Why rainwater is worth collecting

Rainwater is the best drink your garden can get. It is soft, slightly acidic, and free from the chlorine, fluoride and lime that come with mains water. That makes it ideal for ericaceous plants such as rhododendrons, camellias and blueberries, which often struggle with tap water. It also saves treated drinking water for drinking and reduces your bills if you are on a meter. During a hosepipe ban, a full water butt can keep containers, seedlings and newly planted shrubs alive when the tap is off limits.

The maths is simple: 1mm of rain falling on 1 square metre of roof yields 1 litre of water. A modest shed roof of 10 square metres will collect 100 litres from just 10mm of rain. Over a typical British summer, even a small butt will fill many times over. That is a lot of free, plant-friendly water that would otherwise run down the drain.

Choosing and siting a water butt

Water butts come in many shapes: tall slimline models for tight spaces, decorative wooden barrels, and even integrated planters that double as a seat. For most gardens, a 200 to 250 litre butt is a good starting point, but you can link several together to increase capacity. Remember that water is heavy: 200 litres weighs 200 kilograms, so the base must be firm and level. A paved patio or a bed of gravel over compacted soil works well. If the butt sits on soil, place it on a couple of concrete slabs to stop it sinking.

Position it close to a downpipe to keep the connection simple. Some shade is useful because it reduces algae growth, but a sunny spot is fine if the lid is opaque. Leave enough room to get a watering can under the tap, and make sure the overflow can drain away from your house foundations. A diverter kit will send excess water down the drain or into a border. If you have children or visiting grandchildren, choose a butt with a lockable lid or place it behind a screen. Never let small children play near an open water butt.

• Size: 100–250 litres for a small garden; link two or more for larger plots.
• Base: firm, level, and strong enough for the full weight.
• Access: easy reach for a watering can, with a tap at a comfortable height.
• Overflow: directed to a drain, soakaway or planted area.

Connecting to your downpipe

The safest and neatest way to fill a butt is with a rainwater diverter. These fit into a cut section of your downpipe and send water into the butt while allowing the rest to continue down the drain when the butt is full. You will need a hacksaw, a drill, and a little patience. Measure twice, cut once. Most kits include a template and a flexible hose. Fit the diverter so that the inlet is above the highest water level in the butt, or use a sealed connection with an overflow. Never seal the downpipe completely: if the butt overflows and the drain is blocked, water can back up into your gutters and roof.

An air gap between the hose and the water surface prevents any chance of contaminated water siphoning back into the mains. If you use a pump to run a soaker hose, make sure the electrical connections are outdoors-rated and protected by an RCD. You do not need an abstraction licence to collect rainwater from your own roof for garden use in the UK, but check local byelaws if you are in a flat or a listed building. One caution: if your roof has lead flashing or old asbestos sheets, avoid using the water on edible crops. Modern tiles and slates are fine.

• Diverter kit: fits most round and square downpipes.
• First flush: optional, but a simple filter or leaf trap keeps debris out.
• Overflow: essential to prevent flooding and backflow.

Keeping it clean and safe

A water butt is a living thing. Over a season it will collect pollen, dust, bird droppings and the occasional leaf. That is fine for plants, but you should give it a clean once a year, ideally in early spring before the growing season. Empty it, scrub the inside with a dilute solution of garden-safe disinfectant or just hot soapy water, rinse well, and let it dry. Check the tap and seals for leaks, and replace any worn washers. A fine mesh over the inlet and a lid that fits snugly will keep out mosquitoes, which breed in still water. If you see wrigglers, empty and refresh the butt; do not add oil or chemicals.

In winter, a full butt can crack if the water freezes solid. Either drain it down to half full, or insulate it with bubble wrap or a hessian sack. If you leave it full, check it after a hard frost. Keep the tap turned off and the lid secure. For safety, fit a child-resistant tap or a lockable lid if little ones are around. Standing water is a drowning risk, so never leave a butt open and unattended.

Getting the most from every drop

Rainwater is precious, so use it where it counts. Water in the early morning or evening to reduce evaporation. Aim at the soil, not the leaves, and use a watering can with a rose for gentle delivery. For containers and hanging baskets, rainwater is far better than tap water because it does not build up lime in the compost. A soaker hose connected to a butt with a small pump will water a border slowly and deeply. If you have no pump, a watering can is still the most efficient way to target thirsty plants.

• Prioritise: new plantings, seedlings, containers, and ericaceous shrubs.
• Mulch: a layer of compost or bark chips locks moisture into the soil.
• Reuse: rinse tools, clean boots, and top up ponds with rainwater.
• Monitor: check levels after a dry spell and refill cans before you need them.

Do not use rainwater from a butt on leafy salads if you are unsure about your roof material, and never drink it. For everything else, it is a superb resource. A single butt will not drought-proof your garden, but it will buy you days of grace during a heatwave and cut your mains use noticeably.

A steady summer supply

Harvesting rainwater is one of those quiet, sensible habits that pays back all summer. Fit a butt now, connect it properly, keep it clean, and you will have soft, free water on tap when the ground is dry and the hosepipe is banned. Start with one butt and see how often you use it. Most gardeners soon wish they had a second. Link them together, add a diverter, and let the rain do the work. Your plants will thank you, and so will your water bill.