How to Clean a Water Butt (and Keep Your Rainwater Fresh)

If your water butt has turned a bit green inside, or the water coming out of the tap has started to smell musty, it's not a sign anything's gone wrong with your setup — it's just what happens to any container of still water left outdoors for months at a time. The good news is that cleaning a water butt properly takes less than half an hour, needs nothing you can't already find under the sink, and makes a real difference to how pleasant the water is to use on your plants, pots and greenhouse.

Why water butts turn green and start to smell

Three things combine to cause it: sunlight, organic matter, and still water. Algae needs light to grow, so a butt that sits in a sunny spot will turn green faster than one tucked in shade. Leaves, moss and bits of roof grit that wash down the downpipe settle on the bottom and slowly break down, which is usually what causes the musty smell rather than the water itself. And because a water butt has no current or flow running through it, anything that gets in tends to stay put and build up over a season rather than washing away.

None of this means the water is unsafe to use on the garden — rainwater that looks a bit cloudy or has a faint earthy smell is still perfectly fine for watering beds, pots and lawns. It's really about keeping the system working well and pleasant to use, rather than a health concern.

How often to clean a water butt

Once a year is enough for most households, and the best time to do it is either early spring, before the butt starts filling up for the growing season, or early autumn, once the summer's heavy use has emptied it out. If you've noticed a strong smell, visible sludge at the bottom, or the tap running slower than it used to (a sign of debris collecting around the outlet), it's worth doing it sooner rather than waiting for your usual slot.

What you'll need

• A stiff garden brush or an old bottle brush for the inside walls
• A bucket and an old jug or small container for bailing out the last of the water
• White vinegar or a washing-up liquid solution — not bleach, especially if any of the water ends up near edible plants
• A hosepipe for rinsing

Step by step

1. Empty it as low as you can. Use the tap first, then bail or siphon out whatever's left in the bottom — this is usually where the sludge and debris has settled, so don't skip it.
2. Scrub the inside. With the butt empty, go round the walls and base with the brush and a bucket of warm water and washing-up liquid, or a diluted vinegar solution if the smell is stubborn. Pay particular attention to the bottom few inches and around the tap outlet, where grime collects fastest.
3. Rinse thoroughly. Hose it out two or three times until the water running off is clear, tipping the butt on its side if you can to get the last of it out.
4. Check the tap and seals while it's empty. This is the easiest time to spot a tap that's started to drip or seep at the base — if yours has, it's far simpler to swap it now than to do it later with a full butt of water sitting on top of it. A replacement water butt tap with a proper seal takes a couple of minutes to fit and stops both leaks and the slow drip that attracts algae growth around the outlet.
5. Refill and you're done. Pop the lid back on properly — a loose or missing lid is one of the main reasons butts turn green faster than they need to, since it lets in both light and debris.

Stop it building up again

A few small habits make a bigger difference than people expect:

• Keep the lid on and seated properly. It blocks out light (which algae needs) and keeps leaves and insects out.
• Site the butt in shade where you can. Against a north-facing wall or under a tree canopy, a butt will stay cleaner for noticeably longer than one in full sun all day.
• Use the water regularly rather than letting it sit. A butt that's drawn down and refilled through the season stays fresher than one that fills once and sits untouched for months.
• Check the overflow isn't blocked. A clogged overflow keeps old water sitting at the top of the butt rather than letting fresh rain flush through — worth a look alongside your clean.

Cleaning more than one butt

If you've linked several water butts together — whether that's with simple connector hose, as we covered in How to Connect Multiple Water Butts Together, or feeding them from a downpipe diverter like RAINPOD — it's worth cleaning each butt in the chain at the same time rather than just the first one, since sludge and algae can build up anywhere along the line, not only at the point water enters. Disconnect the linking hose first so dirty water from one butt doesn't simply wash straight into the next while you work.

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The short version

A quick clean once or twice a year, a lid that actually fits, and a tap that doesn't drip will keep any water butt running sweet with minimal effort. It's a small job that pays off every time you turn the tap on through spring and summer.