How to Build a Worm Bin for Balcony Composting

If the idea of keeping worms on your balcony sounds strange, that is a normal first reaction. A worm bin is still one of the most useful, least demanding additions you can make to a balcony garden. No smell (seriously), no mess, almost no maintenance, and in return you get the richest, most potent compost you can produce, all from kitchen scraps that would otherwise go to the landfill.

Vermicomposting, composting with worms, is perfectly suited to apartments and balconies because it is compact, odor-free when managed correctly, and produces finished compost in a fraction of the time that traditional composting takes. If you have been buying bags of compost for your balcony containers, a worm bin can dramatically reduce or eliminate that expense.

What You Need

The Bin

You do not need anything fancy. Two nested opaque plastic storage bins (about 10 to 15 gallons each) work perfectly. Drill small holes in the bottom and sides of the inner bin for drainage and airflow. The outer bin catches any liquid that drains through, this liquid, called worm tea, is an excellent liquid fertilizer for your plants.

Alternatively, you can buy purpose-built stacking worm bins with multiple trays. These are more convenient for harvesting finished compost but cost more than a DIY setup. Either approach works well.

The Bedding

Bedding is the carbon-rich material that fills the bin and gives worms a habitat. Shredded newspaper (black and white print only, no glossy inserts), cardboard torn into small pieces, or coconut coir all work excellently. Moisten the bedding until it feels like a wrung-out sponge, damp but not dripping. Fill the bin about three-quarters full with bedding before adding worms.

The Worms

You need red wigglers (Eisenia fetida), not regular garden earthworms. Red wigglers are surface-dwelling composting specialists that process organic matter efficiently in enclosed bins. You can order them online from worm farms or find them at some garden centers. Start with about one pound of worms (roughly 1,000 worms) for a standard 10-gallon bin.

Feeding Your Worms

Worms eat roughly half their body weight in food scraps per day. A pound of worms will process about half a pound of scraps daily. Start slow, underfeeding is better than overfeeding while the population establishes.

What to Feed

• Yes: Fruit and vegetable scraps, coffee grounds and filters, tea bags (remove staples), crushed eggshells, small amounts of bread and grains, shredded newspaper and cardboard.
• No: Meat, dairy, oily foods, citrus (in large amounts, small pieces are fine), onions and garlic (in large amounts), pet waste, diseased plant material.

Chop scraps into small pieces before adding them to the bin, the smaller the pieces, the faster the worms process them. Bury the food under the bedding rather than placing it on top. This discourages fruit flies and keeps the bin tidy.

Harvesting Worm Castings

After 3 to 6 months, you will notice dark, crumbly material accumulating in the bin. This is vermicompost, worm castings, and it is extraordinarily rich in nutrients and beneficial microorganisms.

The simplest harvesting method is to push all the existing material to one side of the bin, add fresh bedding and food to the empty side, and wait 2 to 3 weeks. The worms will migrate to the fresh food, leaving the finished compost behind for you to scoop out.

Use worm castings as a top dressing on your container plants, mix them into potting soil when repotting, or brew them into a compost tea by soaking a handful in a bucket of water overnight. For more on refreshing your container soil, check our guide to balcony plant soil.

Troubleshooting Common Issues

• Fruit flies: Bury food under bedding and cover the surface with a layer of newspaper. If flies persist, freeze food scraps for 24 hours before adding them to the bin, this kills any fly eggs.
• Smell: Almost always caused by overfeeding or too much moisture. Stop feeding for a week and add dry shredded newspaper to absorb excess moisture.
• Worms trying to escape: The bin conditions are wrong, usually too wet, too hot, or too acidic. Add dry bedding, improve ventilation, and reduce citrus scraps. Keep a light on near the bin for the first few days after setup, worms avoid light and will burrow into the bedding instead of trying to leave.
• Bin too wet: Add shredded newspaper or cardboard. Ensure drainage holes are not blocked. The bedding should feel like a wrung-out sponge, never soggy.

A worm bin is one of those additions to a balcony garden that seems unnecessary until you try it, and then you wonder how you ever gardened without one. The worms do virtually all the work. You just feed them your kitchen scraps and collect the results. It is composting made simple enough for any apartment, any balcony, any gardener.