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Vermicomposting: Composting with Worms

Vermicomposting, also know as worm composting, is the digestion of food scraps by worms. It's odorless, easy, and fascinating! Worm composting can be done indoors year-round, making it a great option for complying with Vermont's Universal Recycling Law at home, work, or school.

Red wiggler worms (scientific name Eisenia fetida) are very efficient at breaking down food scraps and generating "castings" (worm poop) that are an excellent soil amendment for house plants and gardens.

To start composting with worms, you need just a few basic materials:

  • Red wiggler worms (Eisenia fetida)
  • Worm bin
  • Bedding (shredded newspaper or other paper)
  • Food scraps

Read on for details about each part of a worm composting system.

Use the right worms

Red wiggler worms (scientific name Eisenia fetida) are the preferred species for vermicomposting. They do well in environments with lots of organic matter at temperatures between 55 and 75 degrees Fahrenheit. Red wigglers will not usually survive below 32 or above 85 degrees Fahrenheit, so keep your worm bin indoors. Common garden worms will not survive in a worm bin.

You may find someone in your community who is already vermicomposting that could give you some worms to get started (try posting on Front Porch Forum). Red wiggler worms can also be purchased at some bait shops, or from suppliers such as Gardener's Supply Company (VT), the Worm Ladies (RI), Red Worm Composting, Planet Natural, or The Worm Farm. When purchasing worms, ensure that the species you are purchasing is Eisenia fetida. 

Worm bins

Ready-made worm bins such as the Worm Factory the VermiHut or the Worm Farm Composter are available for purchase. You can also make a worm bin out of one or more plastic tubs or buckets by drilling holes in them for air flow and drainage. See the instructions below for one method. There are lots of other examples and instructions for do-it-yourself worm bin available online. Feel free to get creative with the materials you have on hand! Just make sure your bin has ventilation and a way of preventing buildup of excess liquid. 

The worm bin should be opaque, because worms do not like to be exposed to light. A worm bin made of one container set inside another will make harvesting the worm castings easier if you add food scraps to only one bin at a time. The worms will migrate towards the food source and leave their castings behind. 

Materials:

  • One or two plastic containers with a lid. An opaque plastic storage tub works well. The volume should be at least one cubic foot.
  • Drill with quarter-inch (or similar size) bit
  • Newspaper, clean cardboard, used paper towels (chemical-free), or paper egg cartons for bedding
  • Fine screen to keep out pests & glue (optional)
  • Tray (optional)

Instructions:

  1. Drill several holes in the lid of one container. If using a quarter inch bit, drill 8 to 10 holes (more if using a smaller bit, fewer if using a larger bit). 
  2. Drill several holes in the sides of the container (both containers if using two) at about an inch or two below the rim of the bin.
  3. Create a system for drainage. If you are using one container, you can drill several holes in the bottom of and use a tray to catch liquid that comes out. If you are using two containers, you can drill several holes in the bottom of both containers and use one of the lids as a tray to catch liquid from the bottom. Or, you can drill several holes in the bottom of one container and leave the bottom of the other container intact. The container with holes in the bottom will be placed inside the container without holes in the bottom. 
  4. Glue screen over holes (optional). If desired, attach fine screen over the holes you drilled to help prevent fruit flies or other small invertebrates from entering. See below for other ways to prevent or control pests.
  5. Make bedding and add worms. Use a shredder or your hands to finely shred or tear the newspaper, cardboard, or paper towels into one-inch pieces or narrow strips to use as bedding. Moisten the bedding by misting it thoroughly with water or soaking it in water and squeezing out the excess. The bedding should be about as moist as a wrung-out sponge. Fill the bin (both layers if using two containers) about two-thirds full of bedding and gently add the worms on top of the bedding. They will burrow down by themselves. Now you're ready to feed your worms!

Feeding the worms

Under ideal conditions, a red wiggler processes half to one times its own weight in food scraps every day, meaning that one pound of worms will process up to a pound of food scraps per day. Feed worms about once a week, and wait to add more food until they have finished their last meal. Pay attention to how much your worms eat on a regular basis, and adjust the amount you feed them accordingly. You can bury fresh food scraps in the bedding already in the bin, or add the food scraps on top and cover with fresh, moist bedding (see below).

Menu

Feed your worms mostly scraps from fruit and vegetables. Chopping scraps into smaller pieces or freezing and thawing the scraps before feeding to the worms can help speed up decompostion and reduce potential for odors.

You can also add coffee grounds and filters, paper tea bags and tea leaves, plain pasta and rice, and crushed egg shells in modest amounts. Breads and pineapple can be added in minimal amounts.

Do not feed worms scraps from meat or dairy products, onions, garlic, leeks, citrus rinds, hot peppers, oils, butter, vinegar, spicy or pickled foods, or more than small amounts of pineapple. You may also want to avoid feeding worms broccoli or other cruciferous vegetables if you notice a strong odor.

Bedding

After adding food scraps, cover with a layer of fresh, moist bedding. Bedding is the necessary substrate for the worms to move through, and it also adds to their diet. Bedding can be shredded newspaper, cardboard, dry leaves, paper egg cartons, or chemical-free paper towels. Bedding should be in one-inch pieces or thin strips to prevent compaction. Moisten bedding by misting or sprinkling with water; squeeze out excess water so the moisture level is about that of a wrung-out sponge. Moisture is critical for the survival of the worms, but too much moisture will prevent them from getting the oxygen they need. You can also add a small amount of soil to provide additional microorganisms and grit for the worms to digest. 

Harvesting the castings

Depending on the type of worm bin, there are multiple ways to harvest castings:

If your bin has only one level, move all the bedding and castings to one side of the bin. Add fresh, moist bedding and food scraps to the other side, and continue feeding the worms on only that side for a couple weeks until all the worms have moved to the side with the fresh food. That should leave mostly worm-free castings on the other side for you to scoop out.

If your bin has multiple levels, add fresh bedding and food scraps to only one of the levels at a time. Once one level is filled with castings, start adding food and fresh bedding to another level. After a week or two, all the worms should have moved through the holes in the bin to the level with the fresh food. That should leave mostly worm-free castings in the other level for you to scoop out.

Another option is to remove all the material and worms from the bin onto a cleanable surface. Separate the material into small mounds, and shine a bright light onto the piles of material. The worms will move away from the light toward the center of the piles. Remove the outer material from the piles, repeating as necessary until you have harvested all you can without also collecting worms. Return the worms to the bin with any remaining material, fresh bedding, and fresh food scraps.

Alternatively, just scoop out material, pick out the worms by hand, and put the worms back in the bin with fresh food and bedding.

Using the castings

Worm castings are rich in organic matter and biological activity, which help improve the structure, nutrient storage and availability, and water-holding capacity of soils. Use the castings just like you would other compost: mix into potting soil, spread on top of the soil in your garden or potted house plants, put some at the bottom of a hole before transplanting, or try making compost tea to water plants. 

Troubleshooting

Contents too wet: Too much moisture will not allow the worms to breath and may create unpleasant odors. Visible mold could also be a sign of conditions that are too moist. If the bedding is wetter than a wrung-out sponge, add dry bedding to soak up excess moisture. You can also reduce the amount of moist food scraps you add.

Contents too dry: Consistent moisture is important to maintain a hospitable environment for the worms. If the bedding seems to be drier than a wrung-out sponge, sprinkle or mist with water, or add more moist food scraps. 

Odor: Make sure all food scraps are covered with bedding to prevent odor. Some foods are naturally odorous when decomposing (such as onions, broccoli, or cabbage). Remove foods that produce unpleasant odors if it bothers you. Don’t add meat, bones, dairy, or oil products. The bin may also smell if it is too wet or not getting enough air. In that case, aerate the bin by gently mixing some fresh, dry bedding into the contents of the bin.

Scraps decomposing very slowly: Chop food into small pieces before feeding to the worms, especially tough materials such as stems. Also try freezing and thawing food scraps to soften them before adding. You may simply be overfeeding the worms. Try feeding the worms less, and wait to add more food until the previous batch is gone.

Fruit flies: Fruit can be microwaved for 60 seconds or frozen to kill any fruit fly eggs before being fed to the worms. Bury fresh scraps under bedding when you add them to the bin to reduce odors that attract flies. Or, don't add any food scraps to the bin. If necessary, make a fruit fly trap by mixing apple cider vinegar and a dash of dish soap in a cup. Optionally, cover the cup with plastic wrap secured with a rubber band, and poke some small holes in the plastic wrap. 

Mites: A small mite population is good, but if you notice large collections of mites you should try to remove them. Remove any food that has a
congregation of mites. Add a few slices of melon (watermelon and cantaloupe work well) to the bin. Mites will congregate on the melon in a short period of time. When the slices are covered, remove, rinse off the mites, and then then return the melon slices to the bin and repeat until the population is reduced. You can also try placing a slice of fresh bread in the bin, waiting until mites congregate on it, and then removing the bread.

Other pests: A small population of invertebrates indicates a healthy ecosystem in your bin. However, if other insects are swarming your bin, the only solution may be to harvest the worms (see above) and start fresh in a clean bin with new bedding and food scraps. 

Few or no worms: If the conditions are not suitable, worms will die and decompose quickly, leaving you with few worms. Unsuitable conditions could be too wet, too dry, too hot, too cold, not enough air, or not enough food. The worm bin should be kept between 55 and 75 degrees Fahrenheit, should be not too wet or dry (see above), and should have air space in the bedding. If contents of your bin seem very compacted, add paper tubes or other bulky paper products such as torn up paper egg cartons to increase air flow. Worms can last a while without being fed, but make sure not to let too much time pass between feedings. 

Sources

“Chapter 14: Vermicomposting, Worm Composting.” The Vermont Master Composter Resource Manual, State of Vermont Agency of Natural Resources, 2017, pp. 33–36.

“Composting with Worms.” CSWD, Chittenden Solid Waste District, 10 July 2019, cswd.net/composting/composting-with-worms/.

NYC Master Composter Manual. NYC Compost Project, 2015.