Many gardeners know that it’s important to keep nutrients and organic matter in place as long as possible. So naturally, many reach the conclusion that sending scraps and yard waste off to be composted and then buying it back in plastic bags is not ideal. Still, urban composting can be tricky because we want to minimize odor, rat buffets, and the amount of space a composting system takes up.
After years of experimentation with various systems, my favorite is still the classic worm bin. Essentially, you provide a home and food for a population of worms, and they tend your compost for you. You’re left with rich organic material to add to your garden (or a friend’s, if you don’t have space yourself).
There are many designs for great worm bins available around the internet. Here are some tips for picking one that’s right for you, and keeping it running smoothly.
Size: I’ve found the larger the better (up to maybe 2’x4’x2’). This gives you the flexibility to keep adding food until you feel like harvesting, rather than being limited by a full bin. It also helps with climate control - worms don’t like to freeze, and normally would go deep into the earth to keep themselves warm. A bigger bin provides more insulation from both heat and cold.
Location: Put your bin anywhere that’s convenient for you, but for climate control reasons it will probably be easier to maintain in a shaded area that’s a bit sheltered from extreme weather.
Material & features: The easiest-to-maintain worm bins I’ve seen are made of wood. There are many designs available that use plastic bins with holes drilled in them, but I’ve found that wood helps regulate moisture and temperature much better. That means you have to worry less about mold or bad smells. In theory, the plastic models allow you to harvest “worm tea,” or the juice that settles out of worm compost, but I haven’t seen this happen in a significant amount.
The other primary design feature to consider is how to protect against rats. Installing hardware cloth on the bottom of the bin and having a sturdy lid that fits well are the best ways to ensure that nobody unwanted has access. Some insect eggs will inevitably be present on your food scraps and may hatch in the bin, so a good lid will also prevent occasional recently hatched flies from traveling freely into your kitchen or around your patio.
Starting off: Once you’ve got your bin put together, it’s time to add bedding. You’ll need some sort of most carbon-rich material. This could be fallen leaves, shredded newspaper or phone books, or anything else on this list. Moisten until it feels like a squeezed out sponge (imagine the natural moisture content of a worm’s body and replicate!). This material will decompose, dry out, and pack down quickly, so add lots! The carbon-based bedding will also help regulate moisture and odor. The most common way to have an icky worm bin is to have insufficient bedding. Once you’ve got plenty of moist bedding, add worms and food scraps!
Once you’ve got your bin put together, it’s time to add worms. You can ask for a donation from a friend or neighbor, or purchase red wriggler worms from a farm supply company. The type of worms you’ll house in your bin, red wrigglers, are not the standard earthworms you dig up in your garden.
Feeding: You’ll likely end up with just a handful of worms to start, so for the first month or two feed them slowly. The worms can eat about half their weight in food each day, so use that benchmark to get started. Feeding them too much isn’t a risk to the worms, but to the pleasantness of your bin, so use your nose to learn if you’re overfeeding. Over time, the worm population will build up to match the amount your household feeds them.
Worms will eat nearly all your food scraps, minus citrus scraps and grease (light amounts of salad dressing aren’t a problem, but keep fats out in general). Some things, like avocado pits, are just too big to get their little mouths on, so you can leave those out too. Egg shells are great for your garden, so add them to your bin even though they won’t get fully eaten.
Moisture: The most frequent issues that arise with worm bins all stem from moisture imbalance. If you’re noticing mold or bad odor (rot, rather than soil-ey dirt odor), chances are it’s too wet in your bin. Add some more bedding material where you notice it’s getting soggy, or all over, and temporarily reduce feeding if it seems like recognizable food is piling up.
Insects: Nature doesn’t let anything go to waste, so where you’re storing food for critters, critters will arrive. Your worm bin will likely have some resident slugs, spiders, and plenty of sow bugs. That’s totally fine - they’re also decomposers, and they won’t harm your worms. Some other insects, like flies, are more pesky. The best strategy to keep them out is to create a cover of some sort, either by keeping a thick layer of bedding or cardboard above the layer of food and bedding where the worms live, or by fitting a fine wire mesh net inside the open bin to block access.
After a few months, you will have some finished vermicompost. It will be very dark and fine, similar to espresso grounds but not gritty. You can remove these from the bin and use them in your garden as needed, or harvest all at once when there’s a sufficient supply. To harvest all at once, you can spread all the worm bin material on a tarp. Sort through by hand and return the worms and recognizable food scraps and bedding to the bin. What’s left is your vermicompost (and probably a pesky fruit sticker or two). Make sure to replenish food supplies and bedding so your worms have plenty of habitat left! To harvest “as needed,” you can just sift through with your hands and find areas of completed vermicompost. Pull it out and use it to fertilize food crops including vegetables and berries.