Harvesting Rainwater for Garden and Allotment Use

Why Rainwater Is Worth Catching

If you garden on a moorland edge or down a country lane, you will know how quickly the ground dries out when a settled spell arrives. Thin, peaty or gritty soils over shale and granite can go from squelchy to dusty inside a fortnight, and that is exactly when the mains tap starts to feel like a guilty habit. Rainwater is free, soft and close to air temperature, so it shocks roots far less than a blast of cold tap water. It is also mildly acidic, usually around pH 6 to 6.5, which suits rhododendrons, camellias, blueberries and hydrangeas better than the hard, limy water supplied across much of lowland Britain. On high ground where rainfall is generous, letting all of it run straight into the drain is a waste you can fix in an afternoon.

Do the Sums Before You Buy

The maths is simple enough to do on the back of an envelope. One millimetre of rain falling on one square metre of roof gives you one litre. A modest shed measuring 3m by 2.5m has 7.5 square metres of catchment, so a 10mm shower delivers roughly 75 litres, and a wettish year of 1,000mm could yield around 7,500 litres from that one roof.

In practice you lose some to splashing, evaporation and overflow once the butt is full. A 200 to 250 litre butt will fill surprisingly fast — often in a single decent downpour off a house roof — which is why most gardeners end up with two, three or four linked together. Run a short length of hose from the overflow of the first into the top of the second, and so on down the line, so they fill in sequence rather than all at once. During a hosepipe ban, stored rainwater is usually still fair game, though it is worth checking the wording of any local restrictions.

Fitting a Butt to a Downpipe

This is a genuinely straightforward job, but the order matters. Get the base right first and everything else follows.

• Stand it up. Every metre of height gives you roughly 0.1 bar of pressure, so a butt sitting on three engineering bricks flows noticeably better than one on the ground. Use a purpose-made stand if you prefer, but make sure it is level and firm — 250 litres weighs a quarter of a tonne.
• Cut the downpipe carefully. Mark a section around 10 to 15cm long and remove it with a hacksaw, keeping the cut comfortably above the top of the butt so you can lift the lid.
• Fit a rain diverter rather than feeding the pipe straight in. A diverter sends water into the butt and, once it is full, pushes the surplus back down the original drain. It also keeps debris out and lets you detach the butt without leaving a gaping hole in the pipework.
• Deal with the overflow. Run a hose from the overflow spout to a second butt, a soakaway, or a thirsty border. Never let it discharge against the wall of a house or shed.

Filtering and Keeping the Water Sweet

Leaf litter and moss are the enemies of clean stored water. A gutter guard is money well spent, and most diverter kits include a mesh strainer that catches the worst of it. If your roof is mossy or sits under trees, add a simple first-flush arrangement so the dirtiest initial run-off is diverted away before it reaches the butt.

• Keep the lid on and the light out, which stops algae turning the water green.
• Empty, scrub and rinse each butt once a year, ideally in late winter before the growing season starts.
• Water at the base of plants rather than over the leaves, particularly for salads and anything you eat raw.
• Treat it as garden water, not drinking water, and wash your hands after filling cans.

Getting It to the Roots

Gravity is a modest friend: a butt on a 40cm stand will happily feed a short hose, a soaker hose or a low-pressure drip line, but it will not drive a sprinkler. For anything longer, use watering cans or a small submersible pump. Whichever route you take, water deeply and less often rather than little and frequently, and spread a generous mulch of bark, compost or grass cuttings over the soil afterwards. A good mulch can cut evaporation losses by half, which effectively doubles the size of every butt you own.

Winter Care and the Long View

Moorland winters are hard on plastic. A full butt that freezes solid can split, so through the coldest months either drain them down or leave them part-filled with the tap open, and lag exposed pipework. Check the diverter and overflow after gales, clear leaves from gutters before the autumn rains arrive, and make sure lids are weighted down against the wind. Look after the setup and it will quietly repay you every summer: softer water for lime-hating plants, a buffer against dry spells, and a lower water bill into the bargain.