Worm composting — also called vermicomposting — is composting with the help of worms. Instead of a large outdoor heap, kitchen scraps go into a bin where composting worms eat them and turn them into vermicompost: dark, crumbly, nutrient-rich material that is one of the best soil improvers there is. Worm composting is fast, odourless when done well, and ideal for small gardens, flats and anyone who wants to recycle kitchen scraps without an outdoor compost heap. This guide covers how worm composting works, how to set up a worm bin, what to feed the worms, and how to harvest and use the finished vermicompost.
Worm composting works because composting worms — usually red wigglers — eat organic matter and excrete castings that are rich in nutrients and beneficial microbes. A worm can eat roughly its own weight in food each day, so a bin with a kilogram of worms can process about a kilogram of scraps a day once it is established. The worms live in damp bedding, eat food buried in it, and the bedding gradually turns into vermicompost. Unlike a hot compost heap, a worm bin does not heat up — it works at room temperature — so it can live indoors, in a garage, on a balcony or in a shed. It produces no smell when managed well, because the worms eat the food before it rots. Our composting basics guide covers ordinary composting, and compost bins covers outdoor bins. If you are unsure about worm composting in your area, check your local extension service for local advice.
Setting up a worm bin
You can buy a worm bin or make one from a plastic storage box. A home-made bin needs a box about 40–60 cm long, 30–40 cm wide and 30 cm deep, with a lid, drainage holes in the bottom, and air holes in the sides and lid. Put the box on a tray to catch any liquid that drains out. Fill it two-thirds full with damp bedding: shredded newspaper or cardboard, torn into strips and soaked in water, mixed with a handful of garden soil or finished compost and a few handfuls of dry leaves. The bedding should be damp like a wrung-out sponge, never wet and never dry. Add the worms — red wigglers, available from worm suppliers or garden centres — and let them settle for a day before feeding. Put the bin somewhere sheltered and frost-free, ideally between about 15°C and 25°C: a garage, a shed, a balcony in mild weather, or indoors. Worms slow down in the cold and die in frost, so bring the bin indoors or insulate it in winter.
What to feed worms — and what to avoid
Worms eat most kitchen scraps, but they have preferences. Good food includes vegetable and fruit peelings, apple cores, banana skins, tea bags (plastic-free only), coffee grounds, crushed eggshells, bread in small amounts, and shredded paper and cardboard. Chop or tear food into small pieces so the worms can eat it faster, and bury it under the bedding rather than leaving it on the surface, which attracts fruit flies. Feed a little at a time — a handful every few days to start — and only add more when the previous food is mostly gone. Overfeeding is the most common mistake: uneaten food rots, smells and attracts flies. Do not feed worms: meat, fish, dairy, fats and oils, citrus fruit and onions in large amounts, spicy food, pet waste, or anything with chemicals on it. Citrus and onion are acidic and worms avoid them in quantity, though small amounts are fine. Do not add garden waste such as grass clippings or woody prunings to a worm bin — those belong in an outdoor compost bin.
Tending the worm bin
A worm bin needs little tending, but it does need attention. Check the bedding weekly: it should stay damp like a wrung-out sponge. If it is dry, sprinkle water over it; if it is wet and smelly, add dry shredded cardboard and stop feeding until it recovers. Keep the bin covered with a lid and a layer of damp newspaper or cardboard on top of the bedding, which keeps it dark and moist and discourages fruit flies. If fruit flies appear, stop feeding for a week, bury food deeper, and cover the surface with dry bedding. If the bin smells, it is usually overfed or too wet — stop feeding, add dry bedding, and fluff the bedding gently with a fork to let air in. Do not stir the bin vigorously, which disturbs the worms. In hot weather, keep the bin in shade and check moisture more often; in cold weather, insulate it with old blankets or bring it indoors. Harvest the liquid that drains into the tray — called worm tea — dilute it about one part worm tea to ten parts water, and use it as a liquid feed for plants.
Harvesting vermicompost
After three to six months, the bedding will have turned into dark, crumbly vermicompost, and it is time to harvest. The simplest method is to stop feeding one side of the bin for a week, then put fresh food and bedding on the other side: the worms will migrate to the fresh food, and you can scoop out the finished compost from the empty side. Another method is to tip the contents onto a sheet in bright light: worms move away from light and burrow down, so you can scrape the compost off the top in layers until the worms are left in a ball at the bottom, then return them to the bin with fresh bedding. Vermicompost is ready when it is dark, crumbly and smells like earth, with no recognisable food in it. Use it as a soil improver, a top dressing, or an ingredient in potting mix: mix it into beds at a few handfuls per square metre, top-dress pots with a 2–3 cm layer, or mix it into potting mix at about a fifth of the volume. Do not use it as the whole growing medium in a pot — it is too rich on its own. Wear gloves when handling vermicompost and wash your hands before eating — see our garden safety guide.
Common mistakes
The most common mistake is overfeeding, which makes the bin smell and attracts flies — feed a little at a time and only add more when the previous food is gone. The second is bedding that is too wet or too dry; it should be damp like a wrung-out sponge. The third is feeding the wrong things — meat, dairy and citrus in quantity. The fourth is using the wrong worms: ordinary earthworms from the garden do not thrive in a worm bin, which needs composting worms such as red wigglers. The fifth is leaving the bin in frost or in hot sun, which kills the worms. The sixth is never harvesting, so the bin fills up and the worms run out of space. And the seventh is leaving food on the surface, which attracts fruit flies — always bury food under the bedding.
Frequently asked questions
What worms do I need for worm composting?
Composting worms, usually red wigglers, which live in organic matter near the surface and eat kitchen scraps. Ordinary earthworms from the garden live deep in soil and do not thrive in a worm bin. Buy red wigglers from a worm supplier or garden centre.
How much can worms eat?
A worm eats roughly its own weight in food each day, so a bin with a kilogram of worms can process about a kilogram of scraps a day once established. Start slowly — a handful every few days — and increase feeding as the worm population grows.
Does a worm bin smell?
No, when managed well. A healthy worm bin smells like damp earth. If it smells bad, it is usually overfed or too wet: stop feeding, add dry shredded cardboard, and fluff the bedding gently to let air in.
Can I keep a worm bin indoors?
Yes. A well-managed worm bin has no smell and can live in a kitchen, utility room, garage or shed, provided it is kept between about 15°C and 25°C and out of direct sun. Keep the lid on and bury food under the bedding to prevent fruit flies.
What is worm tea and how do I use it?
Worm tea is the liquid that drains from a worm bin. Dilute it about one part worm tea to ten parts water and use it as a liquid feed for plants, watering it onto the soil. If you are unsure about worm composting in your area, check your local extension service for local advice.