Vermicomposting relies on earthworms — mainly the red wiggler Eisenia fetida — to break down food scraps into a nutrient-rich amendment. Unlike traditional composting, this process happens indoors in a small bin and requires neither a yard nor a balcony. It works year-round, regardless of outdoor conditions.
How vermicomposting works
Worms convert organic matter into vermicompost — also called worm castings — an amendment rich in humus, minerals, and beneficial microorganisms. The process also produces a liquid known as worm tea or leachate, collected at the bottom of the bin and usable diluted as liquid fertilizer for indoor or balcony plants.
Process efficiency depends on three factors: bin temperature (ideally between 15 and 25°C), material moisture, and balance between fresh inputs and carbon-rich bedding.
A compact vermicomposting bin suited to apartment use. Source: Wikimedia Commons (CC BY-SA 4.0)
Choosing a worm bin
Several models are available at Canadian garden centres and online. Stacked tray bins make harvesting easier without disturbing the worms: they migrate to the upper tray where fresh food is added, leaving the lower tray ready to harvest.
A basic DIY bin made from two opaque plastic containers (one perforated, nested inside the other) is an economical alternative. Recommended capacity for a two-person household is about 10 to 15 litres per tray.
Points to check before buying
- Tight or well-fitted lid to prevent leaks and odours
- Tap or collection tray for leachate
- Opaque material to protect worms from light
- Adequate ventilation without openings large enough for insects
Worms: species and quantity
The species best suited to indoor vermicomposting is Eisenia fetida, also called the red wiggler or manure worm. These worms stay near the surface and tolerate conditions in a closed bin well. They differ from garden earthworms, which need deeper soil layers.
A starting amount of 200 to 500 grams of worms is enough to start a bin. The population adjusts naturally to available food — it grows when inputs are generous and shrinks when the bin is fed lightly. Red wigglers are sold at some garden centres, from local producers, and at specialty shops in several Canadian provinces.
Active earthworms in a compost bin. Source: Wikimedia Commons (CC BY-SA 2.0)
Starting bedding
Before introducing worms, prepare moist bedding made from:
- Finely torn cardboard, moistened
- Non-glossy newsprint, shredded
- Shredded dead leaves
- A small handful of garden soil for microorganisms
Bedding should be damp but not soaked. Test moisture by squeezing a handful: a few drops of water should appear, but not a steady stream. Worms settle into this bedding and begin breaking it down before the first food additions.
Tip: In the first few weeks, feed the bin in small amounts to let worms acclimate. A large initial load of fresh material can trigger acidic fermentation that harms worms.
What can you feed the worms?
Accepted materials
- Vegetable and fruit peels (avoid large amounts of citrus)
- Coffee grounds and paper filters
- Tea bags without staples
- Small amounts of stale bread
- Crushed eggshells (help regulate acidity)
- Hair and nail clippings
Materials to avoid
- Meat, fish, dairy products (odours and pests)
- Large amounts of onion and garlic (too acidic)
- Excess citrus
- Very strong spices
- Oils and greasy materials
Placement and temperature in an apartment
The bin is best placed in a temperate spot: under the kitchen sink, in a closet, or in a laundry room. Worms are sensitive to extreme temperatures — below 10°C their activity slows sharply; above 30°C they can die. In well-heated Canadian apartments, the bin usually stays within a favourable temperature range all winter.
A properly maintained bin does not produce noticeable odours outside the container. A slight earthy smell when opened is normal. Strong odours usually indicate excess moisture or too much fresh material.
Harvesting vermicompost
With a tray bin, harvest by removing the bottom tray, whose contents have been processed by worms that migrated upward. With a single bin, push contents to one side, add fresh material on the other, and let worms migrate naturally before harvesting the empty side.
Harvested vermicompost can be used directly in indoor plant pots or donated to other residents or community gardens.