How to Set Up a Water Butt (and Keep It Safe for Seedlings)

Fit a butt to a downpipe with a diverter or downpipe kit, put it on a firm, level stand with room to fit a watering can under the tap, cover it with a close-fitting lid, and clean it annually. Rainwater suits most plants. Use mains water for seed trays and pots where you can, because the risk to seedlings is debris and pathogens in the butt, not rain itself.

How much rain you will catch in Scotland

Average annual rainfall runs from under 700mm on the East Lothian, Fife and Moray Firth coasts to over 3,500mm over the higher parts of the west Highlands, against about 500mm in the driest parts of eastern England. Unlike other parts of the UK, Scotland tends to stay under the influence of Atlantic depressions for much of the summer too. Autumn and early winter are the wettest seasons. Late winter and spring are normally the driest in the east, and April to June in the west, so a butt can run low just as seedlings and new plantings need water.

Sizing

Even a small roof can collect large volumes of water: in dry south-east England, a small shed roof collects 2,500 to 3,500 litres a year, enough to fill around 10 to 15 standard 240-litre butts.

To add capacity, join multiple butts with connector kits or siphons rather than fitting one oversized tank. That also spreads the weight, since a full 200-litre butt holds 200kg of water alone.

Setting it up

You need a gutter with a downpipe or a rainwater downpipe kit, and you can collect from the roof of a house, garage, greenhouse or shed. Roofs with a decent pitch and a relatively clean surface give the cleanest water. A filter stops leaves and debris entering the butt. Place the butt on a firm, level stand or on bricks, so a watering can fits beneath the tap. If space is short, slimline and decorative models are available.

With a diverter kit, the downpipe is cut about 3cm below the top of the butt, so the butt fills completely without water backing up the pipe (Saga Magazine, July 2024). Once the butt is full, the extra water goes back down the drain, so it does not overflow. Check the instructions that come with your kit for the exact measurement.

Keeping it clean

Cover the butt with a close-fitting lid to keep rain-borne spores of plant diseases out of the water and to discourage algal growth. Uncovered water can stagnate and harbour harmful micro-organisms, and rainwater collected in open tanks can contain damping offA fungal disease that kills seedlings at soil level, usually caused by overwatering, poor airflow, or reused unsterilised compost or trays. Full definition → organisms such as Phytophthora, Pythium, Rhizoctonia and Fusarium. Clean the butt annually, using warm water and a little washing-up liquid, to prevent disease build-up and smells. If the water smells, empty and clean the butt rather than adding chemicals, and avoid water containing bleach, disinfectants, dishwasher salt or harsh cleaners.

Seedlings

For seedlings in pots and trays, use mains water if possible. If you use rainwater, keep the water butt covered and clean it regularly. Utah State University Extension advises using only clean water for irrigation, not pond water or run-off that may have touched soil carrying damping off pathogens. For the other ways to prevent it, see the damping off page.

Rainwater against mains water

Rainwater suits most plants, including acid-lovers like rhododendrons and camellias. The minerals in mains water, especially in hard water areas, can raise the pH of soil and affect nutrient availability, and tap water is often treated. See tap water against rainwater.

Winter

Water in pipes can freeze, expand and crack them, according to Scottish Water, which advises insulating exposed pipes and tanks, especially in outbuildings. A full butt, tap and diverter are exposed in the same way, so check them before a hard frost. It is usual advice to drain the butt and disconnect the diverter in freezing weather, and to keep a secure lid on so children and pets cannot fall in.

Common questions

Sources