The hows and whys of vermicomposting

If you’re looking for a productive and child-friendly summer project, this just might be it.

The Latin word “vermi” means worm, and vermicomposting is the process of using worms to create compost. It’s a small-scale recycling process that converts kitchen waste to “garden gold.” Most horticulturalists consider worm compost to be the best soil amendment, since it’s full of beneficial microorganisms and a long list of nutrients essential to plant growth and development.

The final product of the vermicomposting process is a crumbly, rich, dark brown, earthy material. A thin layer of this compost can be used to top-dress the soil of potted plants. It can be worked into garden soil to improve its texture and nutrient content. And it’s especially useful in the vegetable garden, where it helps feed food-producing plants and encourages the development of healthy roots.

Here’s how to get started.

The worms

Red worms or red wigglers (Eisenia foetida)—often sold as bait—are the best species to use for vermicomposting. In their natural environment, they feed on organic matter that accumulates on the soil surface, and they’re content to live in crowded conditions with frequent disruptions (like feeding and harvesting). Other common worm species aren’t suitable for vermicomposting. The night crawler—often used for bait—doesn’t thrive in captivity, nor does the common earthworm, since they both need plenty of soil, room to wander, and cool temperatures to survive and thrive. 

A worm bin is typically started with about one pound of red worms (roughly 1,000), but it can be started with less than that, as long as the bin isn’t initially overloaded with food. Commercial worm farms and local bait shops are good sources of red worms.

Red wigglers in worm bin bedding material.  (Photo courtesy of Andrea Ikeda)

The worm bin

There are many fancy, commercially produced, and expensive worm bins, but it’s simple and more fun to create your own.

The basic requirements for a well-functioning, easy-to-maintain bin: (1) it should be shallow, with a depth of no more than 12 inches; (2) it shouldn’t hold too much moisture; (3) it should have ventilation; and (4) it should be portable. Various vermicomposting resources have different recommendations, so do a bit of investigation to figure out your own preferences and needs, which are partly related to your volume of usable kitchen waste.

For a small-scale worm bin, a lidded, opaque (not clear-sided) plastic container works well. A sturdy, wooden box with a hinged or removable lid can also work, especially for a large outdoor worm box. Worms like a dark environment since their skin is light-sensitive, so a closed container is essential for their comfort and for pest control.

Vermicomposting guides recommend drilling ventilation and drainage holes in a worm box, the first for air circulation, and the second to prevent excessive moisture build-up. Drainage holes do complicate indoor storage, however, and it’s possible to do without.

The location

A dark, indoor, out-of-the-way place—under the kitchen sink, in a laundry room, inside an unused cabinet, or in an insulated garage—works best as a location for worm boxes. “Pet” worms are happiest with relatively consistent temperatures between 55 and 75°F and a minimum of disturbance. (I once tried maintaining a worm box outdoors, and that experiment failed miserably; in my yard, it was too difficult to keep a consistently cool temperature, and the wriggly little decomposers cooked in the summer heat.)

The bedding

Bedding is the material used to fill and refill the worm bin as needed. It serves several purposes, acting as a refuge where worms can shelter and breed, a moisture control agent, and a backup source of food if plant scraps are scarce.

If you subscribe to the print edition of The Record, you have a head start on creating a worm box. Newspapers that use plant-based, non-toxic inks are one of the best and most readily available sources of bedding material. They’re very easy to tear into narrow strips, they hold moisture well, and they decompose readily. (However, don’t use the slick, full-color advertising inserts.)

Other suitable bedding materials are shredded, uncontaminated, corrugated cardboard (minus any tape or non-biodegradable elements such as wax); coconut coir; plain shredded paper without coatings or chemicals (not magazines, cereal boxes, or other packages); burlap cloth; and shredded leaves.

The bedding material should be loose and slightly moist, like a damp sponge. Fill the box to the top with bedding, and replenish it as it settles and breaks down. 

The worm food

Yes—Raw plant matter (apple cores, banana and carrot peels, vegetable trimmings, wilted lettuce leaves, etc.); plain cooked vegetables; crushed eggshells; cooked rice/grains in small quantities; and dead or wilted leaves from non-toxic houseplants all make good food for red worms. So does that mushy bag of greens that sat for too long in the refrigerator (minus the plastic, of course).

No—Pungent plants such as onions and garlic; meat or dairy products; oily foods; pet waste; or vegetable matter that’s been sauced, dressed, or seasoned. It’s also best to avoid citrus; it contains the chemical limonene, which is toxic to worms, and it can make the bin environment too acidic.

It helps to chop food into small pieces before adding it to the bin. Worms don’t have teeth, so their food must be soft or in pieces tiny enough to swallow. They can eat their own weight every five to seven days. Add a handful of clean sand, coffee grounds (used or fresh), or other gritty material to the box too; worms will swallow it and store it in their gizzards to help grind up the food. Slowly but surely, the worms will covert scraps into compost.

Food scraps should be buried at least one inch deep in the bedding, and preferably underneath it. This discourages potential pests such as fruit flies. Don’t add too much food at once, and monitor your bin to ensure that worms have adequate but not excessive meals.

Worm bin with newspaper-strip bedding pulled aside, showing newly added kitchen scraps (top), and decomposing scraps 5 days later (bottom). (Bin recommendations and photos courtesy of Andrea Ikeda)

The harvest

The worms’ digestive systems break down the food scraps, and the by-product—called “worm castings”—is the end goal of the vermicomposting process. In less delicate terms, it’s worm poop. It looks just like rich, dark, crumbly soil, it doesn’t stink, and it’s an incredibly nutrient-rich, organic soil additive.

There are two simple ways to harvest home-created worm compost. The easiest process is from within the bin. Using close-fitting gloves, move all the food scraps and remaining bedding to one end of the bin, leaving the underlying worm compost exposed. Over the next few weeks, put food in the end with bedding, to encourage the worms to migrate to that side, then remove the worm-free finished compost from the other end, picking out and saving any leftover worms. Spread out the remaining material, add new food and bedding, then repeat the process in another few weeks.

The “cone method” is also easy. Remove a few large handfuls of castings to a portable surface (the box lid works well) and shape them into a cone. Move it to a sunny area for a few minutes. After the worms dig to the bottom to avoid the bright light, harvest the compost from the top of the cone and return the remaining compost and worms to the bin.

A more labor-intensive way to collect worm castings is to empty the entire contents of the bin onto a large plastic sheet. Wearing gloves, carefully pick out the worms, any remaining food scraps, and bedding remnants and return them to the box. What’s left behind will be the finished worm compost. Return some of the compost to the bin, add more food, and top off with fresh bedding to start the process anew.

Other notes

For my current worm bin, I used a 10-gallon plastic container without drainage holes, and drilled a series of closely spaced, 1/8-inch ventilation holes around the top. (This is a smaller diameter than usually recommended, but I’ve had better luck with smaller, more numerous holes.) I also use dry newspaper strips when topping off the already-damp contents of the bin. These measures, plus occasionally loosening the castings, keeps the moisture level under control.

Involving kids in vermicomposting will help them learn about the natural environment, recycling, and caring for living things. Teach them to handle the worms sparingly and gently, and encourage them (and possibly yourself) to overcome any squeamishness!

A healthy, productive worm box shouldn’t have an objectionable odor; it should have a fresh, earthy smell that usually isn’t noticeable unless the box is opened. Therefore, having it in or near the kitchen shouldn’t be a problem, and that makes it easily accessible for regular worm feedings.

If you see little capsules that resemble tiny lemons in your worm box, that’s good! Those are worm egg cases; they range in color from bright yellow to brownish, and each contains from two to twenty worms. They’re a sign that your system is functioning well, and you’ll soon have lots of tiny worms populating your bin.

Other little critters might also inhabit your box, but that’s not necessarily a bad thing. If you see miniscule, whitish insects, those are springtails, another species that aids in decomposition. I have those in my box, and I also have pillbugs, which I intentionally introduced since they help break down food matter into pieces small enough for the worms to ingest. Neither of these have escaped the box or caused any problems. Outdoor worm boxes can attract many other small inhabitants as well, but most are simply part of the natural ecosystem.

Fruit flies and ants, however, are unwelcome pests. They can be discouraged by keeping the worm box filled to the top with bedding, by using only recommended food matter, and by occasionally and gently stirring the contents at the bottom of the box.

Vermicomposting resources

  • The book “Worms Eat My Garbage,” by Mary Appelhof (1936-2005). First published in 1993, it was updated by Joanne Olszewski in 2017 for a 35th anniversary edition.
  • UCCE Master Gardeners of San Joaquin County website, “Composting with Worms”
  • The California Department of Resources Recycling and Recovery (CalRecycle) “Vermicomposting” webpage (Their cute little brochure/bookmark called Worms, downloadable from this website, is what got me interested in vermicomposting many years ago.)
  • North Carolina State Extension’s website “Vermicomposting for Households”

If you’re interested in seeing a commercial worm farming operation, plan a future, when-safe visit to P.J. Dunn Working Red Worms in the nearby city of Galt; it’s the closest worm farm to San Joaquin County. Call in advance to ensure that they have worms and worm compost ready for sale, or possibly even arrange for a group tour.

For gardening-related questions, call the UC Master Gardener office at 209-953-6112, or visit our website.

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