Why a water butt is worth the faff

Rainwater is the closest thing to an ideal drink for your plants. It is soft, free of the chlorine and lime that come out of the tap, and gently acidic, which suits acid-lovers such as camellias, rhododendrons and blueberries far better than mains water. It also arrives free of charge, which matters more than ever for gardeners on a meter — and hosepipe bans, when they come, rarely stop you using a watering can filled from a butt you have already installed.

The numbers are encouraging. Roughly 1mm of rain falling on 100 square metres of roof yields about 100 litres, so even a modest terrace roof will top up a 200-litre butt in a couple of decent showers. That is about twenty watering cans, enough for a fortnight of careful watering on a small border during a dry spell.

Choosing the right spot and the right size

Position is everything. Find a downpipe within easy reach of the beds you actually need to water, because carrying cans any distance quickly becomes a chore. A spot on a north or east wall keeps the water cooler and slows the growth of algae inside the butt.

• Ground: a full butt is heavy — 200 litres weighs around 200kg, more than two adults. Stand it on firm, level paving slabs set on a bed of sand or grit, never directly on soil or decking.
• Height: a purpose-made stand, or a couple of concrete blocks, raises the tap by 30–40cm. That gives you room to slot a watering can underneath and adds a little gravity pressure if you plan to attach a hose.
• Size: 100–250 litres suits most gardens. If you have the space, two linked butts give you far more storage without a huge footprint.
• Access: leave room at the front for the tap and the lid, and check the butt does not block a path or a window.

Fitting a diverter kit

A diverter is what makes the whole system work. It fits into the downpipe, catches part of the flow and sends it sideways into the butt through a short length of hose. When the butt is full, the water simply carries on down the downpipe as normal, so you never flood the garden.

Most kits suit both 68mm round or square and 110mm downpipes, and come with a hole saw for the butt. Measure twice, then cut the downpipe with a hacksaw, allowing a couple of millimetres of clearance so the diverter slots in comfortably. Fit the diverter so its outlet sits slightly higher than the top of the butt, and keep the connecting hose short and with a gentle fall — a sagging loop will hold water and silt up over time. Drill the inlet hole in the butt just below the top rim, then seal the hose with the washer provided.

Do not forget the overflow. Many butts have a built-in overflow outlet you can run to a drain, a soakaway or a second butt. If yours does not, and the diverter is fitted correctly, the downpipe itself will handle the excess.

Taps, stands and getting the water out

Fit the tap as low as you can while still leaving room to fill a can — around 10cm above the base is typical. On a plastic butt this usually means drilling a second hole and sealing the tap with the supplied washer; check the tap is on tight before you fill. If you want to run a hose, a small submersible pump or a gravity feed from a raised butt both work, though gravity alone will only push water a metre or two along a level run.

A simple filter or a mesh guard over the inlet keeps leaves and grit out, which protects the tap from blocking and keeps the water cleaner for the watering can.

Making the most of every drop on your borders

Rainwater is precious, so use it where it counts. Aim for the roots rather than the foliage — a slow, deep soak once or twice a week builds sturdier plants than a daily sprinkle, which only dampens the surface and encourages shallow rooting. Water in the early morning or the evening to cut evaporation.

Mulching with 5–7cm of bark chippings or garden compost after a good watering will hold that moisture for far longer, and it is the single best way to make a butt last through a dry spell. Prioritise new plantings, containers, hanging baskets and thirsty crops such as courgettes and tomatoes. Established shrubs and hardy perennials will usually manage with far less.

A little maintenance goes a long way

• Keep the lid on to stop debris and mosquito larvae getting in, and to keep children and pets out.
• Clean the butt and flush the tap once a year, ideally in spring, and clear any silt from the bottom.
• Check the diverter and inlet filter each autumn once the leaves have fallen.
• In a hard winter, leave the butt part-empty so any ice has room to expand rather than split the plastic.

Set up properly, a water butt quietly pays for itself in healthier plants and lower bills, and there is something quietly satisfying about watering the garden with the last shower's rain.