Setting Up a Rainwater Butt for the Garden

Why a Rainwater Butt Is Worth the Faff

For years I watered my tomatoes with mains water straight from the tap, wondering why the leaves looked sulky and the fruit refused to swell. The answer, of course, was that rainwater is soft, slightly acidic and free of the chlorine and fluoride that keeps our drinking water safe but does nothing for plants. A humble water butt fixed all of that, and it has quietly become one of the hardest-working bits of kit in my garden.

The sums are compelling. A modest house roof can shed hundreds of litres in a single heavy summer shower, and most of it disappears down the drain. Capturing even a fraction of it means you can water during a dry spell without feeling guilty about using treated mains water, and you have a buffer when hosepipe bans arrive. Watering with rainwater also suits our native and ericaceous plants — blueberries, camellias, rhododendrons and heathers all appreciate the softer chemistry, and there is something quietly satisfying about a downpour being an asset rather than an annoyance.

Choosing a Butt and Siting It Sensibly

Most garden butts hold between 100 and 250 litres, with slimline versions designed to tuck into narrow gaps between a wall and a shed. If you have space, link two together with a connector kit so they fill and draw down as one, or consider a bulk tank of 1,000 litres or more on an allotment. Remember that water is heavy: one litre weighs a kilogram, so a full 200-litre butt is 200kg pressing down on whatever is beneath it. It must sit on a firm, level base — paving slabs or a proper stand, not soft soil.

• Position: place it under the downpipe that drains the largest section of roof.
• Height: the higher the outlet, the better your gravity flow. A butt raised a metre gives enough pressure for a soaker hose, though slowly.
• Access: leave room to get a watering can under the tap without doing gymnastics.
• Overflow: plan where surplus water goes before you fill it, ideally towards a border, soakaway or drain — never against the house wall.

Fitting a Downpipe Diverter

You do not need to be handy to do this, but you do need to be careful. Start by checking that the downpipe carries only rainwater and not foul drainage. Then measure and mark a section of downpipe roughly the height of the diverter body, and cut it out with a hacksaw or pipe cutter, leaving clean square edges. Fit the diverter in the gap, securing it to the wall bracket if one is provided, and connect it to the butt's inlet with a short length of hose that slopes gently downwards into the butt.

Most diverters work on a simple principle: when the butt is empty, rain flows into it; once it is full and the water backs up, the surplus carries on down the downpipe as normal. Keep the diverter above the top of the butt so gravity does the work, and check the connection after the first heavy shower rather than discovering a leak in October.

Keeping the Water Clean and the Midges Away

A well-sealed butt stays surprisingly fresh. Choose an opaque, dark-coloured model rather than a translucent one — light encourages algae, and a snug lid keeps out leaves, insects and small wildlife. A mesh filter on the inlet, plus a leaf guard on the guttering, saves hours of scooping. If you can, fit a first-flush diverter, which dumps the first dirty wash of a shower away from the tank.

A few sensible cautions: rainwater is not drinking water, so wash your hands after filling cans and never use it on produce you are about to eat raw without washing it in clean water first. Avoid collecting from roofs with lead flashing or heavily weathered bitumen, particularly for vegetables. Empty and rinse the butt every year or two in autumn, and clean your gutters annually so the tank stays a store of clean water rather than a compost heap.

Getting Every Drop to the Roots

Watering well matters more than watering often. Aim for early morning or late evening when evaporation is at its lowest, and deliver water to the soil rather than the foliage, which reduces disease risk and waste alike. Deep, occasional soakings encourage roots to travel downwards, which makes plants far more resilient in a dry August.

• Mulch beds with garden compost or bark chippings to lock moisture into the soil.
• Sink a hole-punched plastic bottle beside thirsty crops such as tomatoes and courgettes, and fill it from the butt.
• Prioritise new plantings, pots and greenhouse crops, which dry out fastest.
• Group containers together in shade during heatwaves to slow water loss.

Living With the Weather, All Year Round

Rainwater butts are not maintenance-free, but the routine is light. In autumn, fit a leaf guard or temporarily disconnect the diverter before the worst of the leaf fall, and check the overflow is clear. Through winter, the main risk is frost: a full butt that freezes solid can split, so run it down to a low level or drain it entirely if hard frosts are forecast, keeping just a little water to stop the plastic becoming brittle.

Come spring, reconnect everything and give the tank a rinse. During a heavy summer storm, expect it to fill in minutes — that is exactly when an overflow pipe pointing at a thirsty border earns its keep. And when the dry weather bites, you will have a quiet store of soft water waiting, collected free from the sky, ready for the soft fruit, the greenhouse and everything else you are coaxing along. There is no great secret to it: a butt, a diverter, a level base and a little seasonal attention. The garden does the rest.