A pond is the single most effective thing you can add to a small garden for wildlife. It does not need to be a grand, dug-in affair with a liner the size of a rug. A half-barrel, an old sink, a Autumn Rowan sink left in the corner, or even a large glazed pot will do, so long as it holds water, has at least one gentle slope, and sits somewhere with a bit of sun. Frogs, smooth newts, diving beetles, damselflies and a whole cast of tiny, fascinating creatures will find it within a season or two, sometimes within weeks.

Why even a half-barrel counts

Garden ponds have quietly declined across Britain, and many of the species that once relied on farm ponds and field margins now look to gardens instead. A small pond punches far above its weight because it concentrates life. Water boatmen, pond skaters and lesser water boatmen arrive by air within days. Frogs and newts follow, drawn by the smell of damp and the sound of other frogs calling. Damselflies — the large red and the blue-tailed are the most likely visitors — will lay eggs on submerged stems, and you may spot their nymphs clinging to plants later in the year.

Birds benefit too. A shallow edge is a bathing spot for sparrows and blue tits, and a drinking station for hedgehogs, which can swim but struggle badly to climb out of steep-sided water. That is why shape matters as much as size.

Choosing the right spot

Pick a position that gets sun for at least half the day; warmth drives the insect life that everything else feeds on. Some shade in the afternoon is fine and helps keep algae in check. Avoid the drip line of a large tree or overhanging sycamore, because fallen leaves rot down and feed blanketweed. Keep the pond away from where you regularly walk in the dark, and if you have small children, either fence it or plan to supervise — even a 45 cm deep barrel is a serious hazard for a toddler.

Level ground matters. A half-barrel sitting on a slight slope will look as though it is leaning, and the rim will not sit true. Bed it on a firm layer of sand or fine gravel and check with a spirit level before filling.

Shape, depth and a way in and out

The commonest mistake with a small pond is making it a bucket — straight sides, uniform depth, no exit. Wildlife needs a ladder.

If you are lining a barrel or a rigid container, build in two things:

• A shallow shelf about 10–15 cm below the water surface where marginal plants can stand and frogs can sit half-submerged. A stack of bricks, a couple of flat stones or an upturned plant pot will create one in minutes.
• A sloped exit ramp running from below the waterline to just over the rim. A strip of butyl liner draped over a piece of wood, or a run of flat pebbles, gives amphibians and hedgehogs something to grip. Without it, creatures that fall in will drown.

Depth of 40–50 cm is plenty for a half-barrel. It gives larvae and newts somewhere to retreat when the weather turns cold, and it is deep enough not to overheat in a July heatwave. Anything shallower than 30 cm will warm too quickly and turn green.

Fill it with rainwater, not tap water

Tap water contains nutrients and traces of treatment chemicals that give algae a head start. Use rainwater from a water butt if you can, or let a full barrel of tap water stand for a few days before planting so the chlorine dissipates. Rain is free, soft, and exactly what the pond wants. Fill slowly, using a watering can and a sheet of plastic to cushion the flow, so you do not stir up the compost you have just laid.

Let the water sit for a week before you plant. It will look cloudy at first, then clear as the microscopic life settles into balance.

Native plants for a tiny pond

You need three layers of planting, and only a handful of plants.

• Submerged oxygenators: hornwort (Ceratophyllum demersum) or water starwort. These keep the water clear and give damselfly nymphs somewhere to hunt.
• Marginals on the shelf: water forget-me-not, brooklime, marsh marigold and lesser spearwort. All are British natives, all are compact enough for a barrel, and all flower generously.
• One or two floaters: a single water lily in a dwarf variety, or frogbit (but avoid duckweed, which will take over and shade everything).

Plant in aquatic baskets filled with loam-based compost, topped with a layer of grit to stop it clouding the water. Steer clear of invasive non-natives such as parrot's feather, water fern and New Zealand pigmyweed — these can escape a garden pond and cause real harm in the wider countryside.

Looking after it through the year

A pond is not a project you finish; it is a small habitat you keep an eye on. In spring, thin out any plant that has taken more than its share and leave the cuttings on the bank for a day so anything clinging to them can crawl back. Never introduce fish — even a couple of goldfish will eat tadpoles and larvae and turn the water murky. Do not move frogspawn in from elsewhere, as this spreads disease. In winter, if the surface freezes, do not smash the ice; a pan of hot water rested on the surface will melt a hole instead, letting gases escape. And resist the urge to tidy up in autumn. That tangle of dying stems is where larvae, beetles and newts will spend the cold months.

Within a year, a half-barrel can hold frogspawn, a dozen species of invertebrate and the first damselfly of summer. It is the best square metre of habitat you will ever make.