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Why rainwater is worth the effort

Tap water keeps plants alive, but it is rarely what they would choose. Much of the UK sits on chalk or limestone, so mains water arrives hard and alkaline, with chlorine and fluoride added along the way. Tomatoes, cucumbers, blueberries, camellias and rhododendrons all prefer something softer and slightly acidic, and rainwater is exactly that. It is also free, and in a greenhouse it arrives at air temperature rather than the chilly ten degrees or so that comes out of a cold tap in April.

There is a practical angle too. A greenhouse roof is a superb rain catcher: a modest 6ft by 8ft structure presents roughly 4.5 square metres of glazing. With average UK rainfall of around 800 to 1,000mm a year, that roof will shed something in the region of 3,500 to 4,500 litres annually — enough to irrigate a busy greenhouse border almost all season.

Choosing and siting your water butt

For most domestic greenhouses, a 200 to 250 litre butt hits the sweet spot between capacity and footprint. Slimline models squeeze into narrow gaps beside the door; square ones tuck neatly against a gable end. If you have the room, two butts linked together give you far more resilience through a dry July.

• Height matters. Raise the butt on a sturdy stand or a couple of concrete blocks. Every metre of head above the dripper gives you roughly 0.1 bar of gravity pressure — enough for soaker hose, but only if you have the height.
• Shade the water. A dark, lidded butt in partial shade stays cooler and grows far less algae than a translucent one baking in full sun.
• Plan the overflow. Site the butt where a gentle overflow can drain into a border, soakaway or rain garden rather than pooling against the greenhouse base.

Connecting to the greenhouse downpipe

The neatest solution is a rainwater diverter, a small fitting that clamps onto the downpipe. You cut a section out, slot the diverter in, and run a short length of hose to the butt inlet. Most diverters send water to the butt until it is full, then simply pass the rest down the original downpipe — no fiddling about when it rains for three days straight.

Two additions are worth the small outlay. A gutter guard or leaf strainer keeps moss and grit out of the butt, and a first-flush diverter discards the first few litres of a shower, which carry most of the roof grime, before cleaner water enters storage. If your greenhouse has no downpipe because water simply runs off the base, a short length of guttering along the low side feeding a butt will do the same job.

Getting water to the beds

You have two straightforward options, and plenty of gardeners use both.

The watering can. A tap fitted near the base of the butt, with a short standpipe underneath, makes filling a can quick and clean. Water early in the morning, aim at the soil rather than the foliage, and you will cut both evaporation and the risk of botrytis in tomatoes and cucumbers.

A gravity drip line. Run 13mm drip or soaker hose from the butt tap along the beds, with a simple inline tap to control flow. At half a metre to a metre of head, pressure-compensating drippers work best because they deliver evenly along the whole run. A battery timer on the outlet turns the whole thing into a system that waters while you are at work, though check it weekly — drip lines clog, and a dry border is easy to miss under mulch.

For pots and growbags, capillary matting laid on the bench and fed by a wick from a small reservoir is a quiet, reliable alternative that keeps compost evenly moist without any electricity at all.

Keeping the system clean and healthy

Rainwater stored for months will gradually develop a film of algae and a layer of sediment. Once a year, in late winter, drain the butt, scoop out the sludge, and rinse it with a stiff brush and plain water — no detergent. Clean the gutter and strainer at the same time, and check that the diverter is still seated properly on the downpipe.

Keep the lid firmly on. Standing water plus an open top equals mosquito larvae by June, and a lid also stops leaves and sunlight doing their worst. If you use a hosepipe from the butt, drain it after use so it does not become a stagnant sleeve of old water.

Making it last through the year

In a dry spell, even a generous butt runs low. A simple mains top-up valve fitted inside the butt will trickle water in only when the level drops, so rainwater is still used first but the beds never go short. If you would rather stay off the mains entirely, mulch heavily with compost or bark, group pots together to shade each other, and prioritise your thirstiest crops.

Before the first hard frost, decide what to do with the butt. A part-empty butt that freezes can split, so either drain it down and leave the tap open, or keep it topped up and well lagged — a full butt freezes far more slowly than a half-empty one. Come February, a quick clean and a check of the connections will have the whole system ready for another season of soft, free, tepid rainwater flowing exactly where your plants need it.