Worm bin indoors: composting scraps on your balcony

Why a worm bin indoors?

A worm bin indoors is the quiet revolution in sustainable living: you turn kitchen waste into food for your vegetable patch and perennial borders. An outdoor compost bin takes 3-6 months to produce usable compost; a worm bin produces it in 3-4 months and is neater and more intensive. On a balcony, in a kitchen, even in a small room: worm bins do not take much space.

This article is about vermicomposting: worm composting. Red composting worms (Eisenia fetida) eat your food scraps and turn them into fertile, dark-brown compost you can use directly in pots or borders. The liquid that drips from the bottom, worm tea, is ultra-nutrient-rich water for your plants. Not sure which plants would suit the result best? On gardenworld.app you can see a preview of your own garden before you decide.

Choosing a bin: size and material

A worm bin does not have to be complicated. Three layers are the minimum: a base for drainage, a middle section for worms and their food, and a top. Many beginners buy a plastic cube bin, about 40-50 litres, roughly 35 centimetres tall. That is a good start for a household of two to four people.

You can also build your own: two perforated plastic crates stacked, drainage base, holes in the sides for airflow. Important: no light (worms prefer darkness), good drainage and ventilation. Wood looks nice but warps and gives mould a place to grow. Plastic lasts for decades and is easy to clean.

Place the bin on a moisture-resistant surface: tiling, a tray or plastic mat. Underneath, a collection tray for overflow, which might be excess moisture or worm tea.

The right worms: Eisenia fetida

Not every worm suits a bin. Garden worms (Lumbricus terrestris) want to dig deep and feel confined. Red composting worms (Eisenia fetida) are made for this work. They grow fast, eat well and settle happily in stacked layers. They are about 8-10 centimetres long, red-brown, and smell like soil when you touch them.

A good bin needs 500 to 1000 worms to start properly. You buy them online or from a garden centre that breeds them. Cost about 20 to 30 euros for a starter population. They multiply fast. After six months you have roughly double.

Check carefully: many sellers sell "Eisenia" which can be Eisenia fetida (excellent) or Eisenia andrei (also fine but slightly less productive). Ask.

What goes in: the food

Worm bins can handle much but not everything. Vegetable and fruit scraps are ideal: peels, rotten bits, onions, peppers, apples. Coffee grounds and tea bags. Seeds not fully broken down but certainly compostable. Paper, cardboard, thin toilet rolls.

No meat, fish, bones, or oil. Worms do not enjoy fat and it starts to smell strongly. No dairy products in bulk (yoghurt okay, cheese better not). No onions in quantity (the acid slows them). Do not put dry bread directly in; it needs to soften in water first.

The secret: food is made small before entering the bin. Chop fine, let it sit so bacteria begin breaking it down. Then worms work much faster.

Preventing fruit flies

The enemy of every indoor worm bin: fruit flies. They emerge from the food you add. They do not harm the process but they are a nuisance indoors. Three ways to prevent them.

One: cover every new food addition immediately with a layer of brown carbon (dry leaves, paper scraps, cardboard). This stops flies laying eggs on fresh food. Two: bury food deeper in the bin, between existing matter. Three: keep the lid sealed, no ventilation holes at the top.

If your bin is in the kitchen and many flies appear, freeze food scraps a day or two first. Frozen food smells less and attracts fewer flies.

Temperature and air

Worms like mild warmth, 15 to 25 degrees. They also work below 15 degrees but much slower. Above 26 degrees they can stress. An indoor bin in a kitchen or side room is ideal. Avoid direct sunlight (it stresses them) and frost (they can die if it gets too cold).

Air is crucial. Without oxygen, worms die. Holes in the sides, a loose (not sealed tight) lid or small air vents all work well. Test it: fill the bin and wait two weeks. If it smells sharply of ammonia, ventilation is poor.

Worm tea: the liquid gold

Liquid drips from the bottom of your bin: worm tea. This is processed plant food, rich in bacteria and nutrients. Many bins have a tap at the base so you can drain it. Undiluted it is almost toxic to young plants; dilute it in the ratio 1 part tea to 10 parts water.

Use it on your houseplants, vegetable plants or borders. It costs you nothing and your plants grow noticeably faster with it. Do not water newly sown seeds with it; it adds too much salt at once. But for established plants: essential.

Better method: keep worm tea in an open pot and let it sit a few hours before using. The gases escape and the balance restores itself.

The strongest worms: winter and beyond

Worm bins struggle in winter months. At temperatures below 10 degrees the worms become inactive and eat little. In some areas even a garage or shed can freeze over. Four ways to help them through winter.

One: move the bin indoors (garage, cellar, side room) where it stays at least 10 degrees. Two: insulate the bin with wrapping or cardboard boxes around it with an air hole. Three: add no new food from December to February; the bin runs on what is already there. Four: any food you add, pre-break it down (chop it, do not add fresh crushed).

In spring (March-April) as weather warms, they eat again with vigour. You see explosive growth in worm numbers and activity.

Step-by-step setup

Step 1: Choose bin and let it settle

Pick your bin or build your own. Fill the base with a perforated layer (straw, stones, drainage material). Then add a few centimetres of moist organic mix: half seed compost, half fine leaf mould, some extra cardboard.

Step 2: Introduce worms

Gently pour your 500 to 1000 worms into the bin. Do not throw them! Spread them out. They will dig deeper into the darkness themselves. Let them settle 24 hours.

Step 3: First food

After 24 hours add food. Not much! A handful of chopped vegetable scraps, spread around. Cover with brown carbon.

Step 4: Feed regularly

Add food every 2 to 3 days, always covered with brown material. Check that previous food is mostly gone before adding fresh.

Step 5: Check air and light

Keep the bin in half-darkness. Make sure air circulation is good (no ammonia smell).

Step 6: Drain worm tea

After two weeks you can carefully drain the first worm tea. Dilute with water.

Frequently asked questions

I see no movement of worms. Are they dead?

Probably not, they are just settling in. Red worms are most active in darkness. Turn off the light, wait 48 hours, then check with a torch. Plenty of movement means all is well. No movement and a sour smell point to a ventilation problem.

How long until I can harvest first compost?

3 to 4 months. Then the food layer is turned, brown matter is dark and crumbly. Do not remove all worms when harvesting; leave at least a thousand behind.

Can I put the bin outside in winter?

Not outdoors in zones 7-8 in the Netherlands and Belgium. Temperature drops below zero and worms die. Move indoors in winter to maintain at least 10 degrees.

Is there a bad smell problem?

Two causes: the food itself stinks (too fatty, too much meat) or ventilation is poor (ammonia-like). Add brown carbon, wait 48 hours. If it persists: move bin to a better-ventilated spot.

See it in your own garden first

Want to see how compost from your own worm bin can transform your houseplants and vegetable boxes? Upload a photo of your garden on gardenworld.app and view a free example of a new design with this nutrient-rich soil. The sharp image with plant list costs 7.99 euros, one time, no subscription.