What Can and Can’t Composting Worms Eat? A Complete Guide


Worm composting, or vermicomposting, uses a combination of worms and bacteria to convert nutritious waste into fertilizer. Composting worms can eat almost anything that was once living. Nevertheless, some ingredients can harm—even kill—your worms.

The worms in your composting bin can eat almost any decaying organic material. That said, they prefer a varied diet of fruits, vegetables, and starches. Worms can’t eat citrus, oily or greasy foods, animal byproducts, or non-biodegradable materiallike plastic.

In addition to what you can and can’t feed your worms, this article will cover:

  • How worm composting works
  • How to balance your worms’ diet, and
  • How to troubleshoot problems with your bin.
Photo of well lit fruits and vegetables
Photo by Lidija Jankulov

What Can My Composting Worms Eat?

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Technically, your worms can eat almost anything biodegradable; but they perform best on a mixed diet of fruits, vegetables, and other biodegradable ingredients. In addition to balancing your worms’ diet, you’ll need to add bedding and various ingredients that assist their digestive process.

Worm bins also need a specific balance of nitrogen and carbon-rich ingredients to function effectively. The ideal ratio is between 25–30 parts carbon for every 1-part nitrogen.

Green Ingredients

Nitrogen-rich, or “green” ingredients, usually consist of kitchen scraps and fresh plant material. Green ingredients are a significant source of nutrition and moisture for both the worms and the microbial bacteria living in your bin.

Here’s a comprehensive list of nitrogen-rich ingredients that your worms can eat:

  • Apples. Chop your apples and apple cores into smaller pieces to make them easier to eat.
  • Banana peels provide nutrients like potassium, calcium, and magnesium.
  • Broccoli. Cut the stems into pieces or run them through a food processor to make them easier to eat.
  • Cabbage. Tear the leaves into smaller pieces so they decompose more efficiently.
  • Carrots and their shavings might take a while to break down.
  • Celery. Worms favor celery for its fiber, which aids in digestion.
  • Cucumbers, zucchini, and squash are also appropriate foods.
  • Grapes are just fine.
  • Grass clippings. Don’t throw all your grass clippings into the bin at one time. Too many fresh clippings can generate fatally high temperatures. Like other foliage, clippings will gradually dry out and lose nitrogen over time.
  • Hair. Pet and human hair are protein-rich, and their elastic consistency makes them easy to eat.
  • Moldy food. Moldy or rotten fruits and vegetables are fine. However, inedible foods like meat and cheese won’t break down and could spread mold throughout your bin. Mold won’t harm your worms, but it’ll spread if left unchecked.
  • Pumpkins have a high sugar content and are favored by both the worms and the microbes in your bin. Pumpkins are also about 94% water, so be careful not to over hydrate your bin.
  • Soy is fine for your worms in small amounts.
  • Tomatoes are acidic and will introduce a lot of moisture to your bin but are okay in moderation.
  • Watermelon skin. Worms love watermelon, cantaloupe, and honeydew rinds.

Your worms need a variety of green ingredients to produce the most nutritious fertilizer possible. If you’re struggling to find enough variety at home, you can ask your local restaurant or grocery store for food waste and expired produce. Your neighbors might have yard waste or other ingredients you can use, too.

Green ingredients will attract microbial bacteria to your bin that facilitate decomposition and produce heat. If your carbon-nitrogen ratio is higher than 30:1, you’ll risk generating too much heat and roasting your worms to death.

Green foods are often very dense, too, so be conscious of their weight before adding them to the bin. Fresh lawn clippings, for instance, are notorious for causing uninhabitable temperatures in composting bins. Additionally, make sure your yard waste isn’t contaminated with pesticides.

If you’re using kitchen scraps, be mindful that cooked food takes longer to decay than fresh ingredients.

Photo of used teabags stacked on each other with a mug in the background
Photo by Mario Claudio Boh

Brown Ingredients

Carbon-rich, “brown” ingredients are typically dry, wood-like materials—like paper and dry foliage. Brown ingredients help to aerate the bin and are usually used for bedding. You need to have at least 25 ounces (708.74 g) of brown ingredients for every ounce of green ingredients. Keep in mind that most brown ingredients will also contain some nitrogen.

Some common brown ingredients include:

  • Alfalfa can be mixed with cornmeal to create a protein-rich meal for your worms.
  • Coffee filters are safe to feed your worms, and they make a decent bedding material when used without coffee grounds.
  • Glue found in cardboard and other edible brown materials isn’t necessarily a “brown” ingredient on its own, but it won’t harm your worms. In fact, glue is a popular snack for the microbes in your bin.
  • Sawdust takes a while to decompose. Avoid sawdust from pressure or chemically-treated wood.
  • Teabags. Your worms can’t eat fabric or plastic tea bags, but paper bags are acceptable.

Carbon-rich ingredients, especially yard waste and manure, will also contain some nitrogen. If you’re already pushing your nitrogen levels, consider aging your ingredients outside before adding them to the bin. Doing so will dry them out and reduce the nitrogen to a more manageable level.

Brown materials provide more than just nutrition. They also make up the bedding material for your worms.

Bedding

Composting worms don’t live in the soil. Instead, they live in moist, carbon-rich bedding material. Shredded paper and aged manure are popular bedding options.

Here are some other popular bedding options:

  • Brown paper bags provide bedding and are safer than bleached paper.
  • Cardboard. Worms enjoy corrugated cardboard but won’t eat waxed paperboard, like cereal boxes.
  • Coconut coir or peat moss are popular bedding options.
  • Dry leaves. Wet or dry foliage is edible, but only dry leaves should be used for bedding.
  • Egg cartons. Pulp-paper cartons are okay, but avoid using plastic or styrofoam.
  • Manure. Age manure for a year to prevent it from generating too much heat in your bin. Only use manure from herbivores; never use hog, human, or pet manure.
  • Natural fibers like cotton or wool are safe and suitable for absorbing excess moisture.
  • Newspapers are an excellent food or bedding option. The paper isn’t bleached, and the ink is usually soy-based.
  • Office paper is bleached, so use it sparingly. Colored ink is also dangerous, so try to use blank pages.
  • Packing peanuts made from cornstarch or sorghum are fine. Don’t use Styrofoam.
  • Paper towels. You can use paper towels for food, bedding, or to reduce moisture in your bin. Never use contaminated paper.
  • Toilet paper. The cardboard roll is also an acceptable source of food and bedding. Avoid using contaminated paper.

The bedding should be at least six inches deep. Your worms will eat the bedding over time, so you’ll need to replace it periodically. Keep your bedding moist and loose for good oxygen flow.

Digestive Support

Worms don’t have teeth, so they have to wait for their food to decompose and soften before eating it. The food passes through the stomach to the gizzard, where tiny muscles break it down. Then, these broken-down food particles are dissolved in the intestines before finally being sent to the bloodstream and the rest of the body.

Since worms can’t “chew” their food, you’ll need to add grit to their diet. “Grit” is anything that’ll aid your worms’ digestive process, like crushed eggshells or azomite. Worms need grit to break down food in their gizzard.

These are some common grit ingredients:

  • Azomite is volcanic rock dust that worms use to break down their food.
  • Coffee grounds. Moist, ground coffee provides both food and grit. Coffee grounds are highly acidic and should be used moderately to prevent pH imbalance.
  • Eggshells can reduce the acidity of your bin and are a good source of calcium and grit.
An assortment of raw meats, including: beef, chicken, and turkey
Photo by stockcreations

Your Composting Worms Can’t Eat

Your composting worms can, in theory, eat any organic material. That said, some organic ingredients should be avoided. Oil and butter, for instance, will block the pores on your worms’ skin and suffocate them. Some ingredients, like peanut shells, just take too long to decompose. It should go without saying that inorganic materials, like plastic and metal, are entirely inedible.

Here’s a comprehensive list of foods to avoid:

  • Avocado skins. Worms like avocados, but they struggle with the seed and skin.
  • Bread isn’t dangerous on its own, but it can cause significant problems in your bin. Fermenting bread will reduce oxygen, create odors, and spread mold throughout the bin.
  • Citrus. Avoid lemon, lime, grapefruit, and oranges. Citrus is highly acidic, and its peels contain dangerous oils.
  • Dairy products won’t get eaten and will cause odors.
  • Magazine pages. Glossy paper is treated with plastic and won’t break down in your bin.
  • Meat. Worms struggle to digest meat proteins, so any meat will likely be ignored in favor of other foods. This will result in foul odors that attract rodents and other pests.
  • Metal, glass, or other inorganic materials won’t break down.
  • Oils or greasy foods will coat your worms’ skin and suffocate them.
  • Onions. Alliums, like garlic and onion, are technically safe in small amounts. That said, the oils in alliums can harm your worms and should be avoided whenever possible.
  • Pasta. Like bread, pasta is harmless in minimal amounts but should be left out of your bin whenever possible. Pasta also tends to attract weevils, moths, and beetles.
  • Peanut shells. Nuts are oily, and their shells take too long to break down.
  • Peppers. Worms don’t like spicy food or sauces.
  • Pet waste can introduce parasites, and cat litter can cause fatal digestive issues.
  • Pineapple contains too much citric acid, the same compound that makes citrus inedible, for it to be safe.
  • Pine needles are harmless but take too long to decompose.
  • Poison oak, ivy, or sumac. If you wouldn’t touch it, neither should your worms.
  • Potatoes and their peels contain the chemical solanine, which is harmful to worm-friendly microbes.
  • Rice will eventually ferment in your bin.
  • Sauces are either too oily or spicy for worms. Don’t use food waste that still has sauce on it, or you’ll risk killing your worms.
  • Soap is non-biodegradable and will promptly suffocate your worms.
  • Strawberries are very nutritious. Unfortunately, they also contain citric acid and should be used sparingly, if at all.
  • Sugary foods, like breakfast cereals or baked goods, shouldn’t be fed to your worms.
  • Wood ash is highly alkaline and contains the dangerous chemical potassium hydroxide. Only use it in a pinch and in small amounts to balance your bin’s pH.

If you’re not sure about a particular ingredient, just leave it out. There are plenty of safe alternatives, many of which literally grow on trees.

That said, it doesn’t always hurt to experiment with different ingredients. As long as you avoid animal by-products, oils, chemicals, and highly acidic foods, your worms should be just fine. Vermicomposting is pretty forgiving. Even if you make a lethal mistake, the remaining worms will eventually reproduce and get you back on track.

Photo of two hands cupped full of rich soil with earthworms huddled in the middle
Photo by Serezniy

How Does Worm Composting Work?

Most composting methods use heat and bacteria to break down organic material into fertilizer. Worm composting, however, is an aerobic method that primarily relies on worms—usually red wigglers—to eat decomposing organic material. Healthy worms can consume about half of their own body weight every day and produce about ten times their weight in fertilizer each month.

Vermicast, the fertilizer byproduct of vermicomposting, is also known as “castings.” Worm castings might look like soil, but they’re excrement. Adding castings to your garden helps stave off aphids and mites, aerates the soil, and provides essential nutrients to plants. Castings can also be brewed into a “worm tea” that improves pollination, seed germination, and early fruiting.

Unlike other methods, vermicomposting doesn’t require a lengthy curing process, doesn’t generate as much heat and odor, and can be done indoors. That said, composting worms are more sensitive to temperature, moisture, and diet than their anaerobic counterparts.

4 Steps to Feeding Your Composting Worms

New worms will only eat about a quarter of their maximum potential intake for the first week or two. Once the worms are acclimated to their new environment, you can gradually increase their food to up to half a pound per pound of worms each day.

Before you feed your worms, separate, and weigh the ingredients to ensure they adhere to the appropriate carbon to nitrogen ratio.

Once you’ve prepared the ingredients:

  1. Break the food into smaller pieces with a knife or a food processor.
  2. Spread the ingredients across the bedding to make sure they compost evenly.
  3. Bury the ingredients an inch or two into the bedding.
  4. Monitor the bin for pests, odors, unwanted plant growth, and uneaten food.

If you plan on staying out of town for a few days, you can add larger or tougher ingredients to the bin. This will ensure the food lasts throughout your absence.

Never add more than a week’s worth of food (~3.5lbs food per 1lb worms) to your bin. It will decompose faster than the worms can eat it and will cause problems. You can get away with underfeeding your worms once in a while, but you should never overfeed them.

If you use a food processor to break down your worms’ meals, consider adding extra brown ingredients to absorb some of the surplus moisture.

Balancing Your Worms’ Diet

In addition to balancing your bin’s carbon and nitrogen levels, you’ll need to monitor your bin’s pH status. pH stands for “potential of hydrogen” and represents the amount of hydrogen in a given material. You can measure your bin’s pH level with a pH Meter. pH measurements fall between 0 and 14.

Some examples include:

  • Stomach acid: 0 (acidic)
  • Water: 7 (neutral)
  • Lye: 13 (alkaline)

A low pH level (0-6) is acidic, and a high pH (8-14) is alkaline. You’ll need to keep your bin’s overall pH level as close to neutral (7 pH) as possible.

Each ingredient you add to your worm bin has its own pH and will affect the overall acidity of your bin. Most ingredients are acidic. This is especially true for green ingredients, like food scraps. Adding more alkaline ingredients, like limestone (calcium carbonate), can lower your bin’s acidity and neutralize its pH.

farmer hand holding a composting earthworm for producing compost manure
Photo by Psisaa

Troubleshooting Worm Compost Bins – 5 Key Issues

Composting bins are easy to maintain, but that doesn’t mean they’re entirely immune to foul odors or dangerous temperature fluctuations. Luckily, these problems are easy to remedy.

Temperature

The ideal temperature for your bin is between 55 and 77°F (12–25°C). Temperatures around 80°F (27°C) will slow your worms’ activity, and anything over 95°F (35°C) will kill your worms. You can easily monitor your bin’s temperature with a worm farming thermometer.

Compost heat is created by the microbial breakdown of the organic material in your bin. Composting bacteria are attracted to decomposing nitrogen-rich ingredients. An overabundance of these ingredients can increase your bin’s temperature to uninhabitable levels.

You can reduce the heat in your bin by limiting the amount of food and fresh yard waste that you put into it. Alternatively, you can add more bedding or brown ingredients to the bin to balance out its chemical composition. Beware of introducing rapid temperature changes to your bin. Even if it’s too hot, reducing the temperature too quickly can also be dangerous for your worms.

Odor

Over time, decomposing food consumes the oxygen in your bin that its residents need to survive. Restricted oxygen flow promotes anaerobic bacterial growth and will create a vinegar or an ammonia-like odor. This is typically the result of overfeeding. Oils and animal byproducts, like meat and dairy, will also cause an odor and should never be put into your bin.

If there’s a lot of uneaten food in your bin, you’re probably overfeeding your worms. Remember, your worms eat about half their own body weight each day. If you have two pounds of worms (about 2,000 mature worms), you shouldn’t be putting more than a pound of food waste into your bin per day.

If your worms are consistently slow to eat their food, try accelerating the decomposition process before feeding. You can accomplish this by either freezing or microwaving the ingredients. Make sure any treated ingredients are thawed or cooled to room temperature before putting them into the bin.

Pests

Odorous bins can attract rodents and other pests. Typically, this is caused by an overabundance of moisture or uneaten food. Meat, bones, and dairy products will also attract pests. You can prevent attracting mice, fruit flies, and other pests by removing uneaten food scraps from your bin.

Beetles, silverfish, and pill bugs are common sightings in worm composting bins. They aren’t dangerous to your worms, but they can be a nuisance–especially if you’re running your compost system indoors. Ants are another hard-shelled insect that might make their way into your bin and are an indication that conditions are too dry.

You can eliminate beetles and other hard-shelled insects by sprinkling some food-grade Diatomaceous Earth across the surface of your worms’ bedding. You must use food-grade diatomite; filter-grade diatomite has too much silica and will kill your worms.

If you see flies buzzing around the top of your bin, they’ve probably laid eggs. Flies can’t burrow, so adding an inch or two of dry, shredded paper to the surface of your bedding will inhibit their ability to breed and access food.

Weeds are pests, too. Unlike traditional composting methods, vermicomposting doesn’t generate enough heat to destroy live seeds. Pulling weeds from your yard and putting them directly into your bin is a common cause of unwanted plant growth.

Moisture

Excess moisture is probably the most common problem plaguing composting bins. Your worms need moisture to stay healthy, but too much can be deadly. Too much moisture can also create an anaerobic environment, which causes odors and attracts pests.

Sometimes, a thick juice may pool up at the base of your bin. Many people mistakenly consider this juice, called “leachate,” to be beneficial for garden soil. This isn’t exactly true. In fact, the presence of leachate is usually an indicator that you’re over-hydrating your worms.

Your bin’s ideal water content should be between 50-60%. You can test your bin’s moisture without an electronic meter by pulling some of the bedding out and squeezing it in your hand. The bedding should be about as damp as a wrung-out sponge or washcloth. If more than a few drops of water gush out into your hand, then your bin is too moist.

You can reduce the moisture in your bin by limiting the number of hydrated foods, like melons or tomatoes, that you put into it. Adding some more dry material, like shredded newspaper, can help to absorb some of the excess moisture.

Excessively dry bins are also uninhabitable. You can hydrate a dry bin by misting it with a spray bottle until the bedding is evenly moistened. If your bin is consistently too dry, you can retain some of its humidity by covering the top with a breathable fabric—like burlap.

Migration

Are some of your worms trying to escape? Don’t worry, this is normal. However, if more than a few worms are congregating outside or around the top of your bin, you might have a problem.

Some common causes of worm migration include:

  • Moisture. Too much moisture will restrict oxygen, while too little will dry your worms out. Make sure the bedding isn’t heavily saturated and that the bin has adequate drainage. You can also improve oxygen flow by gently mixing the materials in your bin.
  • Temperature. Keep your bin within 55-77°F (12–25°C) to prevent migration.
  • Light. If your worms are just exploring, then putting them under a bright light will drive them back underground. Adding a light layer of dry bedding over the top can also encourage them to move to deeper, moister parts of the bin. Never leave worms under direct light for more than a few minutes at a time.
  • Chemicals. Pesticides, acidic food ingredients, or an overabundance of bleached paper can create an uncomfortable or potentially lethal environment for your worms.
  • Castings. Your worms don’t like living in their own waste for long periods. Harvest finished castings every few months to prevent them from scaring away your worms.

If you’ve checked for these five issues but still find yourself scratching your head, consider whether you may have added anything new or unusual to your worms’ diet. If you’ve removed all the offensive ingredients from your bin, but the worms are still leaving, you can catch some of them by building a wall of bedding material around the bin. Otherwise, it might be time to empty the bin and start over.

Closing Thoughts

Your worms and their microbial partners may be sensitive to many foods, chemicals, and temperatures, but they’re ultimately resilient. They reproduce very quickly, so try not to beat yourself up if you make a mistake here and there. Just remember these key four things:

  • It’s easier to overfeed your worms than it is to underfeed them.
  • Too much carbon is better than too much nitrogen.
  • When in doubt, leave it out.
  • Don’t overthink it!

With the right tools and experience, balancing the ingredients in your worm composting bin will become second nature.

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