Why rainwater beats the tap

Rainwater is the closest thing to what a garden pond has evolved to receive. It is soft, slightly acidic, free of the chlorine and chloramine that keep our mains supply safe to drink, and — most importantly for a wildlife pond — very low in the nitrates and phosphates that fuel algae, blanketweed and duckweed.

Tap water is convenient, but every top-up adds a little more nutrient to a system that is already collecting plenty of its own from fallen leaves, pollen and fish waste. In a small pond those additions build up through the summer, and you end up with green water just when you most want to sit beside it. Rain, by contrast, arrives free of charge and at exactly the moment you need it.

The arithmetic is encouraging. A pond measuring 2 m by 1.5 m can lose 25–40 mm of depth in a week of still, hot weather, which works out at roughly 75–120 litres. A shed roof of 3 m by 2 m will hand you about 60 litres from a single 10 mm shower. Within a season, a modest water butt will cover most of what a small pond needs.

Setting up a collection system that suits a pond

You do not need anything elaborate. A water butt, a downpipe diverter and a stand to raise the butt are the heart of it. Bear in mind that a butt on the ground delivers water at roughly the height of its tap, so if you want to run a hose to the pond under gravity, raise the butt on a proper stand or a stack of paving slabs.

• Divertersfit into a downpipe and send water into the butt only once it is full, which keeps the drain working normally in heavy rain.
• Linked buttsjoined near the base with a short length of hose will roughly double your storage for very little cost.
• Position matters— put the butt as close to the pond as you can while still catching a decent roof, so you are carrying watering cans over the shortest possible distance.
• Greenhouses, sheds and garagesare ideal collection points, especially where the main house roof is shaded by trees.

Keeping the water clean from roof to pond

What comes off a roof is not the same as what falls from the sky. Grit, moss, bird droppings and bits of gutter debris all arrive with the first flush of rain after a dry spell. A simple first-flush arrangement — a short length of downpipe with a screw cap that you empty after long dry periods — will keep the worst of it out of your butt.

A mesh strainer over the inlet, and a filter bag or a layer of gravel and geotextile around the outlet, will do the rest. Keep the butt covered and dark; light and warmth encourage algae, and standing water invites mosquitoes. A well-sealed lid solves both problems at once.

Be selective about the roof you harvest from. Water from old lead flashing, weathered bituminous felt or a freshly painted surface is best kept away from a wildlife pond, particularly one with frogs and newts. Clean, modern tiles and slate are perfectly suitable.

Getting water into the pond without upsetting it

How you add the water matters as much as what you add. A sudden dump of cold rainwater into a warm, shallow pond can shock fish and amphibian larvae, and it stirs up sediment that has settled nicely over the summer.

• Little and oftenbeats one big rescue mission. Top up in the evening, when temperatures are closest to one another.
• Pour onto a stone or a saucerat the pond edge rather than straight into the water, so the flow is broken up and does not gouge the margin.
• Use a watering canfor small ponds, or a hose run from the butt tap for larger ones. A small solar pump and a length of pipe makes the job almost effortless.
• Never top up over spawning— frog and newt eggs and tadpoles are easily dislodged by turbulent water in early spring.

Storing rainwater safely

An open butt is a hazard for small children and a trap for wildlife. A rigid, well-fitting lid is essential, and it keeps leaves and light out too. Where a butt sits beside a fence or wall, leave a clear route so hedgehogs and frogs are not funnelled into a dead end.

Provide a way out of the pond itself. A gently sloping stone, a half-submerged log or a planted marginal shelf gives amphibians, hedgehogs and insects a route to climb out rather than becoming exhausted against a vertical liner. This single addition is one of the most valuable bits of wildlife gardening you can do.

Living with dry spells

Mark a level line on the pond edge with a small stone or a buried peg so you can see at a glance how much you have lost. Most ponds are perfectly happy to drop 10–15 cm in summer; the trouble comes when the deepest zone shrinks so much that frogs and dragonfly larvae lose their refuge from herons and heat.

Priorities when water is short: keep the deepest section topped, keep submerged oxygenators under the surface, and keep marginals damp enough to survive. A bog garden beside the pond can be fed with the last of your butt water and will hold moisture for weeks.

If you must use tap water, do it in small amounts and let it stand in an open can overnight where chlorine is the only additive. Chloramine, which many water companies now use, does not dissipate this way and needs a proprietary dechlorinator. A little planning with a butt or two, though, means you will rarely need the tap at all — and your pond will be clearer, cleaner and far livelier for it.