The Perfect Diet for Compost Worms

Worms Eat My Garbage (Storey, 2017) by Mary Appelhof and Joanne Olszewski walks readers through every step of starting and maintaining a successful worm composting system for their homes. This 35th Anniversary edition is fully updated to help new generations begin their own worm composting systems. The following excerpt is a guide on what to feed and not feed compost worms.

Any vegetable waste that you generate during food preparation can be used: potato peels, grapefruit and orange rinds, outer leaves of lettuce and cabbage, celery ends, and so forth. Plate scrapings might include macaroni, spaghetti, gravy, vegetables, or potatoes. Spoiled food from the refrigerator, such as baked beans, moldy cottage cheese, and leftover casserole also can go into the worm bin. Coffee grounds are very good in a worm bin, enhancing the texture of the final vermicompost. Tea leaves, even tea bags, and coffee filters are suitable.

Eggshells can go in as they are. I have found as many as 50 worms curled up in one eggshell. Usually, I dry the shells separately, then pulverize them with a rolling pin so they don’t look quite so obvious when I finally spread the vermicompost in my garden. Grinding up eggshells also increases its surface area. This makes calcium carbonate more readily available to the microorganisms and other decomposers in the bin and, later, to plants in the garden.

The list below shows some of the variety of food waste that can be fed to worms. It was developed from waste actually buried in worm bins I helped establish at a Michigan nature center during a publicly funded project in the 1970s. Coffee grounds don’t appear on the list merely because none of the six participants’ families drank coffee. Use this list as a guideline only; it is not, by any means, comprehensive.

Food Waste for Worms

• Apples
• Baked beans
• Banana peels
• Bread
• Cabbage
• Cake
• Celery
• Cereal
• Cheese
• Cream cheese
• Cream of wheat
• Cucumber
• Deviled eggs
• Eggshells
• Grapefruit peels
• Grits
• Lemon peels
• Lettuce
• Molasses
• Oatmeal
• Onion skins
• Orange peels
• Pancakes
• Pears
• Pineapple
• Pizza crust
• Potatoes
• Tea leaves
• Tomatoes

Citrus Warning

Excess quantities of citrus will kill worms. If your bin is small and you squeeze a dozen or so oranges for juice, I advise you not to put all the rinds in the bin at once. Lemons, oranges, and limes contain limonene, which is toxic to worms. I have occasionally put an orange peel in my bin, and it disappears, but I generally put citrus rinds in my outdoor compost pile.

A Word about Meat Waste and Bones

When setting up the project at the nature center, we deliberately excluded the burial of meat, in order to avoid foul odors. Because worms do not have teeth, and their digestive enzymes are limited, they rely on the aerobic activity of microbes to start the decomposition of food. This rotting process tends to smell more with meat than with vegetables. We also wanted to avoid attracting insects and rodents (and sometimes even larger scavengers, when the bins were outdoors), prevent possible injury from sharp bones, and enhance the appearance of the final vermicompost. Since the demonstration bins were to be located in a public exhibit area and seen by thousands of visitors, it was important to avoid such potential problems.

Nevertheless, in more than two decades of having a worm bin in my home, I have found that the worms and associated microorganisms can handle some meat in a worm bin. I do bury chicken bones, for example. If I dig too soon into the pocket of bedding containing the bones and decaying meat, the odor is bad. If I don’t disturb it, I don’t notice it. When I harvest the castings after several months, what remains is crumbly vermicompost that smells like damp, rich earth and that contains darkened well-picked bones.

Fellow long-time vermicomposters have had similar experiences. One worm grower I heard from buried the bones from a community chicken barbeque in large outdoor worm bins and said that it took only three weeks for the bones to be picked clean. Dr. Daniel Dindal of the State University of New York at Syracuse suggests adding a good carbon source (such as sawdust or extra bedding) to meat and bones to speed up decomposition time. He finds that if the meat is chopped, ground, and thoroughly mixed with the carbon source, rodents won’t even be a problem. “I do this successfully all the time in outdoor piles,” he says. Several large-scale projects in India use vermicomposting to transform chicken processing waste into valuable natural fertilizer. More recently, studies in India used vermicomposting to break down chicken feathers. Decomposition of feathers usually takes more than five years. Experiments with cow dung and worms composted the feathers in less than three months.

Add Small Amounts and Provide Cover

The examples above indicate that some meat and bones can be successfully composted if sufficient cover is provided. There are some advantages to putting some of these nitrogen-rich materials into your worm bin. Worms require nitrogen in a form they can use. Nitrogen is also required by the microorganisms that do much of the composting and that are, in turn, eaten by the worms. Since meat contains protein, built from nitrogenous components, eliminating all meat from the system could result in a nutrient deficiency for the teeming organisms that constitute a home vermicomposting system. A further advantage of adding some meat is that more plant nutrients will be in vermicompost produced by worms that have consumed a greater variety of materials. Finally, putting meat scraps into your bin means you don’t have to figure out another way to dispose of them.

My personal feeling about burying bones and meat waste in a worm bin is that small amounts are all right. When I clean out my worm bin every six months to a year, I gather the bones into a net bag and hang them in the garage. The next time I clean out the bin and gather more bones, I process the old ones — now completely dry and brittle — by pounding them on concrete with a sledgehammer. These pulverized bones are added to my garden, where plants benefit from their nutrients without my having to purchase bonemeal for nitrogen, potassium, and phosphorus. And centuries from now, the archaeologists excavating my homesite won’t have a clue that I was a meat eater!

For the small system inside my home, I use my own judgment on how much meat and bones to bury. I put more of my meat, bones, and dairy wastes into my Patio Bench Worm Bin, away from my immediate living quarters. My advice to you is either avoid placing meat, bones, and dairy products in your worm bin or experiment cautiously with these high-nitrogen materials. Learn for yourself what your system can take within the design and demands you place upon it.

No-No’s

Since what is obvious to some of us isn’t always obvious to everyone else, let me suggest things that don’t belong in a worm bin: plastic bags, bottle caps, rubber bands, sponges, aluminum foil, and glass. Such nonbiodegradable materials will stay there seemingly forever. They will clutter up your developing vermicompost and make it look more like trash. I have seen the same red rubber band over a three-year period in a large outdoor pit!

Caution: Don’t feed your worms very salty foods. They breathe through their skin and they need to stay moist. Salt will pull moisture from their bodies and can kill them.

Pet Feces

Dog, caged bird, cat, and potbellied pig feces do not belong in the worm bin. The manure of these animals can harbor pathogens and parasites that can be harmful to humans. One example to be guarded against is letting a cat use your worm bin as a litter box. First of all, cat urine would soon make the odor intoler­able. Second, the ammonia in the urine could kill your worms. But the greatest concern with cats has to do with a parasitic disease organism, Toxoplasma gondii, that can be carried in their feces. Tiny cysts of this protozoan can be inhaled by people and hidden in human tissues. Frequently no outward symptoms occur in the infected person, but it is possible that a pregnant woman could transmit the disease toxoplasmosis to her fetus. Although most cats do not harbor this organism, any cat owner should be very careful in disposing of cat litter. In short, if you have cats, keep them from using your worm bin as a litter box. Although there is evidence that vermicomposting can destroy pathogens, it is best to avoid these feces in a home worm bin.

Some people who raise rabbits build a bin underneath the rabbit hutch for the worms to eat the manure and wasted rabbit feed. If you do this, be sure and add absorptive bedding to the bin and watch for concentrations of urine. You may need to dilute the urine so that ammonia does not build up.