Harvesting Rainwater for Wildlife Ponds and Planters

Why rainwater is worth the effort

If you garden for wildlife, the water you give your plants matters almost as much as the plants themselves. Tap water arrives treated and safe for us, but it carries chlorine or chloramine, plus varying levels of nitrate, phosphate and dissolved calcium. None of that troubles a thirsty fern much, but in a pond it feeds algae, and in a planter it gradually makes your compost more alkaline and mineral-heavy.

Rainwater is soft, slightly acidic, free of those additives and — best of all — free. It's also the water our native woodland plants evolved with. Bluebells, wood anemones, ramsons and hart's tongue ferns all expect a steady supply of soft, humus-filtered rain rather than the hard, chalky stuff from the mains. A water butt connected to a downpipe is the simplest way to give them exactly that.

Choosing and siting a water butt

Work out roughly how much you can collect before you buy. One millimetre of rain falling on one square metre of roof yields about one litre. A modest 40 square metre shed roof, then, delivers roughly 400 litres from a single 10 mm shower — far more than a standard 200-litre butt holds, so an overflow is not an optional extra. For most gardens a 100–250 litre butt is plenty; if you have space, link two together and you'll ride out a dry fortnight far more comfortably.

Put it where you'll actually use it, not simply where the downpipe happens to be, and stand it on a firm, level base. Raising it 30–40 cm on a proper stand means a watering can slides easily under the tap. Keep it out of full sun if you can: cooler water grows less algae and is kinder to plant roots on a hot afternoon.

Connecting it to the downpipe

Most butts can be fitted with a rainwater diverter, which replaces a short section of downpipe and sends water sideways into the butt. It's a half-hour job with a hacksaw and a steady hand.

• Cut out a section of downpipe using the diverter's template, then fit the unit so water runs into the butt rather than past it.
• Fit a gutter guard or leaf screen at the top of the downpipe to keep moss, twigs and bird droppings out.
• Check the butt has a tight-fitting lid and a mesh filter over the inlet — this keeps debris out and, more importantly, stops wildlife getting in.
• Run the overflow hose to a border, rain garden or soakaway rather than straight into a drain, so surplus water feeds your soil instead of vanishing.

A linking kit at the base of two butts keeps their levels equal, doubling your storage without a second diverter.

Watering woodland plants in planters

Woodland species in containers are the thirstiest plants in most gardens, and the ones least tolerant of hard water. Think bluebells, wood anemone, ramsons, primrose, wood sorrel, lily of the valley, foxgloves, honeysuckle, and ferns such as hart's tongue and lady fern. All of them relish a moist, humus-rich compost that never quite dries out — and all of them sulk when tap water steadily raises the pH around their roots.

Top the compost with a mulch of leafmould or composted bark to hold moisture, then water in the evening with rainwater at ambient temperature. As a rough rule, a large planter in dappled shade wants a full watering can every evening in high summer, and far less in spring and autumn. Rainwater lets you water generously without any long-term penalty.

Topping up a wildlife pond

Ponds lose water steadily to evaporation through summer, and that's when the temptation to reach for the hosepipe is strongest. Every top-up with tap water adds nitrate and phosphate — a direct invitation to blanketweed and green water — and chloramine, unlike chlorine, doesn't simply gas off as the water stands.

Plumb a butt reasonably close to the pond and use a short length of hose to trickle water in slowly, ideally in the evening. Small, frequent additions are far kinder than a sudden deluge of cool water, which can shock newt tadpoles and the invertebrates living in the silt. Keep the lid firmly on, and if any open water is exposed, provide a ramp or a floating piece of wood so that hedgehogs, frogs and birds can climb out if they fall in.

Keeping the water clean through the year

An annual clean keeps everything sweet. Empty the butt in late winter, scrub the inside with a stiff brush and a dilute vinegar solution — never detergent — and rinse well before refitting. Opaque butts discourage algae far better than translucent ones, and fine mesh over the inlet stops mosquitoes laying in the first place. Any larvae that do appear are free food for newts and dragonfly nymphs, but a good lid is simpler.

Use the water rather than letting it stand for months, and clear your gutters before autumn leaf fall so the first heavy rain isn't washing a winter's worth of debris into your store. During a hosepipe ban, a full butt becomes genuinely precious: prioritise the pond and your most valuable planters, and let the lawn fend for itself. Fill a can, walk the garden, and enjoy the fact that the sky has just done the watering-up for you.