Common kitchen waste, including onion and garlic skins, carrot tops, and coffee grounds, can be composted year-round in an indoor worm bin. A worm bin fits discreetly into a closet, garage, or pantry, making composting quick and convenient, especially for small-space gardeners and apartment-dwellers. Turning food scraps into compost, instead of sending them down the garbage disposal or to the curb, reduces the amount of organic material that ends up in local wastewater treatment plants and landfills.
The process is known as vermicomposting, and it requires a specific type of worm. Unlike regular garden earthworms, which burrow in the soil, red wiggler worms (Eisenia foetid) act as nature’s recyclers, living on or near the surface, where they help decompose organic matter. This habit makes them ideal candidates for living in an enclosed worm bin.
“Red wigglers eat half their weight in food each day,” says Luke Halligan, compost educator with the NYC Compost Project hosted by the Brooklyn Botanic Garden. As they eat, worms produce castings—a crumbly, granular organic matter that is rich in nutrients.
Worm compost, or vermicompost, is a mix of castings and decomposed organic matter. Vermicompost contains a greater diversity of beneficial microbes than traditional compost, which may be why it is linked with increasing plants’ resistance to fungal diseases. The nutrients in worm compost are also more available to plants—a quality that researchers think helps plants grow faster and stronger and resist attacks from aphids, mealybugs, and spider mites.
Ready-made worm bins are available, but it takes less than an hour to build a bin using common household supplies.
- A 14-gallon plastic storage container (approximately 24 by 16 by 12 inches) is usually adequate for a household of two to four people; larger or vegetarian families may need to build two bins to compost all their food waste. (If you prefer not to use plastic, a similar bin can be built from untreated lumber, perforated in the same way as the plastic container for ventilation and drainage.)
- Create a well-drained and ventilated worm bin by drilling a grid of 30 evenly spaced 1⁄4-inch holes in the bottom of the container and its lid. Then drill two horizontal rows of ventilation holes in the sides of the bin, spacing the holes 11⁄2 inches apart and the rows 2 inches apart, with the first one positioned 3 inches below the rim of the bin.
- Place the bin on a tray, with wooden blocks or bricks between the bin and the tray to help air circulate underneath it. The tray should be large enough to catch any liquid that drains from the bin (we used an extra bin lid). Plan to rinse out the tray weekly, using the nutrient-rich liquid in compost tea. Locate the bin out of direct sunlight in a place that stays above freezing but under 75°F.
- Red wrigglers prefer to dwell in leaf litter in their native environment, but the damp paper makes a fine substitute in worm bins. To prepare the bedding, plunge strips of black-and-white newspaper or office paper into a sink filled with water. Wring out the wet strips, fluff them up, and place them in the bin. Fill the bin about two-thirds full with the paper bedding, which should feel as damp as a wrung-out sponge.
- Add some dry leaves, if you have them, and then sprinkle 1 cup of garden topsoil over the bedding. This small amount of grit helps the worms’ digestion and introduces microorganisms that speed the composting process.
- Next, add worms. Garden centers often sell red wigglers, and they can also be purchased online. “One pound of red wigglers will eat approximately 31⁄2 pounds of food scraps per week,” Halligan says. “Households can use this as a guide if they produce more or less food waste.” (To judge how much food waste your family produces, use a kitchen scale to weigh it for a week.) Add worms to the bin by sprinkling them evenly over the bedding. Cover with aerated lid.
Kitchen cleanup is easy with a worm bin—simply bury raw fruit and vegetable scraps, coffee grounds and filters, tea bags, and eggshells under the worm bedding. Avoid adding cooked food or food with sauces, oils, or spices, and never feed worms dairy or meat products. Citrus peels in large quantities may harm worms if they cause the bedding to become too acidic.
“It usually takes about 3 months for the worms to consume all of the newspaper bedding as well as the food scraps and turn it into worm compost,” Halligan says. To prepare the finished vermicompost for harvesting, move it all to one side of the bin and place fresh bedding in the other half. Add kitchen scraps to the new bedding exclusively. After about a month, most of the worms will migrate over to the fresh bedding and food, leaving behind worm-free compost for the collecting.
Use vermicompost to amend potting soil for both indoor and outdoor plants by adding 1 part worm compost to 4 parts soilless potting mix, or simply sprinkle a thin layer around the base of container plants. Use this rich compost in the garden, too, as a nutrient boost in the planting holes of new transplants or a topdressing for established plants.