Worm Bin Composting: It's Gross But It's Beautiful

Claire Splan

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Worms play a big part in the health of your garden’s soil, and you can harness the benefits of their hard work by vermicomposting. If that term sounds intimidating, relax. It just boils down to keeping worms in a bin, feeding them your scraps, and then making use of the nutrient-rich waste, called worm castings, that results.

If it all sounds a little gross to you, let me assure you that the results you'll see in your garden will be beautiful. Soil enriched with worm castings will reward you with healthy, vigorous plants.

Follow these steps to set up, maintain, and harvest the compost from a vermicomposting worm bin.

Set up your worm bin

1. Choose a bin to house the worms. You can purchase a commercially made worm bin that will be well-suited to worm-living, or you can adapt a lidded plastic box on your own. The bin should be 8 to 12 inches deep and have about a dozen or more 1/2-inch air holes. The size of the bin should be based on the size of your household and the amount of food scraps you generate. Two people typically produce enough scraps (about 1/2 pound) each day to feed 1 pound of worms, which need about 4 square feet of space, so a 2 × 2-foot bin would be just right. For every additional two people, add another pound of worms and another 4 square feet, either as a bigger bin or additional separate bins.

2. Once you have the right bin, look for the right place to set it up. Worms need ambient temperatures from 55 to 80 degrees Fahrenheit, so it’s best not to put the bin in full sun or near anything that may throw off too much heat. A spot that is sheltered and shady will work best. You should also consider convenience when choosing the location. You want to make it easy to feed your worms every day or every other day, so you may choose to keep your worm bin in the house (under the kitchen sink, in the laundry, or mudroom), on the back porch, or close to the back door. If, after you’ve established your worm bin, you find that the location isn’t working well, you can move it, but take care. A full worm bin can be quite heavy.

3. With the bin in place, you’re ready to make a bed for the worms. The simplest and most accessible bedding material is shredded newspaper, but you can also use shredded cardboard, coconut coir, or peat. Mix in a couple cups of garden soil or some compost to add some healthy microorganisms to aid the decomposition work.

4. Wet the bedding material with dechlorinated water. (To dechlorinate common tap water, pour a pitcher or bucket of water and just let it sit for a day or two to let the chlorine dissipate.) The bedding material should be completely moist but not sopping. If you squeeze a handful of it and only get a couple of drops of water, that’s just right.

5. You’re ready to add the worms. You should use red worms for composting, not earthworms. Red worms (typically Eisenia fetida or Lumbricus rubellus) tend to dwell in shallow soil, but earthworms like to dig much deeper. You can order red worms by the pound through mail-order sources or sometimes find them at your local nursery. Scatter the worms over the top of the moist bedding material, then give them a day or two to settle in before feeding them.

Feed your worms

To feed your worms, collect food scraps from your kitchen including the following:

  • Fruits and vegetables
  • Cereals and grains
  • Used tea bags, coffee grounds, and coffee filters
  • Eggshells
  • Food-soiled paper towels, cardboard, and paper containers (as long as they are not chemically treated and are torn into small pieces)

Avoid adding the following items:

  • Meat
  • Dairy products
  • Citrus fruits or hard rinds such as melon
  • Bread, pasta, or processed grain products
  • Fats and oils

You can add limited amounts of green waste from the garden (leaves and lawn clippings), as long as it’s chemical-free, broken into small pieces, and added in very small amounts. Adding too much plant debris can lead to the temperature in the worm bin growing too high as it decomposes. Food waste, on the other hand, tends to break down faster through the work of bacteria first, and then the worms finish the job.

Whenever you add food scraps to the bin, cover it with more bedding material and wet it sufficiently to get it back to the original state of moistness. Don’t add more food until what you’ve already given them is mostly gone. As the population of your worm bin grows, the food will disappear more quickly and you’ll need to feed them more often.

Troubleshoot problems

Sometimes something goes awry in the worm bin and you’ll notice a problem. Here are a few tips for troubleshooting worm bin situations:

  • The worms disappear. I’ve had this happen. Usually, it’s because the bin gets too wet, or because something was added that was too acidic or contained chemicals. If only some of the worms have fled, adjust the moisture by mixing in more dry newspaper or remove the uneaten food that may have bothered them. If all the worms have disappeared, remove all the contents of the bin and start over with fresh bedding material and new worms.
  • The worm bin is attracting insects. Whether or not this is really a problem depends on the type of insect. Insects you may not want hanging around can still be helpful in the composting process, but clouds of fruit flies or trails of ants in and around your bin can easily be taken care of. Ants will not go into a bin that is properly moistened so if they’re in your bin, it’s a sign that it’s too dry. Fruit flies will only lay eggs on the surface of decaying food, so if you see significant numbers of fruit flies it’s an indication that you are leaving the food scraps exposed instead of covering them with more bedding material. Most of the other insects that you may find in your bin are harmless and may help with the decomposition process, but if they’re appearing in large numbers it may indicate that something in the food scraps is too acidic or otherwise unsuitable.
  • The worm bin is starting to smell. This can be caused by food that is rotting before the worms can eat it. You may have added too much food at one time for the size of your worm population, or the ratio of carbon to nitrogen may be off. Adding brown material such as dead leaves, chemical-free sawdust, or shredded newspaper will usually fix this problem. An odor problem can also result from the bedding getting too wet, which can create an anaerobic environment. That leads to the production of the wrong kind of bacteria, which creates the bad smell. Again, adding more brown material and stirring the bedding a bit will help it dry out and re-oxygenate, which should get the decomposition process re-started.
  • You need to leave your worm bin untended while you’re on vacation. This isn’t a problem. The worms can take care of themselves for a week or two, probably even up to a month. It’s a good idea, however, to add more bedding material to the bin before you leave and make sure that it is sufficiently moistened.
Use the worm compost

After a few months, your worms should have created a substantial amount of compost, and there may be little bedding or food left. It’s then time to harvest the compost using one of the following methods:

  • Dump the entire contents of the bin onto a large sheet of plastic and start building numerous little piles. Rather than be exposed to the light, the worms will burrow to the bottom of the piles. You can scrape off the top of the piles to use as compost and return the bottom of the piles back to the worm bin with fresh bedding.
  • Set a medium-fine screen over a large tub or box and move the bin contents onto the screen a shovelful at a time. The screen will catch undecomposed bedding and food scraps as well as the worms, which you can set aside to be returned to the worm bin with fresh bedding. What will fall through the screen is the fresh compost.
  • Stop adding food scraps to the bin for a couple of weeks. Then push the contents of the bin to one half of the bin, and fill the other half with fresh bedding and food scraps. Add food scraps only to the new bedding. After a couple of weeks, the worms should have all shifted over to the new bedding where the food is. You can then scoop out the finished compost on the other side.

Finished vermicompost can be used like other compost — as a side- or topdressing on plants or mixed into potting soil for containers. It’s loaded with nutrients your plants will love and microorganisms that will keep your soil healthy.

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Writer/editor. Author of "California Fruit & Vegetable Gardening" and "California Month-by-Month Gardening."

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