Using rainwater butts to irrigate woodland plants

By How to create a woodland garden in a small space
Comments 35
13 Jun 2026

Using rainwater butts to irrigate woodland plants

Why rainwater suits woodland planting

Woodland plants evolved beneath a canopy, drinking rainfall that has filtered through leaves and bark. That water arrives soft, slightly acidic and low in dissolved minerals — the opposite of what comes out of the tap across much of the UK. If your supply is drawn from chalk or limestone, it carries calcium carbonate and bicarbonates that push soil pH upward and lock up iron and manganese. Rhododendrons, camellias, pieris, Japanese maples, blueberries and most ferns and woodlanders show their displeasure with yellowing between the veins, stunted growth and a general sulk that no amount of feeding will fix.

Collected rainwater typically sits somewhere between pH 5.5 and 6.5, free of chlorine and chloramine, with no lime to clog your soil. It is also a better temperature. A butt warmed by the sun through summer delivers water at ambient temperature rather than the chill of a buried main, which means less shock to fine surface roots. The horticultural benefit is the real prize here; the reduction in mains use is a welcome bonus.

Working out how much you need, and where to put it

The maths is straightforward. Take your roof or shed area in square metres, multiply by the rainfall in millimetres, and multiply by about 0.9 to allow for splashing and evaporation. A modest 20 square metre shed roof, catching 10 mm of rain, yields roughly 180 litres. That is a full standard butt from a single decent shower.

Most gardens manage well with 200 to 250 litres per planting area, though a mature rhododendron border will drink that in a fortnight in July. If you have two downpipes, fit two butts and link them near the base with a short length of hose and a tank connector — the water finds its own level, and you double your storage without doubling the cost.

When siting a butt, think about three things:

Proximity. Put it on the downpipe closest to the plants that need it most. Hauling cans across a lawn soon kills the enthusiasm.
Level ground. A full 200-litre butt weighs over 200 kg. Site it on paving, a paved plinth or firm gravel, never on soft soil or a slope.
Height. A sturdy stand raises the outlet and gives you a little gravity. Every metre of head above the tap buys you roughly 0.1 bar of pressure, which is enough for a can but not much else.

Fitting a diverter, filter and overflow

Cut into the downpipe and fit a rainwater diverter rather than simply parking the butt under a disconnected pipe. A good diverter sends water into the butt when it is empty and lets the surplus carry on down the drain when it is full, so your drainage keeps working in a downpour. Connect it with 25 to 40 mm hose, and keep that run short and sloping downwards.

Filtration is what separates a pleasant, usable store from a stagnant soup. Fit a gutter guard to stop moss and leaves entering in the first place, then a fine mesh filter or leaf trap at the butt inlet. A first-flush diverter, which discards the initial dirty run-off from the roof, is worth the small outlay if your roof collects a lot of bird droppings or tree litter.

Finally, plan the overflow properly — a soakaway, a rain garden or a planted swale. Diverting surplus onto a neighbour's patio or into your own foundations is a mistake you only make once.

Keeping stored water clean

Light and warmth grow algae, so choose an opaque butt and place it in dappled shade if you can. Keep the lid firmly on — it also keeps out children and wildlife, which matters more than most people realise. Rinse the interior once a year in late winter, hosing out the sludge that inevitably gathers. A faint tea colour is fine and comes from tannins in leaf litter; it does the plants no harm at all.

In warm spells, check for mosquito larvae. The reliable answer is physical exclusion: a sealed lid and mesh over every inlet. Empty the whole system and refill if you find a heavy infestation. When filling seedling cans, let the water stand for an hour to come up to temperature and splash it in vigorously to aerate it.

Getting water to the plants

Gravity from a stand-mounted butt will happily fill a watering can or feed a short soaker hose laid on level ground. It will not drive a long drip line properly — those need around 0.5 bar or more. For anything ambitious, a small 12-volt submersible pump, run from a solar panel or a leisure battery, costs little and transforms the job.

Whatever the delivery method, water deeply and less often. Twenty litres tipped slowly around the drip line of a shrub does far more good than a daily sprinkle, because it reaches the deeper roots where woodland plants keep their reserves. Water in the morning or early evening, and mulch generously with leafmould or bark chippings to hold that moisture in place. Water the soil, not the foliage, and keep the crown of ferns and hellebores clear of standing water.

A year in the life of a butt

Autumn is the time to clear gutters, clean the filter and empty and rinse the butt before the winter rains arrive. Through winter, keep the store topped up and check that exposed pipework has not frozen or split. In spring, start drawing on it in earnest and give the first cans to anything ericaceous.

By high summer you may run dry, so keep a mains tap nearby as a fallback and consider adding a third butt next year. Many water companies run discounted butt schemes, which makes expansion cheaper than you might expect. A well-sited, well-filtered butt is one of the simplest improvements you can make to a woodland garden — and one of the most satisfying to watch fill.

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5 Comments

Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply
- John Abraham says Jul 21, 2024 at 10:00am
  
  I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
  Reply
  - Robert Sonnysays Jul 21, 2024 at 10:00am
    
    I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
    Reply
Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply

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