Harvesting worms and vermicast

While we all love to rescue our organic wastes from the landfill and playing in the bins, most of us also get into vermicomposting for vermicast and, for some, the worms themselves. Vermicompost is an incredible material: it is rich in humic and fulvic acids plants love, has potent numbers of beneficial soil microbes to foster a healthy soil ecosystem, acts as a gentle feed and growth stimulant to plants, can act against pathogens and disease, and has wonderful structure for improving soil's tilth, aeration and moisture retention.

This is why top-quality vermicast commands prices many times as much as comparable compost products: farmers know that this is a best stuff there is, and that the health and resiliency it lends to soil and horticulture in general will easily return their investment. Once your bin has operated for a while, you'll start to note that the volume keeps decreasing even though you keep adding more food. When much of the bin has been converted to dark, rich vermicompost you will want to start thinking about harvesting the bin.

Types of harvesting

There are a few different ways to separate the worms from the castings. Let's say you have a bin that's a few months old and you have areas that are mostly converted to dark, rich, crumbly vermicast. But other areas have more recently added food and bedding, and worms of course! What's a vermicomposter to do? Well we have some tried and true options; which you use will depend in part of the kind of bin you operate and what your goals are for the vermicast.

"Migration" methods

Migration methods use the worms' natural instincts to gradually separate them from their castings. In this method, part of the bin is essentially left alone and another part of the bin gets the new bedding and food additions. Over time, the worms will move over to this part of the bin leaving the finished vermicast mostly unoccupied. Here's how it can work in a typical Rubbermaid tote-style bin. Once the materials in the bin are less than half of the bin's full volume, move everything in there over to one half the bin. Then add fresh bedding and a light feeding in the other half. The worms will continue to process and eventually finish the "old" half, and head over to the newer material. If the old stuff is really well-processed and finished, this may take only a couple weeks. It's always better to let this process run for longer than to try to hurry it, as there are cocoons and young worms in the old material that continue to hatch for a time that you'd be harvesting along with the castings. Now if your castings are going right into the garden or another use where they're wanted, that's not a bid deal beyond their loss in the bin.

But if you want truly finished castings free of worms and cocoons, or want to make sure a bin doesn't lose come population to harvesting, then you want to be sure they've all hatched out and been "rescued."

"Flow through" bins and harvest

These are actually a different kind of bin (rather than a method) that does its own migration harvesting. These systems, popularized by the efficient bag systems available today and by larger flow-through reactors like the VermBin, Worm Wigwam and others, are based on aligning with the worms' own instincts by feeding the worms on top and harvesting finished vermicast out the bottom. Almost all commercial producers have moved to flow-through production for its efficiency. If properly managed, the worms will move upwards on their own into the new material leaving finished castings below. Thus the material in the bin is constantly but slowly moving downwards in a "continuous flow through" system. These are also called "CFTs" for this reason.

I personally LOVE flow-through systems and use several for my castings production along with outdoor windrow and wedge systems.

The "Light Harvest" system

Light harvesting uses worms' natural aversion to light to separate them, and is used for harvesting worms as well as harvesting everything else. When exposed to light, worms will dig down to escape it. By putting a pile of mixed worms and finished compost in a pile and shining a light on it, in minutes the worms will all have disappeared down into the pile. You then lightly brush off the top material and set it aside until more worms are exposed. Wait again and repeat, and eventually you'll have whittled down a tight ball of just worms at the bottom and you've brushed all the other stuff off! Light harvesting does mean you'll need to "bait out" the hatchlings since all the cocoons will have come along with the finished castings.

Baiting out

Most vermicomposters want to retain all the worms we can in our bins to keep processing our scraps. Unless you are using a flow-through, you'll have this gorgeous pile of harvested vermicompost that has some teeny tiny baby hatchlings in it, and/or cocoons that will hatch over the next few weeks. If you don't mind them going to the garden or whatever destination you have in mind for your vermicast then no big deal.

How to we catch them if we want to keep them though? Well, they can do it for you with only a little work! Though worms can continue to reprocess castings for a long time and be healthy, they are very attracted to fresh bedding and good food. So what we do is place the finished castings into a shallow tray (a mixing tray is the vermicomposter's best friend), and in a pocket add a small amount of moistened bedding mixed with attractive food scraps. Cover and wait. Worm favorites like pumpkin, melon, or the like or really any quality food scraps are great bait stations. Check the scraps weekly and replace as long as you're still getting worms, that's it! To make it even easier one can poke a bunch of holes in the sides of a yogurt cup or similar cup for the scraps so the sides and holes are below the vermicast level. For some reason holes in the bottom tend to find the worms half-in and half-out rather than inside the bait cup, so I only hole the sides on cups. Baiting can also be combined with drying your vermicast a bit for processing and curing: the drying vermicompost will help them drive into the bait cups.

Processing and curing

So you have this lovely, dark, crumbly mix smelling of humus and life. After savoring it for a few moments, you can choose what to do next. Obviously living vermicast and bin materials can go right into soils, making teas or the like. But most vermicomposters will want to take a few extra steps with their vermicast. Often materials will come out of the bin a little wetter than they need to be for our uses. Worms love this, but gardeners might not. Handling wetter materials can feel a little muddy, and wetter castings are a lot more difficult to sift.

Allowing harvested vermicast to dry out a little, particularly if it's on the wet side, is usually a good idea. Spreading it out in shallower trays like mixing trays and then covering with a sand bag or partial cover helps it cure and dry slowly so you will be less likely to over-dry it. This drying can be combined with the "Baiting out" step above as well, and this resting period also allows the microbial communities to settle, mature and prepare for storage.

See also the section on storage in the Using Vermicast page.

Sifting

OK, truth is sifting is NOT necessary for vermicast to be used! However it does make for a uniform product that dries and cures easily, just looks sexy as heck, and grading your vermicast allows you to direct the different grades to specific uses. Sifting also allows you to screen out the occasional fruit sticker or twist tie that made it into the bin. Since I screen, I don't have to be super fussy about picking every last sticker off the fruit peels... from a labor standpoint it is actually much easier to screen out stickers a few weeks later than to pick them off things like bananas or avocados when we eat them.

Once it's dry enough to sift (believe be, you'll know if it's not), you can decide what you'd like to do for sifting. The usual grades of sifting are through a quarter-inch screen and, if you wish, an eighth-inch screen. Quarter-inch gives a nice uniform product and screens out the stickers, larger bits of bark or stems, avocado skins, bones and the like. Eighth-inch goes even further to a really nice, handsome vermicompost if you desire.

Yes, you can sift out worms to harvest them and that's actually how large producers do it with big rotating trommels. I've hand-sifted my share of worms as well as cocoons, and my opinion is firmly that unless the worms are in well-finished material of really consistent particle size it's more of a hassle than it's worth. I try to make use of flow-throughs and migration harvesting whenever possible for this reason.

What do I use?

It's exciting to build your own compost sifter and believe me, I've built a few! "Hardware cloth" is the name of the product most of us use, it's actually welded galvanized wire like we use for rabbit hutches, chicken coops or the like and it's available at your local hardware store. However I'd like to recommend the sifters that I use almost exclusively in both the wormery and my general garden, bonsai, orchids et cetera. This is called a "classifier" for sifting soil, and these particular ones fit right onto a 5-gallon bucket. This makes for WAY fewer spills and it's easy to shake and rock the bucket for the sifting action. Plus you can't drop a handful of un-sifted material into it by accident since it's protected under the screen in the pail (you'd be surprised how easily this can happen in a hurried day of sifting, or maybe that's just me). I have these from half-inch all the way to super-fine for sifting cocoons but all you need is a quarter-inch and maybe an eighth-inch version. No affiliation, but totally worth the money for a wormery or a home garden.

There are plenty of ways to skin a squirrel but these seriously keep the mess down and drop the screened vermicast right into a protected bucket.

Now what to do with the screened-out stuff left over?

I used to feel bad for the screened out materials. I mean, there's nothing wrong with this stuff, it's just not tiny yet right? Well good news, it is just as valuable as the sifted vermicast. It's absolutely packed with microbes so we don't want it to go to waste!

• The simplest thing to do is just return it to the bin, either right away by just dumping it back in or save it as living material to mix with your chopped-up scraps as optimizing material. The worms will gladly re-process this rich material into more castings.
• You can use it as living material to inoculate a new batch of bedding to prepare it for the bin as we talked about in living materials. Again, the worms will re-process it and you also get the added benefit of getting microbes active in the bedding.
• Screenings also are great for making vermicompost tea! Part of the miracle with teas is that the vermicast or screenings you use for making tea can go right back into the bin for re-use after your batch is complete.
• You can also use it as mulch outside or in houseplants. When I have a big pile of screenings and/or there are large items that have gone through several times like well-processed bones, big bark chunks or the like, I'll layer them around new transplants outside and then mulch over to protect the microbes from sunlight. It's kind of like a time-release compost tea addition. Be aware that this stuff smells WONDERFUL, rich in humus, and pets or wildlife may find it attractive.

Depending on my immediate needs I usually use my own screenings for inoculating new WormBOSSes, making up more more bedding, or simply dump back in the worm bins for re-processing. It's all part of supporting a rich microbial diversity that makes high-quality vermicast and keeps worm populations thriving.