Vermicomposting uses worms — primarily red wigglers (Eisenia fetida) — to convert food scraps into vermicast, a dark, fine-textured material with high microbial diversity and plant-available nutrient content. Unlike hot composting, vermicomposting operates at ambient room temperature, making it practical in apartments, condominiums, and cold-climate households where outdoor composting is limited to part of the year.
Finished vermicompost. The dark, granular texture is typical of well-processed worm castings. Photo: Wikimedia Commons.
Worm species selection
Eisenia fetida (red wiggler or brandling worm) is the species used in nearly all vermicomposting applications. It is adapted to processing organic surface material, tolerates wide temperature swings within the range of roughly 15°C to 30°C, and reproduces quickly under favourable conditions. A mature E. fetida population in a well-managed bin can consume close to half its own body weight in food scraps daily.
The European nightcrawler (Eisenia hortensis) is sometimes used as an alternative. It tolerates cooler temperatures slightly better than red wigglers and produces casts of a coarser texture, but it is slower to reproduce and less commonly available in Canada.
Common earthworms found in Canadian gardens (Lumbricus terrestris) are not suitable for bin composting. They require deep soil, do not thrive in shallow containment, and do not process surface organics efficiently.
Bin design and sizing
A basic worm bin can be constructed from a food-grade plastic storage container. The bin should be opaque (worms avoid light), well-ventilated through small drilled holes in the sides and lid, and have drainage holes in the base above a collection tray for excess leachate.
Basic bin dimensions for a typical household
- Dimensions: 60 cm × 40 cm × 30 cm (approximately 60 litres) for 2–4 person household
- Worm starting weight: 0.5 kg for 0.5 kg of weekly food scraps
- Bedding depth: 15–20 cm of moist bedding at start
- Temperature range: 15°C – 30°C (optimal around 20°C–25°C)
- Location: indoors year-round in Canada; outdoors in summer only
Stacked flow-through systems (commercial designs with multiple trays) allow worms to migrate upward as fresh food is added in the top tray, leaving finished castings in lower trays for harvesting. These designs reduce the need to separate worms from finished material by hand.
Bedding materials
Worms live and move through the bedding layer, not just through the food material. Suitable bedding provides structure, moisture retention, and a carbon buffer to offset nitrogen-rich food inputs.
Commonly used bedding materials include:
- Shredded corrugated cardboard (no wax coating) moistened to roughly 70–80% moisture
- Shredded newspaper (non-glossy) — though this is less recommended as newsprint quality varies
- Coir (coconut fibre) — widely available at Canadian garden centres and has consistent moisture retention
- Aged leaf litter, shredded to 2–5 cm pieces
Bedding should be replaced or supplemented periodically, particularly when the bedding layer has largely converted to castings and lost its structure.
Feeding and food management
Red wigglers process most fruit and vegetable scraps effectively. Certain inputs require specific handling or should be avoided entirely in bin composting:
Red wigglers in active vermicompost. Photo: Wikimedia Commons.
Accepted and excluded inputs
- Accepted: fruit peels, vegetable scraps, coffee grounds, tea bags (paper only), eggshells, small amounts of cooked grains and pasta (no sauce)
- Use in moderation: citrus peel, onion, garlic (tolerated in small amounts but can stress worms in large quantities), bread and starches (can attract pests)
- Exclude: meat, fish, dairy, oily or highly salted foods, pet waste, large amounts of citrus
Burying food scraps 5–10 cm under the bedding surface reduces odour and discourages fruit flies. Rotating feeding locations within the bin distributes worm activity and avoids concentrated hot spots.
Moisture and ventilation
A healthy worm bin should feel moist throughout but not drip freely when compressed. Excess moisture indicates overfeeding, poor drainage, or too much wet feedstock. A layer of dry shredded cardboard on top of the feeding zone absorbs surface moisture and reduces fruit fly pressure.
The leachate collected below the drainage tray is a dilute liquid. It can be used as a soil drench when diluted further with water (at a ratio of roughly 1:10), though its nutritional content is variable and depends on bin conditions. It is not equivalent to aerated compost tea.
Harvesting vermicast
Vermicast is ready to harvest when the bedding and food have largely converted to dark, uniform castings with a soil-like smell. Two common harvest methods:
- Light migration: move finished material to one side of the bin; add fresh bedding and food to the empty side. Over two to four weeks, worms migrate toward the fresh material. The depleted side can then be harvested.
- Dump-and-sort: empty the bin onto a tarp under bright light. Worms burrow away from light, allowing gradual removal of surface castings from the pile.
Finished vermicast can be applied directly to garden beds at rates comparable to finished compost, or blended into potting mixes at low concentrations (roughly 10–20% by volume) to avoid salt build-up.
Canadian winters and indoor operation
Vermicomposting is one of the few composting approaches that functions without modification through a Canadian winter when the bin is kept indoors. Worm activity slows below 15°C and essentially stops below 5°C, so bins kept in unheated garages or sheds in January in most provinces will need protection or temporary relocation.
Some households maintain a smaller bin inside and a larger outdoor bin in summer, transferring worms inside in autumn before the first frost. The indoor bin handles kitchen scraps through winter; the outdoor bin resumes in late spring.