Worm Composting

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Let Worms Eat Your Kitchen Waste

worm-bin1-2 

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. An opaque plastic storage box purchased from a local retailer can serve as a great worm bin. Look for a bin that is relatively square. A long, thin, plastic storage bin that fits underneath a bed has a lot of surface area and will not provide your worms with any thermal protection. In a more square-shaped bin, your worms can migrate to the interior of the bin if the outside temperatures become too hot or cold and the surrounding bedding can provide some thermal insulation for them.

A tight-fitting lid is essential to keep pests out and protect 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—but you will need to make sure there are no gaps around the rim of the lid or any holes larger than 1/16 inch so that flies cannot access your worm bin and produce maggots.

Drill 10-20 small holes (no larger than 1/16 inch) in the lid of your worm bin so the worms receive fresh air. Drill another 10-20 holes into the bottom of your worm bin to allow excess moisture to drain out of the worm bin (this drainage, or "worm tea" is an excellent liquid fertilizer).

Locating Your Worm Bin

worm-bin-inside

Composting worms prefer temperatures from 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. Worms are susceptible to high temperatures and should never be placed in the sun. If your worm bin receives any sunlight at all, the heat from the sun can build up in the bin, creating a greenhouse effect and killing all of your worms.  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—on a hot day the radiant heat from the paved surface can 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-bedding

Common bedding materials include shredded newspaper, corrugated cardboard or coarse peat moss. Worm boxes should be filled to the top with bedding to provide the worms with a place to live. Just as a fish tank needs to be filled with water, the worm bin must be filled to within 2-3 inches of the top with bedding.

Shred paper through a vertical shredding machine (do not use cross cut paper shreds) or tear paper into 1/2 inch strips or smaller. 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 bin. It will take a LOT of paper to fill a worm bin. Add a handful or two of soil to provide grit that will help the worms digest food particles.

The Worms

red-worms

Red worms, also known as red wigglers, 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 bin. 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.

Locally, red wigglers are available from the City of Davis after completing this course (see details below), through UC Davis Project Compost (cce@asucd.ucdavis.edu) and often through local garden and hardware stores.

Feeding the worms

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Worms will eat fruit and vegetable peelings, coffee grounds and filters, tea bags, rice, pasta and bread. Treat your worms as vegans and do not feed them any meat, dairy products, greasy foods, salad with dressing or pasta with a meat sauce. Be careful adding acidic material to your worm bin—coffee grounds and citrus scraps are tasty to worms, but too much acidic food can kill them. If your worms are avoiding the coffee grounds or citrus scraps in your worm bin, stop adding acidic material.

Be sure to bury your food scraps in the bedding to discourage molds and fruit flies. Never leave food on top of the bedding. Bury the food in a different corner of the bin at each feeding. Before feeding your worms, check to see if they've eaten the food you gave them last time. If they have eaten more than 60% of what you gave them, you can feed them a little more food. If they have not eaten most of the food from the last feeding, wait until they have eaten most of it before feeding them again. Do not overfeed your worms! Too much food in a worm bin can cause odors and may create an unhealthy environment for your worms.

You will need to add more bedding every few months as the worms eat the bedding as well. Always moisten the bedding before adding it to the bin. Always make sure there is enough fresh bedding to provide at least 2 inches of cover over the food scraps in the worm bin. If the food scraps are exposed, flies may gain access to the food and your bin may develop maggots. 

If you develop maggots and/or flies in your bin, add more bedding to cover exposed food scraps with fresh bedding. Check to see if there are any holes in the top of your bin that are large enough for flies to get in (any hole larger than 1/16 inch). Check for holes in the side or the bottom of your bin where flies may be able to access and lay eggs directly into your bin. Cover unwanted or large holes with duct tape and wait for the maggots to pupate, turn into flies and leave. Do not spray pesticides into or around your worm bin.

The Harvest

worm-compost

After a few months, you will notice that the original bedding is disappearing and is being replaced with rich, dark worm compost. When you have at least an inch of worm compost at the bottom of your worm bin you can harvest it.

To harvest the compost, put your worms on a diet; let them eat all the food in their bin. Then, place food in one corner of the bin. By the next day, most of the worms will be on that side of the bin eating. Remove the worm-free side of the box and set it aside. Spread the remaining contents across the bottom of the worm bin and refill the bin with fresh bedding. It's not a good idea to harvest the entire bin at once since worm cocoons and young will be hidden in the bedding and compost. Harvesting can also be stressful for the worms, so it's a good idea to leave some of them in the bin during harvest.

When scooping the contents of the worm bin, you probably noticed worms, some food and bedding still mixed in with the compost. Most people prefer to separate the compost before using it. To do this, place the contents that you removed from the worm bin in a pile on a flat surface. The worms will bury themselves deep in the pile to avoid light, so you can scrape off materials from the side of the pile a little at a time. Place the un-composted bedding, food and any worms you find back inside the worm bin.

Lay the worm compost out in the sun to cure and dry. Once the compost has been cured in the sun, it can be added to your garden—a homemade, natural fertilizer!  If any fresh food scraps you fed your worms contained seeds, the seeds are likely still viable and may sprout in the worm bin or in the finished compost.

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 in begging materials. Add more fresh, damp bedding.
Worms aren’t eating Too much food Feed less
Too acidic Stop feeding citrus peelings and coffee grounds.

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 (cce@asucd.ucdavis.edu) and Davis Ace Hardware (758-8000).