sits already in winter, unless you want a bog garden instead.

Digging, depth and the shallow ledge

The single most important design detail is the shallow ledge. Frogs, newts, beetles and even thirsty hedgehogs need to be able to walk in and out of the water. A vertical-sided pond is a trap. Build a shelf around at least one side of the pond, roughly ten to twenty centimetres deep and thirty centimetres wide, so that plants can sit on it and animals can climb onto it.

In the centre, aim for a depth of sixty to seventy-five centimetres. That gives frogs and newts somewhere to overwinter below the frost line, and it means the pond is far less likely to freeze solid in a hard January. Smooth, sloping sides are better than steep ones, and where a slope is not possible, lay a few flat stones as a ramp.

Use a butyl or EPDM liner for a natural shape, and finish the edges with turf, gravel or stones so the liner is not on show. Fill the pond with rainwater from a water butt rather than tap water, which carries nutrients and chlorine; if you must use tap water, let it stand for a few days first. Then leave the whole thing alone for a fortnight before planting.

Planting: native, oxygenating and not too much

Plants do three jobs: they oxygenate, they shade, and they provide cover for spawn, tadpoles and newt larvae. Stick to native British species and avoid the invasive non-natives that wreck small ponds — floating pennywort, parrot's feather, water fern and New Zealand pygmyweed are all best left out.

• Oxygenators: hornwort, water starwort, spiked water milfoil and common water crowfoot. These sit below the surface and keep the water clear.
• Marginals for the shelf: brooklime, water forget-me-not, marsh marigold, lesser spearwort and ragged robin.
• Floating foliage: a small water lily or frogbit, but keep it to a third or half of the surface at most.

Basket the vigorous growers so they do not take over, and let the rest root into the silt naturally. Do not add fish. Fish eat spawn and tadpoles, and a wildlife pond is far richer without them.

What arrives, and when

Common frogs usually spawn in February or March, laying clumps of jelly in the shallows where the water warms first. Newts are later and more discreet; smooth newts breed from April into June, and the females wrap each egg individually in a folded leaf of an oxygenating plant. You will often see the males displaying in the shallows, crests raised.

Do not be tempted to move frogspawn in from another pond. It risks spreading ranavirus and invasive plant fragments, and the wildlife will find your pond on its own — often within a few weeks of filling. A first summer of green water and a little blanketweed is normal; it settles as the plants establish.

Caring for the pond through the year

Spring is the time to thin oxygenators if they have filled the water, and to scoop out duckweed with a net. Never empty and scrub the pond — that destroys the whole community of invertebrates you have built up.

In summer, top up with rainwater and leave the grass around the edge a little long. In autumn, net out some fallen leaves but leave a few; a light layer of debris is habitat, though too much will starve the water of oxygen. In winter, if the surface freezes, do not smash the ice — the shock waves can kill frogs resting in the silt. Float a ball or stand a pan of hot water on the surface to keep a breathing hole open instead. Sit back, keep a notebook, and watch what turns up.