How do I use a wormery?

Setting up and running a wormery for kitchen waste in a small garden or on a balcony, and diagnosing the four things that usually go wrong.

Difficulty: beginner
Time: 30 min
Read: 3 min

Short answer

A wormery uses composting worms — tiger or brandling worms, not earthworms — to turn kitchen waste into a rich compost and a liquid feed. Feed it small amounts often, keep it damp not wet, keep it shaded in summer and insulated in winter, and it needs almost no attention.

A wormery suits exactly the situation a compost heap does not: small quantities of kitchen waste, no space for a heap, and a wish to deal with cooked-food-free scraps without a bin that attracts rats.

What you'll need

Wormery with a tap and tray system
Composting worms
Bedding material such as coir or shredded card
Wormery lime mix (optional)

Step by step

Set it up with bedding first.A layer of damp coir, leaf mould or shredded cardboard for the worms to live in, about 100mm deep. Add the worms and leave them a few days before feeding.
Site it out of extremes.Shade in summer, sheltered in winter. Worms slow down below about 10C and die above about 30C, and a black plastic wormery in full sun cooks.
Feed small amounts often.A handful or two every few days at first. Overfeeding is the commonest cause of failure — uneaten food ferments, goes sour and drives the worms out.
Chop what you add.Smaller pieces are processed far faster. Whole apples and thick peel sit for weeks.
Keep the mix balanced.Add torn cardboard, egg boxes or shredded paper along with the kitchen waste. All-green feeding goes wet, acidic and smelly.
Leave out the wrong foods.No meat, fish, dairy, cooked food or oily leftovers — they attract vermin and go putrid. Go easy on citrus, onion and anything strongly acidic.
Drain the liquid regularly.Open the tap weekly. The liquid is a concentrated feed, diluted roughly one part to ten before use. Left to accumulate it drowns the worms.
Add the lime mix if it goes acidic.Wormeries drift acidic. A scattering of the calcified seaweed or lime mix sold for the purpose, every few weeks, keeps conditions right.
Harvest the compost.In a tray system, worms migrate upwards to fresh food, so the bottom tray can be emptied once they have left it. The result is a dark, rich material used sparingly as a top dressing rather than as a potting compost.
Overwinter it properly.Move it into a shed or garage, or wrap it in an old duvet or bubble wrap. Feeding drops right down in cold weather and that is normal.

Tips

Composting worms are a different species from garden earthworms — digging some up does not work.
The liquid is a concentrate, not a ready feed. Dilute it heavily.
Smelly wormery almost always means overfed, too wet, or both.

Common mistakes

Filling it up on day one — A new wormery has a small population that cannot process much. The excess ferments, the pH crashes and the worms try to escape. Build up gradually over the first two months.
Adding cooked food or meat — It putrefies rather than composting, smells badly, and attracts rats and flies. Wormeries take raw plant material and paper only.

If it doesn't work

Sour or rotting smell

Cause: Overfed and too wet — Fix: Stop feeding for two weeks, add plenty of dry shredded card, and open the tap.

Worms climbing the sides or escaping

Cause: Too acidic, too wet, or too hot — Fix: Add lime mix and dry bedding, drain the liquid, and move it into shade.

Tiny flies everywhere

Cause: Exposed food on the surface — Fix: Bury each feed under the bedding and cover the surface with damp cardboard.

Nothing happening in winter

Cause: Worms dormant in the cold — Fix: Normal. Insulate, feed much less, and expect activity to resume in spring.

Questions people ask

Can I use worms from the garden?

No. Wormeries need composting species — tiger or brandling worms — which live in decaying surface material. Earthworms from the soil will die in a wormery.

What do I do with the liquid?

Dilute it to roughly one part in ten and use it as a general liquid feed. Undiluted it is far too strong.

Is worm compost the same as potting compost?

No. It is very rich and dense, so it is used as a top dressing, mixed at perhaps ten per cent into a potting mix, or dug into planting holes — not used neat in pots.