What vermicomposting is
Vermicomposting — also called vermicasting — is the processing of organic waste by earthworms. It is a natural, odourless, aerobic process that differs from a traditional pile: the worms ingest waste and excrete a dark, nutrient-rich material (castings) that works well as a soil conditioner. Because the worms self-aerate the bedding, the system does not need turning.
Temperature is the limiting factor
The worms used are red wigglers (Eisenia fetida), and their workable range sits roughly between 15°C and 25°C. They do not survive freezing. In a Canadian winter this is the central constraint: an outdoor bin cannot simply be left out, so the practical options are to bring the bin inside or to insulate it heavily.
When temperatures approach the low single digits Celsius, move the bin indoors — a basement, heated garage, laundry room, or kitchen — or insulate it well enough to stay above freezing.
Setting up a bin
A common starting point is a shallow, wide plastic storage bin, roughly 20–30 cm deep.
- Drill several small holes in the base for drainage and a few in the lid or upper sides for air.
- Raise the bin on blocks and set a tray underneath to catch excess liquid.
- Keep it out of direct sun and away from heat sources.
Bedding
Use a mix of moist carbon-rich bedding such as shredded newspaper, cardboard, and dry leaves. A couple of handfuls of soil or sand adds grit, which the worms use to grind food since they have no teeth. Keep the bedding as damp as a wrung-out sponge.
What to feed — and what to leave out
Vermicomposting systems work best on plant matter only. Bury scraps under the bedding rather than leaving them exposed.
| Suitable | Avoid |
|---|---|
| Vegetable and fruit trimmings | Meat and fish |
| Coffee grounds and tea | Dairy |
| Crushed eggshells | Oily or greasy food |
| Soft garden trimmings | Large amounts of high-acid citrus |
In cooler conditions the worms eat less, so feed more sparingly and add dry bedding if the bin becomes too wet. Checking the bin about once a week is enough.
If you keep a bin outdoors
Where moving the bin inside is not possible, insulation becomes essential. Common methods include surrounding the bin with straw bales, wrapping it in foam board or bubble wrap, piling dry leaves around and over it, and placing a damp cardboard or newspaper "blanket" on top of the bedding. A sheltered, south-facing position helps it absorb daytime warmth. Even so, an indoor location is the more reliable choice through a cold Canadian winter.
Using the castings
Finished castings are a ready-to-use soil conditioner. Mixed into potting soil or worked lightly into garden beds, they add organic matter and improve the soil environment for growing plants. Because nutrients are released gradually, castings can be applied generously without the risk associated with stronger fertilisers.
References
- Government of Ontario — Vermicasting (or vermicomposting).
- Saskatchewan OER Composting Toolkit — System Selection Guidance.
- Manitoba Master Gardener Association — Composting Through the Growing Season in Manitoba.