Why rainwater is worth the space

Rainwater is the single best thing you can give your garden, and harvesting it is one of the simplest habits a wildlife-friendly garden can adopt. It is soft, slightly acidic and free of the chlorine, chloramine and fluoride that come with tap water. Plants notice the difference: lime-hating shrubs such as blueberries, camellias, rhododendrons and heathers will sulk if you water them from the mains, and ferns and woodland plants look noticeably fresher after a summer of stored rain.

The practical argument is just as strong. During a hosepipe ban, mains water through a hose is usually restricted, but water you have collected yourself can generally still be used — always check the details of any local restriction, as rules vary. And a surprising amount is involved: roughly one litre of water lands on every square metre of roof for every millimetre of rain. So a modest 100 square metre roof will collect something in the region of 800 litres from a single 10 mm shower. Spread over a dry July, that is several weeks of watering for pots, beds and veg.

Choosing and siting your containers

A standard water butt holds between 100 and 200 litres, which is a good starting point for most gardens. For something narrower, slimline models fit neatly along a wall or behind a shed. If you have more space and a decent roof, linking two to four butts in series gives you serious capacity without a huge footprint. Whatever you choose, follow three rules.

• Stand it on something solid. Water weighs a kilogram per litre, so a full 200 litre butt is a quarter of a tonne. Paving slabs on level ground will do; decking absolutely will not.
• Leave room for a watering can. You want 20 to 30 cm of clearance beneath the tap, which is why a proper stand matters more than most people expect.
• Shade it if you can. Warm, sunlit water grows algae and turns green. A north-facing wall, a trellis of climbers or a shrub planted alongside will keep it cooler and cleaner.

Opaque, dark-coloured butts stay cleaner than translucent ones. Larger tanks of 650 litres or more suit big plots, but they are heavy and best filled in place.

Getting the rain from roof to butt

The simplest route is a downpipe diverter, a small kit that fits into your existing downpipe and sends water into the butt until it is full, after which the flow carries on down the drain as normal. It is a half-hour job with a hacksaw and a screwdriver, and it saves cutting the pipe in a way you cannot undo.

A fine mesh or leaf guard over the inlet keeps moss, twigs and bird muck out. If your roof or gutters are particularly dirty, a first-flush device sends the first few litres of a shower to waste, which is often the grimiest. Give some thought to the overflow too: when the butt fills, the excess has to go somewhere. Linking it to a second butt makes use of it, but directing it to a planted bed or soakaway, well away from the house foundations, is the safer choice — especially on clay.

Keeping stored water clean

Clean water is not fussy water. Keep the lid on to exclude light, keep mesh over the inlet and overflow, and keep the tap closed when you are not using it. Give the butt a proper clean once a year, in late winter or early spring before it fills again: drain it, scrub the inside with a stiff brush and a diluted vinegar solution, rinse thoroughly, and leave it to dry. Skip the bleach, which will do your plants no favours.

• Earthy-smelling water is normal and perfectly fine for ornamentals.
• A rotten-egg smell means it has gone anaerobic. Use it on established shrubs and borders, not on edible roots or seedlings.
• Never add plant food or fertiliser to the butt — it feeds algae and turns the whole store green.
• Treat it as non-drinking water and keep lids weighted and secure if there are small children or pets about.

Making every drop count in a drought

Use a watering can rather than a hose. It is slower, but that is the point: you aim precisely at the roots, you know exactly how much each plant has had, and nothing is wasted on paths or leaves. Water in the early morning or evening, when evaporation is lowest.

Prioritise ruthlessly during a dry spell. Lawns will go brown and recover on their own when rain returns. Concentrate your stores on new plantings, pots and containers, vegetables, and any lime-hating plants that struggle. Mulch beds with 5 to 8 cm of compost, leafmould or woodchip to lock moisture into the soil, and group pots together so they shade one another. Watering less often but more deeply encourages roots to go down rather than sit near the surface. A gravity-fed seep hose, run from a butt on a raised stand, will trickle water slowly along a border while you get on with something else.

The best time to prepare for next summer is this winter. Empty butts in spring, add more capacity while the wet months are still ahead of you, and by the time the ground starts cracking you will have a garden that barely notices.