Using Rainwater to Top Up Your Pond

Every pond is a small, self-contained world, and every drop you add to it shifts the balance slightly. Tap water is treated to be fit for us to drink, and the things that make it so — chlorine, chloramine in some areas, and the dissolved lime and minerals picked up from our chalk and limestone aquifers — are not what pond plants and pond creatures have evolved with. Rain arrives soft, mildly acidic and free of the disinfectants and nitrates that feed green water and blanketweed.

Rainwater also matches what your pond already experiences naturally. Every shower that falls straight onto the surface is rainwater, so topping up from a butt keeps the water chemistry and temperature roughly where your pond already lives. There is a practical bonus, too: dry spells are exactly when hosepipe restrictions tend to appear, and a full butt is your own private reservoir. Most restrictions still allow a watering can, so a butt, a tap and a bit of patience can keep a pond topped up when the mains tap cannot.

How much water you actually need

The sums are easy enough once you know them. Roughly 1 mm of rain falling on 1 square metre of roof yields 1 litre in the butt. A small shed roof of about 4 square metres will give you 40 litres from a 10 mm shower; a garage roof of 15 square metres will give 150 litres from the same rain. Even a modest downpipe can fill a 200-litre butt in a wet week.

Losses are surprisingly modest, but they add up in a hot spell. An open pond in still air loses perhaps 3 to 4 mm a day in warm weather, and considerably more if you have a waterfall, fountain or splashing stream. On a pond of 6 square metres, 4 mm a day works out at roughly 24 litres daily, or getting on for 170 litres across a dry week. Knowing that stops you panicking at a small drop in the level — and stops you dumping in far more than you need.

Setting up your butts properly

A little preparation at the outset saves a lot of trouble later. The key is to keep the stored water as clean and cool as the rain that fell.

• Fit a gutter guard and a first-flush diverter so the first dirty wash of a shower bypasses the butt, taking moss, grit and bird droppings with it.
• Keep the butt covered and dark. Light and warmth turn stored rainwater green with algae, and an open butt collects leaves that rot into a nutrient soup.
• Site it in shade if you can, and link two butts with a connector so you have more reserve without more clutter.
• Raise the butt on a firm plinth so gravity gives you a decent flow at the tap, and use a downpipe diverter to send winter overflow to the drain.
• Be cautious with runoff from a brand-new felt or bitumen roof, roofs with lead flashing, or any roof recently treated with a moss killer. Let the first few months of rain go elsewhere.

Before you use stored water, smell it. Fresh rainwater has no aroma; a whiff of rotten eggs means it has gone anaerobic in the butt. Aerate it by pouring between buckets and let it stand, or tip it on the borders instead and use fresher water for the pond.

Slow and steady wins every time

This is the rule that matters most. A pond is a stable environment, and sudden change is what harms fish and amphibians — not the water itself. Two dangers come with a fast top-up: a sharp temperature difference and a sudden shift in pH.

Leave a filled watering can or bucket standing beside the pond for an hour or so to equalise temperature. As a guide, a difference of more than 2 to 3°C is enough to shock goldfish, rudd, tench and orfe, and cold butt water tipped into a sun-warmed pond in July can easily be 10°C cooler.

Add the water gently, and never empty a whole butt in one go. Trickle it over a marginal shelf, a flat stone or a floating saucer so it enters the pond in a thin film rather than a torrent. Pouring into the shallow, planted edge is kinder than pouring straight into the deep centre, and the plants will slow the flow and help disperse it. Replace no more than a centimetre or two at a time, then wait an hour or two before adding more. A big deficit is best made up over two or three days rather than one afternoon.

Timing your top-ups

Early morning or after dusk is ideal. Avoid the middle of a hot afternoon, when surface water can be at its warmest, and avoid topping up just before forecast rain — you will simply overflow your hard-won reserve.

Mark your normal water level with a painted line or a settled stone. A steady drop over a hot, breezy week is evaporation and worth topping up. A sudden overnight loss is not evaporation at all, and no amount of rainwater will fix a torn liner or a slipped edge.

Fill your butts during the wet weeks of spring so the reserves are there when you need them in July. In hard winters, if ice has lowered the level and a top-up is genuinely needed, pour onto a floating object or into an ice-free patch — never straight onto the ice, as the shock waves can distress fish.

Water level is not water quality

Do remember that topping up dilutes; it does not clean. If your pond is pea-green, the problem is nutrients, not depth, and rainwater will only help if you are also keeping leaves, uneaten food and lawn runoff out of the water.

Check the pH occasionally, keep a shallow beach and a gently sloped ramp at one edge so hedgehogs, froglets and insects can get in and out, and leave the marginal plants to do their quiet work. Soft rainwater, added slowly and thoughtfully, is one of the kindest things you can give a British garden pond — and the wildlife that has moved in will repay you handsomely.