Why rainwater is worth collecting

Rain is, quite simply, the best water your garden can get. It is soft, slightly acidic and free of the chlorine and dissolved lime that come with tap water, which makes it ideal for lime-hating plants such as blueberries, camellias, rhododendrons and most ferns. Stored rainwater also feels less of a jolt to seedlings and greenhouse crops than a can of cold mains water straight from the tap.

The maths is compelling. Every square metre of roof gathers roughly one litre of water for each millimetre of rain, so a modest shed roof of six square metres will produce about 60 litres from a single 10mm shower. Even a small 100-litre butt will fill several times over in a wet British winter, and that is water you would otherwise be paying for and sending down the drain.

Choosing a butt and siting it well

Water butts come in sizes from around 100 to 500 litres, and it is worth buying the largest you can reasonably fit. A 200-litre butt sounds generous until you have emptied it over a border in twenty minutes. Slimline and square designs tuck neatly against a wall, while decorative wooden-effect or terracotta styles are less of an eyesore on a patio.

Position matters as much as capacity:

• Put the butt on a firm, level base — paving slabs or a purpose-made stand, not soft soil.
• Raise it 30–45cm above ground so a watering can fits underneath and gravity does some of the work.
• Choose a downpipe fed by the largest roof area you can reach, ideally close to the beds and greenhouse.
• Keep it out of full sun where possible; shade reduces algae and keeps the water cooler.

Connecting to a downpipe

The neatest method is a rainwater diverter kit, available from most garden centres and DIY shops. It fits into the downpipe, usually after cutting out a short section with a hacksaw, and channels water through a flexible hose into the top of the butt. When the butt is full, the diverter simply sends the surplus on down the drain, so you never have to remember to switch anything over.

A few practical points make the job much easier:

• Mark the downpipe carefully before cutting, and check the diverter's instructions for the height it needs.
• Cut square and clean, then smooth the edges so the seals seat properly.
• Drill or cut the inlet hole in the butt lid to suit the hose, and fit the supplied grommet.
• Make sure the hose runs downhill all the way — a dip will trap water and silt.
• If you rent, or want something reversible, a simple downpipe-to-butt hose with a rain trap works almost as well.

Check the joint after the first heavy rain. A small weep is easy to fix with a twist of the connector, but a badly seated diverter can wash soil away from the base of the wall behind the butt.

Linking butts, overflow and safety

Once you have one butt, a second is almost inevitable. The simplest arrangement is to cascade them: run the overflow from the first into the top of the second, slightly lower, so the first fills completely before the second starts to take water. Alternatively, link them at the base with a connector kit so the levels equalise, though this needs both butts at exactly the same height.

Always fit a lid. It keeps out leaves, limits algae and, most importantly, stops small children and wildlife falling in. If you have hedgehogs or birds in the garden, float a piece of timber or a rope in the water so anything that tumbles in has a way out. Fit a tap rather than scooping with a bucket — it is cleaner, faster and far kinder to your back.

Getting the most from stored water

Rainwater is precious in a dry spell, so use it where it counts. Give priority to pots, hanging baskets, the greenhouse and newly planted shrubs, which dry out fastest and suffer most. Beds with established plants can wait a little longer.

Water deeply and less often rather than little and often, aiming at the root zone rather than the leaves. Early morning is the best time, before the sun lifts evaporation. A layer of mulch over damp soil will hold that moisture for days, and a seep hose run from a raised butt can irrigate a whole border with barely any effort.

For greenhouse plants, fill your cans the day before if you can, so the water reaches air temperature. Cucumbers, tomatoes and tender seedlings in particular dislike a cold shock at the roots.

Looking after the butt through the year

Autumn is the moment to check the inlet and clear fallen leaves, or fit a leaf trap above the diverter. Once a year, drain the butt, scrub the inside with a stiff brush and a dilute solution of vinegar, then rinse well — never use bleach or detergent if the water is going on plants. Keep the lid on and a little shade around it to discourage green algae.

In a hard frost, plastic can crack as the water inside expands. Either drain the butt completely before winter or leave it part-filled and disconnect the hose so ice has somewhere to push. A few minutes of attention now means a full butt and a healthy garden when the dry weather finally arrives.