Vermicomposting 101

Vermicompost, or castings, is worm manure. Worm castings are considered by many in horticulture to be the very best soil amendment available. The nutrient content of castings is dependent on the material fed to the worms–and worms are commonly fed materials with high nutrient content, such as food waste and manures.

Worm composting is using worms to recycle food scraps and other organic material into a valuable soil amendment called vermicompost, or worm compost. Worms eat food scraps, which become compost as they pass through the worm’s body. Compost exits the worm through its’ tail end. This compost can then be used to grow plants. To understand why vermicompost is good for plants, remember that the worms are eating nutrient-rich fruit and vegetable scraps, and turning them into nutrient-rich compost.

The vermicomposting needs specific details in the environment.

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.

There are several housing options for vermicomposting. When choosing a house in which to compost with worms, you should keep in mind the amount of food scraps you wish to compost, and where the bin will be located.

Your bin needs to be only 8 to 16 inches deep, since compost worms are surface feeders. You can build your own bin by using a washtub, dish pan, used shipping crate or a commercially available worm bin. Just be sure your bin has a lid to keep out flies and rodents. It also needs holes in the bottom (a quarter inch or smaller), for ventilation and drainage.

The rule of thumb for bin size is two square feet of surface area per person, or one square foot of surface area per pound of food waste per week. Because worms like moderate temperatures, place your bin in a shady location where it will not freeze or overheat. Some good locations include:

  • Kitchen corner
  • Garage
  • Basement
  • Patio
  • Outside the back door
  • Laundry room

Plastic or wooden containers are preferred. Because of their photosensitive skin, using glass or clear plastic wouldn’t be very sensitive. We used a Styrofoam box. Initially we had a one from Subway’s dumpster, but it was cracked and the stuff was leaking through and we couldn’t find another, so we had to by one. Still, I would check around in some dumpsters before choosing to go out and buy one. No matter what material you choose, make sure to rinse out the container before using. Drill a hole every couple 1 ½ inch or so. For wooden bins, line the bottom with plastic (e.g. from a plastic bag or old shower curtain). Cover the bin with a loose fitting lid. This lid should allow air into the bin.

Such a system can even be kept indoors. With the exception of holes for drainage and ventilation, worm bins for indoor use are typically completely enclosed, with a lid of some sort to cover the top. Outdoors, worms can be turned loose in a pile in your compost bin, or contained in a worm bin built specifically for vermicomposting.

To feed only use raw fruit and vegetable scraps and stay away from meats, oils and dairy products. They are more complex materials than fruits and vegetables, so they take longer to break down and can attract pests. Cooked foods are often oily or buttery, which can also attract pests.

Avoid orange rinds and other citrus fruits, which are too acidic, and can attract fruit flies so don’t use them. Try to use a variety of materials. Stay away from onions and broccoli which tend to have a strong odor.

Some municipalities, fearful of rodent pests and the diseases they may carry, discourage or even prohibit the composting of food wastes in open piles, recommending enclosed worm bins instead. A sturdy outdoor worm bin is protected from pests, and produces compost quickly during the warm season (or year-round in mild climates).

The next thing you have to think about is bedding. Newspaper strips or leaves, will hold moisture and contain air spaces essential to worms. Black and white newspaper is the most readily available and easy-to-use bedding material. Tear it into strips about one inch wide and moisten so it is as damp as a wrung-out sponge. Cow or horse manure can also be used to lighten bedding and absorb excess moisture.

The general idea is to provide a cool, moist bedding (some kind of ‘brown’ compost ingredient such as shredded leaves or paperboard) for the worms to live in, and then bury kitchen wastes in the bedding. As bacteria and fungi begin to decompose the materials, the worms graze on the bacteria and fungi, and also break up the ingredients with their movement through the bedding.

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.

Now you’re ready for your worms.

A typical earthworm from the garden won’t do. Vermicomposting requires a species that is adapted to living in decomposing organic materials rather than in the soil. Two species are Eisenia foetida also known as the redworm, manure worm or red wiggler and Lumbricus rubellus. Eisenia foetida is often available at bait shops (ask for red wigglers), can be ordered from a worm farm, and governments and organizations that promote vermicomposting may maintain ‘worm banks’ as a low-cost source of worms for the general public.

The best kind of worms for composting are “red worms” or “red wigglers.” They are often found in old compost piles, but are different from the earthworms you would normally find in the ground. These worms have a big appetite, reproduce quickly and thrive in confinement. They can eat more than their own weight in food every day!

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.

A handful or two of soil, ground limestone or well-crushed eggshells every few months are good for providing grit and calcium. Fill your bin with moistened bedding, toss in a few handfuls of soil, and you are ready to add the worms and food. Over time, the bedding and food are eaten by the worms and turned into dark worm compost.

Begin feeding your worms only a little at a time. As they multiply, you can add larger quantities of food waste. Bury the waste into the bedding regularly, rotating around the bin as you go. When you return to the first spot, most of the food you buried there should have been eaten. If not, don’t worry. Just feed the worms less for a while.


If you take care of your worms and create a favorable environment for them, they will work tirelessly to eat your “garbage” and produce compost. As time progresses, you will notice less and less bedding and more and more compost in your bin. After 3-5 months, when your bin is filled with compost (and very little bedding), it is time to harvest or remove the finished product from the bin. After several months, worms need to be separated from their castings which, at high concentrations, create an unhealthy environment for them.

To prepare for harvesting, do not add new food to the bin for two weeks. Then try one of two methods for harvesting:

Push all of the worm bin contents to one half of the bin, removing any large pieces of undecomposed food or newspaper. Put fresh bedding and food scraps in empty side of bin. Continue burying food scraps only in freshly bedded half.

Over the next 2-3 weeks, the worms will move over to the new side (where the food is), conveniently leaving their compost behind in one section. When this has happened, remove the compost and replace it with fresh bedding. To facilitate worm migration, cover only the new side of the bin, causing the old side to dry out and encouraging the worms to leave the old side.

Hands-On Method:

Dump the entire contents of the worm bin onto a sheet of plastic or paper. Make several individual cone-shaped piles. Each pile will contain worms, compost and undecomposed food and bedding. As the piles are exposed to light, the worms will migrate towards the bottom of the pile. Remove the top layer of compost from the pile, separating out pieces of undecomposed food and newspaper. After removing the top layer, let pile sit under light for 2-3 minutes as the worms migrate down. Then remove the next layer of compost. Repeat this process until all of the worms are left at the bottom of the pile.

Regardless of which method you choose, the compost you harvest will most likely contain a worm or two, along with old food scraps and bedding. If you are using the compost outdoors, there is no need to worry–the worms will find a happy home and the food scraps and bedding will eventually decompose. If you are using the compost indoors, you may want to remove old bedding and food scraps for aesthetic purposes and ensure that there are no worms in the compost. Though the worms will not harm your plants, the worms may not like living in a small pot.

For both methods, you may continue to compost your food scraps after harvesting. Just add fresh bedding and food scraps.

Using Your Worm Compost

Worm compost is more concentrated than most other composts because worms are excellent at digesting food wastes and breaking them down into simple plant nutrients. Use it sparingly for best results. To mulch with worm compost, apply a one-inch layer to the soil around plants. Be sure the worm compost is not piled against plant stems. To amend soil, worm compost can be spread one-half to two inches thick over garden soil and mixed in before planting, or mixed into the bottom of seeding trenches or transplanting holes. You can also mulch your worm compost into:

  1. Houseplants: Sprinkle worm compost around the base of plants to fertilize. Each time you water, plant      nutrients will seep into the soil.
  2. Potting Mixes: For healthy seedlings, mix one part worm compost with three parts potting mix or three      parts sand and soil combined. Peat moss, pearlite and worm castings are also good ingredients to add.

You can also make “compost tea” with your compost. Simply add 1-2″ of compost to your water can or rain barrel. Allow compost and water to “steep” for a day, mixing occasionally. Then water plants as you normally would. The resulting “tea” helps make nutrients already in the soil available to plants. Also sometimes there is tea in the bottom.

Additional things you should know:

Worms can live for about one year in the worm bin. If a worm dies in your bin, you probably will not notice it. Since the worm’s body is about 90% water, it will shrivel up and become part of the compost rather quickly. New worms are born and others die all the time.

The baby worms live in the egg case for at least 3 weeks, sometimes longer depending on the surrounding conditions. For example, in the winter time, baby worms may stay in the cocoon for many weeks until the temperature worms up again. When the baby worms eventually crawl out, they are the thickness of a piece of thread and possibly 1 cm 1/4″ long. Usually the worms appear white, as they have not yet developed pigmentation, or do not have enough pigmentation (or blood) to be seen.

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. We didn’t start out with that many. We only had six and we’ve just got more), which will soon multiply to 2,000–3,000 if conditions are good. Conversely, if one or more of the above conditions are unacceptable, the worms may crawl out leaving the bin or die off.

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 increases greatly. 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.

Warning Signs

Some symptoms that your worm composting is not going as well as it could are:

  • If your worms are dying (at an unnatural rate)
  • If your bin smells rotten and/or attracts flies

Worms Dying

If your worms are dying there could be several causes:

  1. It may be that they are not getting enough food, which means you should bury more food into the      bedding.
  2. They may be too dry, in which case you should moisten the box until it is slightly damp.
  3. They may be too wet, in which case you should add bedding.
  4. The worms may be too hot, in which case you should put the bin in the shade.
  5. The bedding is eaten, and it is time to add fresh bedding.

Bin Smells

If your bin smells rotten and/or attracts flies, there may be three causes:

  1. First, it may be that there is not enough air circulation. In this case, add dry bedding under and over      the worms, and do not feed them for two weeks.
  2. Second, there may be non-compostables present such as meat, pet feces or greasy food. These      should be removed.
  3. Third, there may be exposed food in the bin. In this case, secure the lid, cover food scraps with      bedding, and cover worms and bedding with a sheet of plastic.



2 responses to “Vermicomposting 101

  1. What a great well detailed post. I’ve been raising worms on a heavy dose of coffee grounds with great results, along with banana peels and other fruit.

    Keep up the great work.

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