There are a few VERY important principles to keep in mind when feeding a worm bin. Refer to the fundamentals section as well to review how feeding impacts bin management. The main inputs for a worm bin are feeding and bedding.
First is to feed on the WORMS’ schedule, not yours, especially with a newer bin or a smaller bin. Overfeeding is by far the most common reason bins fail, become odorous, or attract pests.
By that I mean nearly every serious problem in home vermiculture comes from overfeeding… I cannot over-emphasize this point.
Please pause, and read that sentence again.
What does it mean to feed on their schedule? Wait until the last feeding is at least 90% processed before adding more food scraps. You’ll know it’s ready when there are no recognizable food scraps from the last addition remaining, and the surface of the bedding has a smooth worked-over appearance. It's OK if some durable materials (avocado skins, hard woody material like stems etc) are still there. You really cannot in practical terms underfeed worms… they are not like a household pet. They can reprocess material for months they’ve already worked over, deriving further nutrition from it (and in fact this is desirable).
It is really common to produce more scraps than a brand-new bin can consume especially at first, or have a larger than normal amount occasionally (after for example a holiday meal, canning day or other event). More than any other, the most common error as a vermicomposter is to treat your bin like a compost pile that just gets added to because you have scraps rather than because the worms are ready for them. Your worms will quickly scale up if you do this right, don’t worry! But what to do with this great stuff in the meantime?
This is the reason I strongly encourage all vermicomposters to also have a “regular” composting system. It can be a pile, a bin, or a tumbler, but it will do several things for you and for your worms:
Not everyone can also have a typical composter in addition to their worm bin. Alternatives include freezing the scraps and adding them in later as the bin allows, just putting them in the trash, or donating them (for example on Share Waste).
Second, to understand feeding, we need to understand a concept called “bedding” as well. Since most of the food scraps and other things we want to convert to vermicast are 1) high in moisture, and 2) high in nitrogen, we need to balance those inputs to keep our worms healthy and productive. Too much moisture causes not enough oxygen (from air) to be in the bin, harming the worms and causing odor problems. Too much nitrogen will cause the bin to heat up (like a hot compost pile), cooking the worms and releasing ammonia (which is toxic to them at very low levels).
Side note: if you are familiar with hot composting, you’ve heard of the “carbon to nitrogen” ratio or C:N. Where in hot or "thermophilic" composting we typically want around 25:1 or 30:1 carbon to nitrogen, we want rather more carbon in a worm bin (more like 40 to 1, but seriously don’t get hung up on the numbers). This prevents heating and ammonia release, and also provides a more diverse microbial environment (which is good for worms and for the nutritional value of our vermicompost). Vermicomposting is a "mesophilic" process conducted at temperatures in the 60F to 80F range.
Luckily, and this is part of the magic of home vermicomposting, dry and carbon-rich materials for bedding are another “waste” product that we can divert into the bin to convert to vermicast!
Things like corrugated cardboard and paper products are readily available in most households and are perfectly suited as worm bedding! People also use peat moss and coconut coir which also have great properties and are cheap: having some around as a backup is a good idea. However they do have some drawbacks. Peat is not renewable on a human time scale. Coir is renewable but can contain higher levels of salts than we want, and part of the reason many of us vermicompost is to try to use local materials that don't have to be processed, packaged and shipped to us (more than has already happened anyway). Both coir and peat cost some coin, and re-using locally-available products we already have like cardboard and paper just makes sense ecologically, especially since we do need them in the bin and they work so well!
My favorite bedding (with/after living materials) by far is shredded corrugated cardboard. The worms just LOVE it: the corrugations ensure that it stays aerated even when really wet, it’s a great slow-release carbon source, and in my experience it enhances breeding productivity. It is also an already-used product, essentially waste sawdust that’s re-constructed into corrugated cardboard using waste starch as glue, which provides a slow-release food source! Corrugated can be shredded by soaking in water and then just ripping into pieces (it’s SO much easier when wet). An office shredder that can handle 16 sheets of paper can eat through corrugated easily, and makes a wonderful uniform bedding. A 12-sheeter can do it, but processing too much as a time can overheat the shredder. Shredders for the serious vermicomposter can be found inexpensively on Craigslist, FaceBook marketplace and the like. My 16-sheeter was a dumpster-dive and has been my boon companion for years!
Shredded paper like newspaper or office paper is also great, but it can kind of mat together into clumps once wet so it takes a little more care to mix in with food scraps. Avoid shiny/glossy paper like magazines, paperboard that’s glossy like cereal boxes, and the plastic “windows” in envelopes, but otherwise most paper products are good bedding once torn up/shredded. Tom Perkins (who sews and sells VermiBags) makes pre-made bedding along these lines. You do not need to use all the ingredients as he does, but Tom has some good tips for making bedding as well as effectively using shredded office paper in this video.
You almost cannot add too much bedding. If it's WAY out of whack all that carbon without some nitrogen as "fuel" might take a long time to break down, but worst-case you're creating lots of living material nicely inoculated for later use. Over-bedding is SO MUCH LESS RISKY than over-feeding that it's always wise to have bedding at the ready: use bedding early and often.
As we learned in fundamentals, having a balance of moisture and aeration is one of the critical habitat needs for our worms. Every feeding addition should be accompanied by a bedding addition. The bedding keeps the habitat open and aerated, adds carbon to balance the nitrogen, and provides a long-term food source for the worms. Coarsely chopping your scraps and mixing well with the bedding addition REALLY helps keep things aerobic and comfortable for your worms. Adding living materials with each feeding as well, if you’re able, speeds the process dramatically.
Since watering to add moisture to a bed can be hit-or-miss, it is advantageous to adjust moisture when you feed: prepare the bedding/food mix to be wetter if you need more moisture, and make it drier if your bin is running wet.
My feeding process is like this: I coarsely chop the food scraps (the tougher the food, like broccoli stems, the more I chop it up). Worms have no teeth, so exposing more of the food surface area to microbial attack speeds decomposition. I mix the chopped food in with shredded bedding and/or living materials until evenly distributed. This mix I spread evenly over the whole surface of the bin. If the bin is running dry, I sprinkle some water on top at this point. Add some more bedding over the surface (and/or replace your surface cover) and close up the bin.
The “do” feed list is a lot longer than the other one.
If you eat it your worms can probably eat it too, and some more things besides! Things they’ll love to process for you include:
Then there are the "yes, but careful..." foods
A robust bin ecosystem can handle many of the usual “don’t feed” items, in moderation and if they’re properly prepared. It is definitely a good idea to avoid some of the more challenging scraps until you have a sense of your worm’s feeding schedule. Items that are more challenging for a bin to process include:
Smaller and younger bins definitely can struggle with the "advanced" materials above, where a larger system can rock through them with little difficulty. The grammarian's rule for commas is well-suited for the beginning vermicomposter: "when in doubt, leave it out".
It is important to understand that most of these things can absolutely be processed in your bin! For example when we fry bacon, I’ll compost the paper towels we use to wipe out the pan (the towels add carbon and keep it from just being a blob, and then I chop and mix it well before adding). But when we make soup stock, the big disc of fats from the cooled stock goes into the pre-composter to be broken down first before hitting the worms. If it were mixed in to enough bedding or especially living materials it could absolutely be added to the bin, but for a big glob of fat like our large bone-stock batches that’s one of the few things I sometimes just pitch. We do add bones to our bins and they come out dry and clean a few weeks later having added valuable phosphorus, calcium and more to the vermicompost. For some folks bones and meat would be objectionable in the bin, and that's fine too.
Lastly there’s an item that is on one hand a kind of natural for vermicomposting, and also a caution: animal wastes. Pet manures like cats or dogs, and even human waste, can actually be a wonderful composted product. However since these animals can share pathogens and parasites with humans the vermicomposting of their waste is an advanced topic recommended only for people willing to have a dedicated system for these wastes, and who are willing to strictly separate the vermicompost from these systems so they won’t be used on any edible crops (landscaping only). There are several interesting ways to recapture these wastes in a composting system that are perfectly safe, and I’d be glad to talk more about it with folks who are interested. Just contact me!