Why does my water butt smell and go green?

The two problems have separate causes and separate fixes, which is worth knowing because people usually treat them as one. Algae is a light problem. Smell is an organic matter problem. A butt that is dark and free of leaf litter has neither.

Safety

What you'll need

• A properly fitting lid
• Gutter guard or leaf filter
• Stiff brush and hose
• Shade — a shrub, a screen or a shaded position (optional)
• Fresh water for rinsing (optional)

Step by step

1. Fix the lid first, for safety and for algae.A lid that fits and cannot be lifted off or slid aside by a child is not optional. It also excludes light, leaves and mosquitoes at the same time, which is most of the problem solved.
2. Keep the light out.Algae needs light. A translucent butt in full sun goes green within weeks. An opaque butt, or a position in shade behind a shed or under a shrub, prevents it entirely.
3. Keep leaves out at source.A gutter guard, or a leaf filter in the diverter, stops the material that becomes sludge. That sludge is what makes the water go anaerobic and smell, and it blocks the tap.
4. Clean it out once a year.Late winter, when it is at its emptiest and nothing needs watering. Empty it, scrub the inside with a brush and water, rinse the sludge out of the base and refit the lid. Never try to move a full butt.
5. Check the diverter is doing its job.A proper rainwater diverter fills the butt and returns the excess to the downpipe. A butt fed by a cut pipe overflows down the wall in every storm and never flushes properly.
6. Use the water on the right things.Established plants, shrubs, borders and lawns: fine, and better than tap water for lime-hating plants. Seed trays and seedlings: use mains water instead, because butt water carries the organisms that cause damping off.
7. Draw from the tap, not the surface.The cleanest water is in the middle of the butt. Skimming the surface picks up the algae and the debris.
8. Do not create a spray or mist with it.Legionella multiplies in stagnant warm water and is a risk when it is aerosolised. Water at the base of plants with a can or a hose end, and keep stored rainwater out of pressure washers and fine misting sprays.
9. Leave the mosquito larvae alone if the lid is on.A properly lidded butt does not develop them. If it has, the fix is the lid, not a chemical — and never put anything into a butt whose water goes onto edible crops.
10. Draw it down before a hard freeze.A full butt can split when the contents freeze. Use it or partly empty it before a prolonged cold spell.
11. Check the stand every year.Ground settles. A stand that has started to lean under 200kg needs sorting before it fails, and it needs doing when the butt is empty.

Tips

• Shade and a lid together remove almost all the algae problem, and both are free or nearly so.
• Butts on the shed and the greenhouse get used; a butt at the far corner of the house does not.
• Rainwater for camellias, rhododendrons and blueberries. Mains water for seed trays. That is the whole rule.

Common mistakes

• Leaving the lid off or unsecured — It is a drowning risk to a small child, and it also lets in the light, leaves and insects that cause every other problem here.
• Putting bleach or disinfectant in a butt — It goes onto your plants and into the soil, and it does nothing about the sludge that is causing the smell. Emptying and rinsing is the fix.
• Using butt water on seed trays — It carries the fungi that cause damping off, and seedlings are the least able to withstand them.
• Standing a butt on soft or uneven ground — A full butt is over 200kg. As the ground settles it leans and eventually goes over, which is dangerous as well as messy.

If it doesn't work

Questions people ask