Worm Composting

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Let Worms Eat Your Kitchen Waste


Another method of composting, separate from the backyard bin or compost pile method, is known as vermicomposting, a.k.a., worm composting. This is the practice of using worms to do your composting. Worms are macroorganisms that feed on your fruit and vegetable trimmings, and decompose them for you. The goal of vermicomposting is to create the ideal environment for worms to thrive so they can efficiently decompose the material you feed them. Worm composting is simple. You need a box, bedding, fruit and vegetable trimmings, and worms.

The Worm Bin

worm-boxYou can either buy a worm bin or make your own. A plastic storage box purchased from a local retailer can serve as a great worm bin. A homemade worm box should be a sturdy wooden box with a tight-fitting lid. It can be made from scrap wood or constructed with plywood and two-by-fours. A tight-fitting lid keeps pests out and protects the worms. (The worms will not escape from their box. They are not attracted to light; hence, they migrate toward the center of their box.)

You will need to drill small holes in the top, sides and bottom of your worm bin so the worms to receive fresh air and to allow drainage bottom of the box (this drainage, or "worm tea" is an excellent liquid fertilizer). The holes in your worm bin should be very small, 1/16 inch or less, to prevent flies from entering your bin.

Locating Your Worm Bin

worm-bin-insideThe worms can tolerate temperatures from 50°-84°, but they prefer 55°-77°. They reproduce and compost the fastest when the weather is warm, so if you have your bin outdoors, keep in mind that the worms will not compost food scraps as quickly during the fall and winter as they do in the spring and summer. Also, if your worms are kept outside, be careful not to freeze or overheat them. If your bin is outside, keep it in the shade of a tree or bush during the hot summer months. Do not place the bin on concrete or asphalt—the radiant heat from the paved surface will kill your worms. If your bin is outside in the wintertime, place it in a protected area close to your house so that your worms will not freeze. You may want to move your worms into the garage during the winter to protect them from freezing temperatures. If you are using a plastic storage bin as a worm bin, get an extra bin of the same size, fill the bottom with shredded newspaper, and place your worm bin inside the other bin to prevent any liquid (known as "worm tea"--an excellent fertilizer) from leaking onto your garage floor.

The Bedding

worm-beddingCommon bedding materials include shredded newspaper, corrugated cardboard, coarse peat moss, and partially decomposed leaves. Worm boxes should be filled with bedding to provide the worms with a balanced diet as well as a damp, aerated place to live.

Tear newspaper or cardboard into one-to-two-inch strips before using as bedding. Bedding material should be thoroughly moistened by soaking it in water for several minutes. Squeeze out the excess water before adding bedding material to the worm box. Eight or ten inches of bedding inside your bin will provide the worms with a damp, aerated place to live. Add a handful or two of soil to provide grit that will help the worms digest food particles.

Always cover food waste with a few inches of bedding to prevent flies from becoming a problem. Rotate the location where food is buried in order to provide the worms with a balanced diet of kitchen scraps and bedding.

The Worms

red-wormsRed worms, known as red wigglers or manure worms, are the best for vermicomposting. They thrive on organic material such as fruit and vegetable trimmings. Red worms are not the same as earthworms or night crawlers, which need mineral soil to survive.

One to two pounds of worms are needed to start a worm box. Worms regulate their own population based on how much food is available. In other words, if they have a lot to eat, they increase their population. If food is scarce, their population decreases until the food supply increases. Worms reproduce very quickly. It is estimated that eight worms can multiply to 1,500 worms in six months. Once your system is established, there will be plenty of worms to help your friends get worm bins started.

Red wigglers are given out for $5 after attending a worm composting class taught by the City of Davis Recycling Program staff. For class dates and times, contact the City of Davis Recycling Program at 757-5688, or email pwweb@cityofdavis.org.  Locally, red wigglers are also available through UC Davis Project Compost (754-8227) and Davis Ace Hardware (758-8000).

Feeding the worms

worm-bin-picWorms will eat fruit and vegetable peelings, coffee grounds and filters, tea bags, crushed eggshells, rice, pasta and bread. To avoid odors, raise your worms as vegans and do not feed them any meat or dairy products. Do not feed your worms salad leftovers if there is any dressing on it, or pasta with a meat sauce. Be careful not to add too much acidic material to your worm bin—coffee grounds and citrus scraps are tasty to worms, but too much acidity in the bin can kill your worms. If your worms are avoiding the coffee grounds or citrus scraps in your worm bin, stop adding acidic material—the bin may be too acidic. Be sure to bury your food scraps in the bedding to discourage molds and fruit flies. Bury the food in a different corner of the box at each feeding. The next time you look, it should be gone. If not, try feeding your worms a little less for a while. Do not overfeed your worms! Too much food in a worm bin can cause odors and may create an unhealthy environment for your worms.

DO feed the worms Do NOT feed the worms
  • fruits and vegetables
  • bread, rice, pasta
  • coffee grounds and filters
  • small bits of plant material
  • tea leaves with bags
  • crushed eggshells
  • meat, dairy, bones, fish
  • greasy foods
  • peanut butter
  • pet/animal litter/feces
  • salad dressing
  • vegetable oil

The Harvest

worm-compostMake sure that there is always at least 3 inches of space between the top of the bedding and the lid of the worm bin to allow your worms enough fresh air to breathe. After a few months, you will notice that the original bedding has disappeared and has been replaced with rich, dark worm compost. You may also notice that you have 3 inches or less of space at the top of your bin. It's time to harvest the compost!

Put your worms on a diet. In a few days, you can either 1) separate the worms from the compost by hand and start anew with fresh bedding, or 2) coax your worms to one side of your box by feeding only on that side for a few weeks. Then harvest the worm-free side of the box, replace it with fresh bedding and do it again. When you start scooping out the compost, you may notice that there are still a lot of worms in it. Place the compost in a pile on a flat surface (as long it is not too hot!). The worms will bury themselves deep in the pile to avoid the sun, so you can scrape off the compost and put the worms back in the bin. Once you have removed the compost, dry it out in the sun and screen out any un-composted scraps left. Those can be thrown back inside the worm bin. Once the compost has been cured in the sun, it can be added to your garden—a home-made, natural fertilizer!

Trouble Shooting Guide for Worm Composting

Symptoms Problems Solution
Worm Bin smells bad Too much food Feed less
Too Wet Check drainage holes/add dry bedding.
Fruit Flies Food isn’t buried Bury food. Cover surface of bedding w/carpet or a sheet of newspaper
Worms aren’t eating Too much food Feed less
Too acidic Stop feeding citrus peelings. Sprinkle w/crushed oyster shells

More Information

  • More information about worm composting can be found on the CalRecycle web site.
  • Copies of Mary Appelhof's book Worms Eat My Garbage (1982) are available for loan at the Davis branch of the Yolo County Public Library, 315 East 14th Street, 757-5591.
  • Red wigglers are given out for $5 after attending a composting class taught by the City of Davis Recycling Program staff. For class dates and times, contact the City of Davis Public Works Department at 757-5686, or email PWWeb@cityofdavis.org.
  • Locally, red wigglers are also available through UC Davis Project Compost (754-8227) and Davis Ace Hardware (758-8000).