The formation of vegetable mould through the action of worms with observations on their habits — Darwin’s last book, 1881

Composting with Worms

VermicompostingJust what you always wanted — a worm farm in your basement. The idea is enough to make some gag. Won’t they be slithering around all over the basement floor? Won’t the bin smell? Won’t all that garbage attract flies?

The answer to all three questions is, of course, no. (If it were Yes, it’s hard to see how worm composting could ever have become popular.) Worms don’t like light, they don’t like hard, cold surfaces, and they seem to have a minimal instinct for exploration and travel. Most species stay happily in their bins unless something goes wrong, so worms on the loose is a clear sign that there’s a problem. Once it’s solved, the worms will settle right back down.

As for the other worries, a well-maintained bin does not have flies and it most certainly does not smell. (Or rather, it doesn’t stink. It’s not entirely odor-free, though, which is one reason many people keep the bin in the basement.) If a bin is surrounded by a swarm of flies, or if a god-awful stench greets you when you go to feed your worms, then clearly the bin needs more attention than it’s been getting.

Worm CompostingIn general, worm bins require very little attention. Worms are surprisingly low-maintenance housemates. They don’t need to be fed every day, they make no noise, and their bins only need to be cleaned every three to six months.

Even if one of the problems above does occur, its cause is usually easy to diagnose, and the solutions are pretty straightforward.

One more point before proceeding to the practical issues below: As Appelhof points out in the quote that heads this section, a worm bin is not merely a worm bin. It is home to a much more diverse population of creatures than many people realize. (If they did realize, they might change their minds about the whole proposition.) The most important residents of the bin are indeed the worms and they are the only ones whose health or comfort you need consider in setting up your bin.

Getting Started

The essential questions — how big (a bin), how many (worms) — depend on how much worm food your household produces in a week. This depends on a long list of variables: How often do you eat out? Do you buy prepared foods (including canned spaghetti sauce, frozen vegetables, and apple sauce) or do you usually cook from scratch? (Cooking from scratch will produce more worm food in the form of carrot ends, strings from beans, tough bits from broccoli and asparagus, and so on.) Is your diet high in fruits and vegetables or does it lean towards meats and potato chips?

Since answers to these questions vary so widely, it’s impossible to predict accurately how much worm food a household will produce in a week. The only real way to find out is by weighing it, ideally for three or four weeks.

Homemade Bins
Bins can be made of wood or plastic and can be quite simple. It’s important that the material be opaque as worms do not like light. A tight-fitting lid isn’t necessary; worms don’t generally try to flee their quarters. Some sort of cover is important, both to shut out light and to keep moisture in.

Bin size depends on the number of worms you’re planning to house and the amount of garbage you want them to recycle. A couple of structural rules pertain to all. First, though some people have made a successful worm bin out of a small garbage pail, worms generally need floor space rather than head room. (Commercial bins that look tall usually consist of several shallow trays stacked on top of each other.) A one-room bin need be no more than 12-18 inches (30-45cm) deep.

Ventilation: Drill a line of holes a couple of inches above the bottom and a couple of inches below the top of the bin. These should be quite large — at least an inch in diameter. If you plan to use a tray to handle excess moisture, then drill holes in the bottom of the bin for drainage as well. These should be smaller (1/3-1/2″ in diameter) so that bedding and food and worms don’t fall through them. They don’t need to be smaller than the worms since the worms will not try to get out of the bin unless something is amiss.

Commercial Bins
A wide variety of commercial worm bins are available for composting, from a simple, ventilated box to various “stacked” versions. The bins with layered trays make harvesting finished compost very easy; this is the primary advantage they offer over most home-made bins and one-room bins.

In a stacked worm bin, the trays are used in succession, each one coming on line after the one below fills up with compost. When it’s time to move the worms, food and bedding gets placed on the next tray up; the trays are designed so that worms can indeed migrate to the next level. When they’ve left the old one behind, it can be removed and emptied — and then replaced at the top of the stack. As long as it’s left empty, it has nothing to attract the worms. Only when it’s been made hospitable will worms move in.

Moisture/ Drainage
For worms, moisture is essential to the most basic function of life, breathing. Lacking lungs, worms “breathe” through their skins, something that is only possible in a moist environment. Their bedding should therefore be damp. But if moisture starts collecting in the bottom of the bin, it can be a problem. The classic solution is to set a couple of low blocks (1-2″ high) in a large tray and put the bin on the blocks so that liquid can drain from the holes in the bottom of the bin. This is fine as long as you empty the tray frequently enough to prevent any really nasty microbes from breeding there. Check from time to time to be sure that bedding has not blocked these holes.

(Microbes can’t breed in the bin itself, where the antibiotic-producing beneficial microbes hold sway.) A turkey baster may be the best way to drain the tray without lifting off the bin.

Mary Appelhof suggests several other ways to deal with excess water. If your bin has no drainage holes, the baster can again be pressed into service. But if you simply stick it down into the bin and try to suction up the liquid, it will almost certainly clog. Instead, scoop away the bedding from one area and lower a small strainer into the bin — you until you can see moisture rising in it – and then bring the baster into play.

Another ingenious way to deal with excess moisture is the coir sock. Fill an old sock or stocking with dry coconut coir and lay it in the bottom of the bin. Check it from time to time. If it becomes water-logged, remove it and squeeze it out saving the liquid, of course, for plants.

Make bedding out of strips of newspaper or shredded grocery bags, cardboard, or egg cartons, (no glossy paper), composted manure, old leaves, coconut coir, or a mixture of any of these substances. Just be sure that the material is clean and non-toxic as the worms will eat the bedding as well as the table scraps you feed them. If you’re working with cardboard or paper, soak the chosen material in water until it is easy to work with. Then rip it up into fairly small pieces and wring them out thoroughly. The bedding should be damp but not wet.

Half-fill the box, but loosely, with bedding and add a handful or two of dirt as well as some crushed eggshells. The dirt provides roughage, the eggshells calcium. Fluff the bedding up as you put it into the bed. The worms need a place to burrow and you need a place to bury their food. This will keep odors and insects at bay.

Since worms are quite sensitive to both light and noise, a corner of the basement often works best for their home. They thrive at temperatures between about 55°-77°F (13°-25°C) which means that most basements should fit the bill. During summer months it’s possible to keep worm bins outside (at least in some places) as long as they’re in the shade. People have found ways to keep a worm bin in the kitchen, and even in the living room. Seattle Tilth has developed a bin that doubles as a bench, and has posted the instructions online (PDF format).

What Kind of Worms?
The worms used most frequently for vermicomposting are Eisenia fetida (or foetida), known as red wigglers, red worms, manure worms, or tiger worms. Mary Appelhof, author of the indispensable Worms Eat My Garbage, lists all these names and then eight or nine more. This may explain why she stresses that you should ask for the scientific name of any worms you’re planning to buy. Common names are just too imprecise.

E. fetida and its close cousin, Eisenia andrei, (the two are often bred and sold together) make good composting worms as they’re more tolerant of the various and varying conditions that may exist in bins. (In other words, they won’t up and die on you when you change the bedding from soil to newspaper or pick them up while doing that.) These are not, alas, the worms in your backyard, which are more likely to be Lumbricus terrestris. The essential difference, besides adaptability, is that L. terrestris is a deep-soil dweller (as its name suggests), while worms for vermicomposting are litter-dwellers that neither need nor want several feet of earth in which to delve.

According to Appelhof, several other species can also be used for composting. Lumbricus rubellus can sometimes be found in compost and manure piles or under cow patties. If you want to collect worms for vermicomposting, this is your best bet.

In warmer climates, Perionyx excavatus and Eudrilus eugeniae make the best choices as they tolerate higher temperatures than do the other varieties. Not surprisingly, they’re also more vulnerable to cold; while temperatures below 50°F (10°C) will slow down other species, they’ll kill these. One warning: most worms don’t wander from their bins unless something is wrong, but these two seem to be fond of the occasional stroll outside the box.

How Many Worms?
While everyone agrees on what kind of worms to use, there seems to be significant disagreement on how much they eat and therefore on how many to buy. Toronto says a pound of worms can eat a pound a day; the United States EPA says half that.

Mary Appelhof recommends buying enough worms to have a 2:1 ratio of worms to daily garbage production, so she appears to be siding with the EPA. She does concede that in a well-maintained bin the worms will breed and their overall weight will increase so it is possible to buy fewer. However, the bin will get off to a slower start this way. If you want to be sure that you will be able to dispose of all your kitchen garbage in the bin, stick to the 2:1 ratio.

To get an accurate idea of how much worm food your household produces on a daily basis, weigh potential worm food for several weeks, add up the weight of the food, then divide by the number of days you kept track. This will give you an average daily worm food weight.

Whether you were working in pounds or in kilograms, double that amount and you’re looking at how many pounds (or kilograms) of worms to buy. Just be sure that the dealer you order from is working with the same units you are, or that one of you performs the necessary conversions or you may end up with twice (or half) as many worms as you expected!

Introducing the Worms
When the bin and bedding are in place, dig a shallow depression in the bedding, and place the worms in it. Then leave them, with the lid off or askew and a low light on overhead. The light will encourage them to burrow into the bedding. Leave the worms to acclimate for a week or so before feeding them. Food left out too soon will just rot and smell — not a good beginning for the new venture.

Feed the worms for the first time about a week after they have arrived. Be sure to bury the food in the bedding rather than just scattering it on top. Again, leave them for a week, then check on whether the worms are eating and adjust quantities accordingly.

Maintaining a Worm Bin

Worms, being living things, do require care but of the most minimal sort. They need to be fed but they are not picky about regular hours. If food arrives within several days of the regular time, they’ll be fine. Nor are they finicky about what they’re fed. Too much citrus fruit has been found to sometimes cause problems sometimes but beyond that they’ll eat whatever you have on hand. Also, their living quarters require cleaning — two to four times a year. All in all, they are not especially demanding.

Once the worms have acclimated, they should be fed every few days. The feeding schedule need not be rigid. After a week or two without feeding, some worms may die but the bin does not stop functioning.

Since too much food in the bin can lead to a foul odor, it’s probably best to err on the side of less than more and increase feedings gradually as you learn what your worms can dispose of.

As you read through the trouble-shooting section below, you’ll notice that citrus fruits and peels can cause no end of unlikely problems: mold on the bedding, escaping worms, fruit flies. Avoid these problems by feeding your worms only a limited diet of citrus fruits and peelings until you learn what they and the system as a whole can tolerate.

Uncovering last week’s meal while you’re burying this week’s is counter-productive (and may disturb the worms), so always bury the food in the bedding. Covering it thoroughly will reduce odors and insects. To keep track of where last week’s food went, Mary Appelhof recommends dividing the bin up as if you laid a grid over it, and then rotating feedings amongst them. She uses a 3 x 3 grid, which yields nine spots.

What to Feed Worms

Virtually every source, including the one above, warns against feeding meat to worms. The one dissenter, Mary Appelhof, routinely included small quantities of meat to her worms. According to her, the secret is to bury the meat completely and to leave it buried until it’s had a chance to be eaten. Her grid-feeding system ensures that she doesn’t accidentally expose the meat which may not smell very good.

Cleaning and Harvesting
When to collect the compost depends largely on whether you want to continue the operation year-round or shut it down for the summer. If you’re planning to vermicompost only through the winter, then you can set your bin up in the fall, feed your worms for three to four months, and then leave the bin untouched for another month or two while the worms eat through what remains of their bedding and any left-over food in it. Most will eventually die off and decay and what will be left will be almost pure vermicast, with very few worms left in it.

If you plan to keep vermicomposting even through the summer, you will need to move the worms to a new clean home after the third or fourth month. The vermicompost you harvest will contain bedding and bits of old food as well as a high proportion of worm castings. Though not as pure as the vermicast left after most of the worms die, it nevertheless has high nutrient value, perhaps higher than the vermicast, which has passed so many times through worm guts.

But — just how do you harvest this compost? How does one separate a couple of thousand worms from their bedding?

The most frequently recommended method is to scoop all the worms and litter to one side of the cage, put down fresh bedding on the other side, and start feeding on the new side exclusively. Within a couple of weeks, all the worms should have migrated to the clean side leaving the dirty side free for harvesting.

A less patient vermicomposter developed an accelerated version of this technique. He let the worms go hungry for a week or two and then put down the new bedding and fresh food. He assured his readers that the worms practically scampered over to the clean half of the cage.

Of course, there’s always the dump `em out and pick `em out option. But Appelhof, as usual, has an elegant variation on this. It may take most of an afternoon but it would be highly entertaining, especially for children.

Spread a large piece of plastic on the floor, dump the entire contents of the vermicomposter on it, and then make nine or twelve little piles out of the one big pile. As you work, you’ll see any left-behind worms hurrying to escape the light by diving into the closest pile.

Now brush the dirt off the top of one of the piles, exposing some of the worms which will promptly bury themselves again. Brush off as much compost as you can without dislodging any worms and then move on to the next pile. What you remove can just fall to the plastic sheet as long as it doesn’t get deep enough to attract worms.

Keep working from one pile to the next, brushing away dirt and exposing worms. By the time you get back to the first pile, the worms you had exposed there will have disappeared. Brush away more dirt, exposing more worms, which will of course head for the center of the dirt.

By the time you’ve made several rounds of the piles, each one will consist of a tangle of worms inside a doughnut of compost. This process will take a while, so there’s plenty of time for some member of your team to clean out the bin and prepare fresh bedding for it. If you’re working alone, it probably makes sense to take care of this task after you first dump out the dirt or after forming the smaller piles. A bright light over the plastic will ensure that the worms won’t go anywhere while you prepare their bedding.

When you have brushed away as much compost as possible from the piles, pick up the worms and put them into the fresh, clean bedding then lift the corners of the plastic and carefully collect the vermicompost in a bucket.

As mentioned earlier, commercial bins with stacked trays make harvesting easy. When it’s time to harvest one tray, you stop feeding in it. Instead, put new bedding and food in the tray above, and wait for the worms to migrate. When they’ve made their move, you take out the old bin, dump the compost, clean the tray, and return it to the stack.

Using Worm Compost

Vermicompost is the mixture of bedding and castings that you will harvest if you wait three to four months to change the bedding. Vermicast is the almost pure worm castings that remain after six months if you’ve stopped feeding the worms and they’ve devoured virtually all of their bedding.

If compost is the soil amendment, vermicompost is THE soil amendment — the best of the best, the creme de la creme, the quintessence, the apotheosis of amendments. It makes little sense, therefore, to use it as a garden mulch; it’s too rare, and too rich for such a use. Virtually no one would waste vermicompost on a lawn.

In the garden, save it for your most delicate crops and flowers. Like compost, it can be used as a top or side dressing.

Vermicompost also makes an excellent top-dressing for houseplants. Alternatively, mix it with potting soil when you are preparing containers for house, patio or balcony. Most container plants need the finest soil since they have access to so little of it so vermicompost is an excellent choice here. Be aware that there are occasional exceptions such as orchids which do not do well in rich, moist soils.

To Sterilize or Not to Sterilize

It’s not clear where the idea that vermicompost should be sterilized first cropped up but it continues to do so here and there at garden question centers every year. Probably this myth originates in a mis-directed but understandable reaction to the fact that vermicompost is, after all, feces. Surely, therefore, it must be full of bacteria — disease-causing bacteria.

Well, yes and no. Yes to bacteria but no to disease-causing bacteria. The bacteria present in vermicompost are those that aid in decomposition. But there is very little chance that dangerous pathogens will proliferate in a worm bin. Vermicompost does not need to be sterilized.

To sterilize it would eliminate all the micro-organisms that make it such a powerful soil amendment. Everything said of compost in the section on garden soil is true twice over of vermicompost: it works biological, chemical and physical changes on the earth. But most of those effects are due to the microbes that live in it. Kill them, and all of the biological benefits are lost. So are most of the chemical benefits which depend on the biological population. What’s left is a substance whose physical structure can to some extent improve your soil structure. That’s all.


Sometimes a vermicompost bin will develop a rotten smell. It’s important to realize that this is not the smell of the compost or of the worms; it is the smell of rotten food. Most often, this happens if the worms are being fed more than they can eat. But it may be simply that the food is not buried deeply enough. Either way, make sure the food is buried and stop feeding the worms until they catch up, or remove the rotten food, wait a few days, and start again on a smaller scale.

If the odor doesn’t go away, check that the holes around the bottom of the bin are clear. If the bin has drainage holes, make sure it is up on its blocks, so that the underside is ventilated.

Worms on the Loose!
Worms trying to flee the bin is a clear sign that something has gone wrong,. It’s usually one of two things: either the castings have built up too deeply or the bedding is too acidic. Obviously, if the problem is castings, the response is to harvest. If you don’t have time to do a complete bin change, tear up some extra newspaper or other bedding material and toss it into the bin. This may hold the worms until you have time to harvest the castings and set up new bedding.

If that doesn’t seem to be the problem, try adding crushed eggshell to the bedding to reduce acidity. Too high a proportion of peat moss or coconut coir (especially peat moss) can make bedding acidic, as can too much citrus fruit or peels in the diet. Mix more shredded newspaper or cardboard with the bedding and cut out all citrus fruits.

Fruit Flies and Gnats
The first trick is to figure out whether you’ve got fruit flies or gnats. Or both. They’re both small, flying insects but the fruit flies tend to be rounder and paler. Gnats are fairly slender and quite dark. Often the tell-tale difference is behavioral: gnats resist flying, frequently trying to scramble away rather than taking to the air.

Fruit flies: Completely buried food should not attract flies. A fly population indicates that food is exposed or rotten, which may mean that there is too much of it. Citrus fruit, especially, will attract flies. Clean out some of what is there and wait for several days or a week before feeding the worms again. When you do, give them less and bury it deep. This should bring the fruit fly population under control.

One suggestion for trapping fruit lies comes from the Massachusetts Department of Environmental Protection: put a banana peel inside a plastic bag and make numerous holes in the bag with a toothpick or small knitting needle. Put the bag near the bin and wait several days. Because fruit flies are particularly drawn to bananas, they will find their way to the banana peel but the vast majority will not be able to find their way out again. Don’t feed your worms any bananas while you’ve got this trap set; you don’t want the flies to be distracted from the one source of banana.

Gnats: These can be remarkably pesky for unlike fruit flies, which hang around the fruit, gnats like light (your computer screen) and damp places (your nose). Furthermore, unlike fruit flies, they can damage plants. So even if you don’t worry about the few in your bin, it’s important to eradicate them from finished compost before using it. This involves inoculating the batch with beneficial nematodes.

Beneficial nematodes (Steinernema feltiae) can also be used in the bin itself, though eventually the worms will eat them along with just about everything else in reach. With luck, they’ll get the gnat larvae before the worms get them.

Coffee grounds seem to discourage gnats as well. Fruit flies, however, love them — which is one reason why it helps to know which one you’re dealing with. Sticky traps, fly paper, and traps baited with apple cider vinegar or red wine can all help control a problem with either gnats or flies.

Mold in the bedding, curiously, indicates not that the bedding is too wet, but once again that it is too acidic. Change the bedding and cut out citrus fruit completely until the problem is solved; reintroduce it slowly and carefully.

YouTube Preview Image