Using rainwater to top up a garden pond

Why tap water is rarely the best choice

Mains water is treated to drinking standards, not pond standards. Chlorine and chloramine are added to make it safe for us, and neither is kind to the microscopic life that underpins a healthy pond. Then there is the mineral content: tap water across much of southern and eastern England is hard, carrying dissolved calcium and magnesium that gradually push up pH and general hardness. Over a season of topping up, that can shift your water towards conditions that suit fewer plants and leave fish and amphibians coping with swings they would never meet in a natural pool. Add the traces of nitrate and phosphate that find their way into mains supplies, and you are effectively feeding blanketweed and green water every time you reach for the hose.

Rainwater is different. It is soft, mildly acidic across most of the UK, free of chlorine, and almost devoid of the nutrients that fuel algae. It is, quite simply, what your pond was designed to be filled with in the first place.

Setting up a rainwater supply that is safe to use

Not every roof is suitable. Water from slate, clay, concrete tile and most metal roofs is fine. Avoid collecting from roofs with lead flashing, from old bitumen felt, or from any surface recently treated with a moss killer or timber preservative. If your roof is thick with moss, fit a first-flush diverter so the initial run-off — the dirtiest water of all — is sent to a drain rather than your butt.

• Fit a diverter into a downpipe so the butt fills automatically and overflows back into the drain.
• Stand the butt on a firm, level base; a full 200-litre butt weighs over 200kg.
• Link two or three butts together with a connector kit to build a useful reserve.
• Fit a fine gauze or leaf filter to the inlet and keep a lid firmly on top.
• Choose a dark butt, or site it in shade, to limit algae growing in the stored water.

A single butt will not go far on a large pond, so plan for capacity before you need it.

Keeping stored water fresh

Rainwater that sits for weeks can turn anaerobic, especially in warm weather. If it smells sulphurous when you draw it off, do not tip it straight into the pond. Aerate it first by pouring it from a height into a watering can, or use it on the borders instead and let the butt refill. Water that has been sitting in the dark with a lid on, drawn down regularly and topped up by rain, stays perfectly usable.

How to top up without shocking the pond

Amphibians, fish and invertebrates cope with gradual change far better than sudden shifts. Top up little and often rather than in one dramatic gesture, and add the water slowly — a watering can dribbled over the surface, or a hose from the butt tap set to a trickle. Early evening is ideal, when the water is cool and evaporation is at its lowest.

Do the arithmetic before you start. A pond measuring 3m by 2m loses roughly 120 litres for every 2cm of water level, which in a hot spell can mean a butt a week. Store enough, or accept that in a drought year you may need to let the level drop a little. Mark your target level with a stone or a discreet line on the liner so you can top up to the same point each time. If you must use tap water in an emergency, treat it with a dechlorinator — leaving it to stand overnight will not remove chloramine.

Top-ups through the year

• Spring: levels rise naturally; check for overflow and make sure surplus water has somewhere to go.
• Summer: the main topping-up season. Trickle in rainwater weekly, ideally in the evening.
• Autumn: leave the pond to fill itself; clear leaves from the gauze so the butt keeps filling.
• Winter: little or no topping up needed. Fit a gentle spillway so overflow cannot wash fish over the edge.

Rainwater stored in a butt is usually unaffected by hosepipe bans, though it is always worth checking your water company's current rules.

Where the surplus should go

Any excess is a gift for the damp end of the garden. Run an overflow pipe into a bog garden or a shallow marginal shelf planted with Caltha palustris, water mint, Iris pseudacorus and purple loosestrife, and you have a planted area that drinks soft water gladly and gives wildlife somewhere to shelter. Soft rainwater also suits acid-loving shrubs elsewhere in the border, so a spare butt is never wasted. Collect it, store it well, add it gently, and your pond will thank you with clearer water and healthier plants.