There is something quietly satisfying about watching a summer downpour and knowing that every drop sliding off your roof is being banked for the weeks ahead. A rainwater butt turns a wet afternoon into a resource, and for wildlife gardeners it does double duty: it cuts the amount of treated tap water you pour onto your plot, and it gives you a store of soft, chlorine-free water that plants, ponds, frogs and garden birds all prefer.

Why rainwater suits wildlife better than tap water

Mains water arrives treated and, in many parts of the UK, carries chlorine or chloramine along with traces of the minerals that make it "hard". None of that troubles a human drinking a glass of it, but it is not what a bog garden, a wildlife pond or a seedling tray has evolved with. Rainwater is naturally soft, slightly acidic and free of the disinfectants that can irritate amphibian skin and young plant roots.

That softness matters more than many gardeners realise. Bog plants such as marsh marigold and ragged robin, acid-lovers like blueberries, rhododendrons and camellias, and thirsty woodland ferns all respond well to rainwater. So do the shallow dishes you keep topped up for hedgehogs and the bird bath outside the kitchen window — birds seem to prefer a bath that has not just come out of a tap.

The arithmetic is encouraging, too. Roughly one litre of water lands on every square metre of roof for each millimetre of rain. A modest 40 square metre roof in a region with 700mm of annual rainfall can shed something in the order of 28,000 litres a year. You will not capture all of it, but even a couple of 200-litre butts will see you through most dry spells.

Choosing and siting your butt

Water butts come in every shape from a slim column that tucks behind a shed to a barrel-shaped classic. Capacity usually runs from around 100 to 250 litres. Two or three smaller butts spread around the garden are often more useful than one enormous one, because you can place them near the beds that need water most and link them in series with a connecting kit.

Position matters. A full 200-litre butt weighs the best part of a quarter of a tonne, so set it on a level, solid base — paving slabs on a bed of sand are ideal. A wobbly butt leaks, tips and eventually splits. Raise it on a sturdy plinth if you can, so a watering can fits under the tap without gymnastics.

• Site the butt close to the downpipe it will serve, and close to the beds you water most.
• Keep it out of full midday sun where possible; shade reduces algal growth inside.
• Choose a dark, opaque butt over a translucent one — light feeds algae.
• Plan where the overflow will go: a bog garden, a soakaway or a planted swale is far better than a paved patio.

Fitting a diverter properly

A diverter is the small unit that sits in the downpipe and channels water into the butt. When the butt is full, the surplus simply continues down the downpipe as normal, so you never get a flooded base or a soggy wall. It is a job most gardeners can manage in an afternoon.

Put a bucket under the downpipe, mark the height you want the diverter at, then cut a section out of the pipe with a hacksaw. Fit the diverter body, seal it as the instructions direct, and run the flexible hose to the butt inlet with a gentle, continuous fall — a hose that sags will hold water and silt up. Check the alignment with a jug of water before you call it done.

Consider fitting a mesh filter or a first-flush arrangement at the inlet. It keeps moss, grit and roof debris out of the butt, which means cleaner water for your pond top-ups and fewer blocked watering can roses.

Safety, lids and the midges question

An open water butt is a hazard as well as a habitat. Always keep the lid firmly secured — a child or a curious dog can drown in surprisingly little water, and hedgehogs, fledglings and small mammals fall in easily and cannot climb out. If your butt has no lid, fit a rigid mesh cover, or build a ramp of rough twigs and netting that reaches from the water surface to the rim.

Standing water also breeds mosquitoes. A tight lid, or a fine mesh over the inlet, removes the problem at source. Do not be tempted to add bleach or detergent to "clean" the water — it will damage your plants and harm any wildlife that drinks from a dish you fill later.

Getting the most from every drop

Use rainwater deliberately rather than casually. Water in the early morning or evening so less evaporates, and pour it at the soil rather than over the leaves. Newly planted trees and shrubs establish better with a slow, deep soak from a can than with a quick blast from a hose, and rainwater is the kindest choice for seedlings and greenhouse plants.

For ponds, a summer top-up with rainwater is far better than tap water, which adds nutrients that feed blanket weed and algae. Fill bird baths and shallow ground dishes with it, and refresh them every day or two in warm weather. If you garden on heavy clay that bakes hard in July, a buttful saved in May is what keeps a border alive in August.

Through autumn and winter, clear leaves from the inlet and check the overflow runs freely. In a hard frost, a full butt is usually fine, but avoid drawing water when the tap is frozen solid. Empty and rinse the butt once a year, and it will quietly serve your garden — and its birds, frogs and hedgehogs — for a decade or more.