How to make a worm composter

Worm composting - also known as vermicomposting - relies on worms and microorganisms to transform organic waste into compost. The worms eat food scraps, garden waste, and some brown materials and excrete a high-quality, fine-grain compost, ready for use. It is suitable for indoor use and shared households, and it is pretty much odorless. After starting it, there’s low maintenance, as worms work for you and mostly prefer to be left alone. ("Some say vermicomposting results in the very best compost out there!") We will show you how to build your own compost.

A worm composter made of three large flowerpots on a moveable trolley.
A worm composter made of three large flowerpots on a moveable trolley. The first version of our worm composter. Last update: 8 October, 2024.

Step 1: Preparing Materials

For your home composter, we suggest to use four three containers that can be easily stacked. To give the worms enough space to work, we recommend choosing a container size of around 15-20 liters (about L30cm x W30cm x H30cm) per person or more. For larger households, you may want to build several composters or create larger boxes. For our worm compost, we used terracotta flower pots.

( ! ) Composters works from the top down, so remember that the top container must fit into the bottom one to avoid leakage of material moving downwards. This is where the final resource, the compost tea, is found.

Step 2: Preparing the Containers

Drill Holes: Drill small holes (about 5-10 mm) in the bottom of the top three containers to ensure good ventilation and the drainage of excess liquids. The bottom container does not need any holes.

Prepare the Lid: Drill a few small holes in the lid of the top container for ventilation. This keeps the compost airy and provides the worms with enough oxygen.

Step 3: Setting Up the Composter

Technical drawing showing the three layers of our composter.
Technical drawing showing the three layers of our composter. Drawing: Sebastian Schilbach. Last update: 8 October, 2024.

Bottom layer: For a tea composter, fill the bottom container with a thin layer of gravel or sand to catch excess liquids and ensure good drainage.

Middle layer: This is the worm casting. Place moist, shredded paper or cardboard, lightly moistened coconut coir, or old leaves in the second container as bedding for the worms.

Add Worms: Place the compost worms (e.g., Eisenia fetida, also known as red wigglers) on the prepared bedding. A starting population of about 500 worms (around 500 g) is sufficient for most households.

Step 4: Feeding and Maintenance

Feeding: Add the Bedding Material Add kitchen scraps in the top layer such as vegetable peels, fruit skins, coffee grounds, and tea bags. Avoid meat, dairy products, oily or heavily spiced foods.

Covering: Cover the scraps with a thin layer of moist paper or cardboard to keep odors and fruit flies away.

Maintenance: Regularly check the moisture level of the composter. The bedding should feel like a wrung-out sponge. Add water if necessary or balance excess moisture by adding dry material (paper, cardboard).

Step 5: Harvesting the Compost

Harvesting Compost: After about 3-6 months, the compost (vermicompost) is ready to harvest. The finished worm compost is dark, crumbly, and smells pleasantly earthy.

Separating the Worms: Push the finished compost to one side and add fresh food to the other side of the container. The worms will migrate to the new food source after a few days, making it easy to remove the finished compost.

Collecting Worm Tea: The bottom container catches the excess liquid (worm tea). This can be used as a liquid fertilizer by diluting it with water in a 1:10 ratio.

Conclusion

Worm composting is an efficient and eco-friendly way to recycle kitchen scraps. With a little care and attention, you can produce high-quality compost and worm tea that will benefit your plants. Happy composting!