Oliver Radcliffe
Why Rainwater Beats Tap Water in the Garden
There is something deeply satisfying about watching a summer storm roll in and knowing that every drop landing on your roof is free, soft and ready for the beds. Rainwater is naturally slightly acidic and free from the chlorine, fluoride and chalky minerals that come with mains supply in many parts of the UK. That makes it a favourite for ericaceous plants such as rhododendrons, camellias and blueberries, and it helps avoid the white crust that tap water can leave on pots and greenhouse staging.
Beyond plant health, harvesting rain is a practical way to cut your metered water bill and ease pressure on reservoirs during dry spells. When hosepipe bans arrive, a full water butt often means the difference between a thriving allotment and a parched one. Even if you are not on a meter, storing rain reduces the energy used to treat and pump drinking water.
Choosing and Siting a Water Butt
Water butts come in many shapes, from slimline models that tuck behind a shed to rugged 250-litre drums. For a typical garden, 100 to 200 litres is a good start; on an allotment, linking two or three butts together gives you a buffer that lasts through a fortnight of dry weather. Look for an opaque body, a secure child-safe lid, a sturdy tap and a wide opening for cleaning. A stand or plinth raises the tap so you can fill a watering can easily, and it helps if you later attach a soaker hose.
Site your butt on firm, level ground close to a downpipe, but not so close that it blocks access or overhangs a path. Remember that a full butt is heavy — a 200-litre one weighs over 200 kilograms — so a paved or concrete base is safer than soft soil. If you have a greenhouse, a butt inside or beside it keeps water handy and slightly warmer than an exposed one.
Connecting to a Downpipe
The neatest way to fill a butt is with a rainwater diverter, a simple fitting that replaces a section of downpipe and sends water down a flexible hose into the butt. You will need a hacksaw, a drill, a spirit level and a marker. First, hold the diverter against the downpipe and mark the cutting lines, remembering that the butt inlet must sit below the diverter outlet. Cut away the marked section, smooth the edges, and slot the diverter into place. Then drill a hole for the hose, connect it to the butt inlet, and check that the flow runs downhill.
Most diverters include an overflow that returns excess water to the downpipe once the butt is full, but it is wise to fit a separate overflow pipe that directs water away from your house foundations. On an allotment shed, the same principle applies: a short length of guttering and a downpipe is all you need. If you cannot cut the downpipe, a rain saucer or open gutter can catch water, though less efficiently.
Keeping Algae at Bay
Stored rainwater can turn green if light reaches it, so an opaque butt with a tight lid is your first defence. Keep the butt in shade if possible, and fit a leaf guard or a piece of fine mesh over the inlet to stop leaves and debris from rotting in the water. A mesh bag of barley straw floated in the butt inhibits algae naturally; replace it every few months.
Once a year, in late winter or early spring, drain and scrub the butt with a dilute solution of garden disinfectant, then rinse thoroughly. Clean the gutters and downpipe too, as a build-up of moss and bird droppings will feed algae and clog the diverter. If you notice a smell, empty the butt and start again — stagnant water is not kind to plants.
Using Stored Water Wisely
During dry spells, water early in the morning or late in the evening to reduce evaporation. Aim for the soil, not the leaves, and give plants a thorough soak rather than a daily sprinkle; this encourages deep roots. Prioritise new plantings, containers, hanging baskets and thirsty crops such as courgettes, beans and tomatoes. Rainwater is also ideal for filling a pond, topping up a water feature, or rinsing tools and boots.
If you have a large allotment, consider a gravity-fed drip line or soaker hose attached to the butt via a timer. Even a simple watering can routine can be made easier by keeping a second can next to the butt so you always have one warming in the sun. Rainwater is safe for edible crops; just wash produce as usual.
Practical Maintenance and a Little Maths
To work out how much rain you can collect, multiply your roof area in square metres by the rainfall in millimetres. One millimetre of rain on one square metre gives one litre. So a shed roof of 3 metres by 2 metres (6 square metres) will collect 60 litres from a 10mm shower.
- Check taps, seals and connectors each spring for leaks.
- Insulate or drain plastic butts before a hard frost to prevent cracking.
- Secure lids firmly, especially if children or pets are about.
- Keep a spare diverter and hose clip in the shed for quick repairs.
- Label your butts if you use one for drinking water — though most gardeners keep them for plants only.
With a little planning, your water butts will quietly fill themselves all winter and see you through the driest weeks of summer. There is no better feeling than watering your runner beans with the sky's own supply.
