Vermicomposting: Composting with Worms


Traditionally, worms have been raised for fishing bait as well as a protein and enzyme source for various products, including animal food and biodegradable cleansers. Worms have also been used to manage agricultural wastes such as dairy manure. They convert waste into worm manure (also known as worm castings), a nutrient-rich, biologically beneficial soil product.

Vermicomposting is the use of worms as a composting method to produce vermicompost. Vermiculture is worm farming for the production of worms. In recent years, worm farming has been practiced on both a small and large scale with three complementary goals in mind: waste diversion, vermicomposting, and vermiculture.


What is vermicompost?

Vermicompost, or castings, is worm manure. Worm castings are considered by many in horticulture to be one of the best soil amendments available. The nutrient content of castings is depends on the material fed to the worms–and worms commonly feed on highly nutritious materials, such as food waste and manures.

Worm castings provide a variety nutrients helpful to promote plant growth and in a form readily available for plant uptake. The biology of the worm’s gut facilitates the growth of fungus and bacteria that are beneficial to plant growth. In addition, many chemical compounds are found in castings that are thought to promote plant growth.

Much of the content of worm castings and their effects on plants are still being studied. Nonetheless, farmers and soils blenders know the benefits of worm castings from their actual effect on plants and product sales, even when the worms are fed low-nutrient materials such as paper fiber.

What kind of worms are used for vermicomposting?

Most worm farms raise two main types of earthworm: Eisenia foetida and Lumbricus rubellis. These worms are commonly used to produce vermicompost, as well as for fish bait. Both are referred to by a variety of common names, including red worms, red wigglers, tiger worms, brandling worms, and manure worms. These two species are often raised together and are difficult to tell apart, though they are not believed to interbreed. While several other species have been successfully bred in recent years, this fact sheet focuses primarily on the use of these species.

The night crawler (Lumbricus terrestrius) is also harvested and sold for fishing bait. This species does not breed well in captivity and is generally harvested from wild stock.

What do worms need?

Worms can survive a wide variety of temperatures, but they thrive best at temperatures between 55 and 77 degrees Fahrenheit (13–25 degrees Celsius). They need a moist, organic substrate or “bedding” in which to live. They will eat the bedding and convert it into castings along with other feed. Moisture and oxygen are vital and bedding should be about as damp as a wrung-out sponge. A worm’s skin is photosensitive and therefore they need a dark environment.

Because worms have no teeth, they need some type of grit in their bedding that they can swallow and use in their gizzard to grind food, much like birds do with small stones. A little soil or sand will work, but it should be sterile so that no foreign organisms are introduced. Common additives used include rock dust or oyster flour (ground up oyster shells).

Since oyster flour is basically calcium carbonate, adding too much will raise the pH in the worms’ environment. Worms prefer a slightly acidic pH level of about 6.5. For a typical worm bin, no more than a tablespoon of grit is needed, which should not significantly alter the pH.

What will worms eat?

Worms will eat a wide variety of organic materials such as paper, manure, fruit and vegetable waste, grains, coffee grounds, and ground yard wastes. While worms will eat meat and dairy products, it is best not to feed these materials or oily foods to worms, due to potential odor and pest problems. Worms will consume limited amounts of citrus scraps, but limonene, a chemical compound found in citrus, is toxic to worms, so it is best to limit or avoid feeding them this material.

Since worms have no teeth, any food they eat must be small enough to swallow, or soft enough for them to bite. Some foods may not be soft enough initially for them to consume, but they quickly degrade so that the worms can consume them.

Where should you keep worms?

Worms can be raised on a small or large scale, depending on your goals. If you are trying to manage food scraps for yourself or your family, a small 12-to 20-gallon worm bin should be adequate. The bin should be dark and opaque and should, have a lid, drainage, and aeration holes in the bottom. Small 1-inch legs and a tray underneath the bin are also helpful.

If you are trying to manage larger amounts of organic materials or produce large amounts of worms or vermicompost, worms can be managed in low-mounded rows called worm beds or “ricks,” or in large in-vessel continuous-flow systems available from suppliers. Worms burrow into the bedding to protect themselves, and they will not come out to sunlight unless bedding conditions are intolerable.

How do you harvest worms and vermicompost?

Large-scale worm farmers using worm beds generally use harvesting equipment to separate worms and castings. In-vessel “continuous flow” systems are generally designed to produce vermicompost. They rely on the surface-feeding tendency of red worms to incorporate a casting harvest mechanism on the bottom of the system, below the active feeding area. Food and additional bedding are added to the top, encouraging the worms to continue feeding upwards.

Smaller scale worm bins are harvested in a variety of ways. In all cases, harvesting should begin when the bedding and consumed food has turned a rich dark brown, with a consistency of coffee grounds. Waiting longer can result in a sludgy material that is difficult to harvest and may become anaerobic and odorous.

One commonly used method of harvesting is to dump the bin onto a tarp in bright light, allowing the worms to burrow down to escape the light. Castings can then be separated by slowly scraping them away, pausing periodically to let the worms burrow further. Eventually, you are left with a pile of worms.

Some will harvest by placing new bedding in one half of the bin, and feed exclusively on that side. Eventually (sometimes over a period of several weeks) most of the worms will move to the side with the new bedding, and the finished compost can be harvested.

One simple method is to place a large amount of food in one area of the bin. Within a few days to a week, this should become a writhing mass of feeding worms. By turning a plastic bag inside out over your hand, you can then “reverse harvest” the worms by simply grabbing the mass of worms and turning the bag right-side out. You then have enough worms to start your bin again. Some worms and egg cases will be left in the castings. This should be no problem if the castings are used soon for indoor potted plants. Castings should be cured before outdoor use.

Harvested castings can be mixed into potting soil soon after harvest for best effect on indoor plants. If they are to be stored or used for outdoor plants, they should be cured in an aerobic environment to dry, eliminate the potential to introduce new species and prevent mold.

Will I need to buy more worms?

Red worms are hermaphroditic, but they need two worms to procreate and exchange DNA. A small egg case, usually amber in color, is produced which can contain from 2 to 20 baby worms.

Worms will regulate their own population according to the conditions of their environment. These conditions include space, moisture, pH, temperature, bedding material, and amount of food available. A typical household worm bin might start out with one pound of worms (approximately 1,000 adults), which will soon multiply to 2,000–3,000 in favorable conditions. Conversely, if one or more of the above conditions are not provided, the worms may crawl out leaving the bin or die off.

What other organisms live with worms?

Worms do not live in isolation. In addition to microscopic organisms like bacteria and some fungi, you may notice several other creatures, such as springtails, mites, pot worms (small white worms often mistaken as baby red worms), and an occasional fungus gnat. These organisms generally stay in the bin, live in harmony with the worms and cause little problems. Consistently burying the food in the bedding will minimize the attraction of unwanted species.

Keeping the bin moist and stirring the castings and bedding periodically will minimize the growth of fungi and the potential of fungal spores. If the bin is not stirred, full-sized mushrooms can grow.

If a bin is kept outside, the number of organisms that find their way into a bin greatly increases. Slugs and snails, ants, spiders, soldier fly larvae, fruit flies, pill bugs, centipedes, even frogs, salamanders and some small rodents have found their way into outdoor worm bins. Rarely will more than three or four of these cohabitants occupy a bin. Most do not hinder the functioning of a bin, and they are not bothersome. It is best to keep outdoor bins outside to prevent the introduction of unwanted animals into your house.

The most common “pests” in worm bins are ants and fruit flies. Keeping the bin moist, stopping feeding for a week or two, and stirring the bin every day can eliminate ants. Fruit flies can be more problematic, and sometimes can only be eliminated by starting over. Short of that, stopping feeding for a couple weeks and using flypaper or other fly traps can work if the population of flies is not too high.

Worms raised in worm beds can also attract predators such as birds and moles. Birds can be deterred in traditional ways such as placing scarecrows near the beds, or the beds can be covered with cardboard or other material. Moles can breed quickly and can eat a lot of worms. They can be deterred either by raising the worms in an in-vessel system, on a cement pad, or placing a wooden or plastic barrier several inches into the soil around the beds. The barrier should stick out of the soil an inch or two to prevent the moles from finding a way over it.

Are vermicomposting facilities regulated?

Vermicomposting is defined as an “excluded activity” from California solid waste regulation. However, an “exclusion” recognizes that a given activity is involved in solid waste handling and therefore must comply with fundamental health and safety codes.

According to California food and agriculture regulations, worms can be considered livestock, much as cows are livestock in a ranching or dairy operation. Within reason, certain organic wastes can be viewed as feed. However, the handling of compostable material prior to and after use as a growth medium is subject to regulation under solid waste regulations.

If a large worm operation becomes a nuisance by taking in more waste than can be effectively fed to and processed by the worms (resulting in odors, for instance), the activity could be viewed as a solid waste facility. Concerns about a particular circumstance should be directed to a community’s environmental health department.

Where can I get more information on vermicomposting?

Individuals interested in pursuing more information on vermicomposting or vermiculture, as well as worm and soil market issues, can contact an existing worm farm, soil blender, or organic waste processor to discuss the present and future possibilities of worm enterprises. Explore the rest of this site for more information on worms.

For more information contact: Organic Materials,