How to Separate Worms from Their Castings

This is Part 2 of a 2-part harvesting vermicast series. Be sure to also check out Common Questions About Harvesting Worm Castings.

When it finally comes time to put your beautiful black gold to good use in the garden (or even just to harvest it for storage), it will usually make sense to separate out the worms, so you can continue to enjoy the benefits of their amazing waste-conversion skills! As touched on in our “Common Questions About Harvesting Worm Castings” article, CFT systems are designed to do this as part of the vermicomposting process, making additional separation strategies less important. That being said, there are still many cases where material from continuous-flow systems can benefit from further separation efforts (e.g. stacking bins are notorious for having plenty of worms staying in the “finished” trays).

In this article, we’ll look at the 3 main ways to separate worms from castings (and other materials in your system).

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The Light Harvesting Method (LHM)

Likely the most widely-known small-scale worm-separation strategy, the “light harvesting method” (LHM), relies on an earthworm’s sensitivity to light and, more specifically, its tendency to move away from it as quickly as possible.

The basic idea here is that when you shine a bright light over top of your worm-rich material, and start loosening the upper layers, the worms will dive down. You then remove these upper layers gradually over time, and eventually end up with mostly-castings separated out from a heap of mostly-worms.

Key Factors Influencing the Success of The LHM Approach

• Brightness of the light source.
• Temperature of the environment and the worm-rich material.
• Moisture content of the worm-rich material.
• Frequency of your activity (i.e. how often do you go back to remove layers).
• Partner strategies used (vibration, airflow, migration), if any.

Let’s look at each of these in more detail.

Light Source

The type and intensity of the light source used can play an important role in determining how effective your LHM efforts will be. As expected, brighter lights tend to work better than lights with less intensity. So, a fluorescent light fixture or set of wide spectrum LED grow lights positioned directly above the material will naturally work a lot better than a single 60 watt incandescent ceiling fixture (just as an example). Sunlight is extremely effective, but with some important caveats to keep in mind. Too much sun exposure can actually harm or kill your worms, especially during the summer months. On a similar note, it’s also very important to remember that solar radiation can heat up a system as well - so we don’t recommend attempting light harvesting outdoors in full sun on really hot summer days.

Temperature

Generally speaking, fairly warm habitat temperatures (20-30 C / 68-86 F) will work best for light harvesting as well, since the worms will be more responsive to the light and other cues. When temperatures start to dip below, or go above this range, the worms can get more sluggish, and the process less effective as a result. Temperature differences between ambient and the worm habitat can have an effect as well. E.g. If you move worm-rich material from a cold outdoor system to a warm indoor environment, you may find that the worms become quite restless (prone to leaving their habitat), making it a lot more difficult to perform light harvesting.

Moisture Content

Excessively wet, muddy materials can make light harvesting much more challenging. It will be difficult to loosen the upper layers of materials and remove them, and the worms won’t move down nearly as quickly. Be sure to check out our “Common Questions About Harvesting Worm Castings” article (Part I of this harvesting series) for some wet-castings strategies. Excessively dry habitat can also make the worms less active, and therefore less likely to move down and away from the light.

Your Activity Level

The light harvesting method is a fairly passive approach. Just how passive you are with it can also determine how effective it is. Aside from the light itself, it is the roughing up of the surface layers and removal of upper material that helps to drive the worms down. If you leave everything to sit for extended periods, you may end up with even more worms just under the surface, especially as you near the end of the harvesting process, and the worms get concentrated in a smaller amount of material. It’s great if you can find something else to do during your light harvesting sessions (unless you are harvesting a lot of material at once, and have multiple bins on the go - something we will circle back to shortly), but aim to get back to the bin every 10-15 minutes if you can. Feel free to adjust this time based on how quickly the worms seem to be going down.

Partner Strategies

As we’ve touched on, there are other factors, aside from light, that can encourage worms to dive down, such as vibration and overall activity in the upper layers of material. Another one worth mentioning is airflow. Air blowing over the surface of the material you are loosening up can help to drive the worms down, since they are highly sensitive to any sort of air movement over their body. Outdoors - especially on a windy day - you’ve likely got this covered. Indoors, you may want to set up a fan to blow air over your system. This is especially recommended in cases where you need to shut off the lights and leave things to sit overnight (or longer), since it should help to discourage the worms from roaming. This has the added benefit of helping to dry out your castings as well.

One other partner strategy that works really well with light harvesting is migration harvesting (covered in the next section), where we encourage the worms to move from a less favourable environment to a more favourable environment.

Larger-Scale Light Harvesting

A great way to use light harvesting when working with larger volumes of material is to set up a series of harvesting trays (we love black plastic mortar mixing trays), and simply move from one to the next, to the next, and so on - basically as a continuous process.

How continuous the process is will depend on the number of bins you are working with, but even with as few as 3-4 trays, you may find you are able to scrape off the next layer by the time you make it back to the first tray. You might be surprised by just how much material you can work through using this rotation method - especially if you are able to do it outside on a nice day!

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Migration Harvesting

This is a harvesting method that relies on an earthworm’s tendency to move from areas that are less favourable into areas that are more favourable. As exciting as it is for us to see more and more castings accumulating in a system, this actually means the quality of the habitat is becoming worse for the worms (no one wants to live in their own wastes, after all). If given the opportunity, many of them will move into a higher quality environment.

This is the basis of the continuous-flow-through (CFT) vermicomposting concept, but it also provides us with a handy way to separate the worms from the castings once it comes time to harvest.

The approach can also be accelerated when combined with light/vibration/air harvesting tactics.

The Double-Bin Migration Method

If you’ve read our DIY Worm Composting Bins article, this approach may remind you of the double bin system we described - but the design and approach will be somewhat different.

You will need two plastic tubs that are exactly the same size and type - one with holes in the bottom, the other only with holes in the lower walls. Rubbermaid Roughneck totes or Home Depot HDX bins in the 50-80 L (~13-21 gal) range should work well.

The bin with the holes in the bottom will need 20 or more - ideally, drilled with a ¼” bit. NOTE: it is very important to remove debris from these holes and make sure there are no sharp edges, since the worms will be crawling through them. This will be our upper bin - where we will dump in the contents of the system(s) we want to harvest.

The lower bin is going to be our receiving system. We need holes in the lower sides of this one because we want to make sure the worms have an aerobic environment to migrate down into (the weight of the upper bin may cause the two bins to nest together quite tightly). Drill 5-10 ¼” holes in the sides, and 2 or 3 on each end, approximately 2” up from the bottom of the bin.

Once both bins have their drill holes, it’s time to get your lower tub set up in a way that encourages the worms to move down into it. This doesn’t need to be quite as involved as actually setting up a new system, but keep in mind that the more tempting you make the habitat, the more eager the worms will be to move down!

This mix should be nice and damp, but not sopping wet, and be made up mostly of safe bedding and living materials, with a small amount of food. A simple guideline to follow is 4 parts bedding, 2 parts living material, 1 part food - but even a lot of damp bedding, with a small amount of fast-to-breakdown fruit/veggie scraps (e.g. chopped up melon, cucumber, leafy greens), along with leftover bedding and any other unprocessed material from the system you are harvesting - should work really well. A very small handful of aged livestock manure (eg rabbit droppings or bedded horse manure) can also serve as a strong attractant for the worms.

Aim to fill your lower bin about ¾ full. You want to make sure there is enough habitat for your worms to move into while still leaving enough space for the upper bin to actually be able to nest into it. The weight of the upper bin will cause the materials in the lower bin to compress, so we don’t want to underfill that bin.

Once the lower bin is ready, you can place the upper bin on top (the holes should be sitting directly over top of the habitat/food materials in the lower bin) and empty the contents of the system you are harvesting into it. Make sure to clean out your system bin well, to get all the worms and material transferred over (manually, not with water). A small hand rake (like our Worm Rake) can help with this.

NOTE: If the system you are harvesting has a bigger volume than these harvesting bins, you are best to do your harvesting in stages, rather than attempting to overfill the upper bin.

Unlike with the double bin vermicomposting system (and other CFT systems), the worms will migrate in a downward direction. This makes the most sense for harvesting because: 1) it allows any excess moisture to drain away from the castings, and 2) we can greatly accelerate the migration by using the light harvesting method in the upper bin.

Position your double-bin harvester under a bright light, or at least in a brightly lit location. You may want to blow a fan over the surface of the material in the upper bin to help accelerate the process, but this isn’t mandatory. Rough up the surface using a small garden hand fork or your fingers, before leaving everything to sit for at least 10-15 minutes. Then, you simply proceed with a typical light harvesting approach. This is best done gradually over time, especially if the material in the upper bin is quite wet. It will give the worms a chance to move down, and the castings will have the chance to reach a moisture level closer to “ideal”.

Simplified Version of the Double-Bin Method

You don’t even need to use two bins for this migration method to work. A sheet of plastic with holes punched through it - or a sheet of burlap - laid over the surface of the material you want the worms to migrate into can work too. In this case, you will want to have less material in the receiving bin, since you need to also have enough room to dump in the contents of the worm bin you are harvesting.

Double-Tray Method For Stacking Systems

If you are using some form of multi-tray stacking system, a very similar approach can be used to separate out any straggler worms that are left in your lowermost tray when it is ready for harvesting. Simply fill one of your empty trays with lots of new (moistened) habitat material and some tempting foods like chopped up watermelon (and maybe a very small handful of aged manure, if you have some).

NOTE: It is a good idea to first line this tray with some sheets of newsprint or kraft paper, since we won’t want the worms crawling out the bottom of it.

Once the new tray is ready to go, place the finished tray directly over top. Make sure there is enough material in the new tray so that the bottom of the finished tray is in direct contact with it - this will make it easier for the worms to migrate down.

Next, we simply start light harvesting the upper (finished) tray. Shine a bright light directly over top (or move trays outside on a sunny day), rough up the upper layer with a small garden fork, and gradually start removing layers of material. The light and vibrations up top will send the worms further and further down, and the lower tray will give them a tempting safe haven to move into. By the time you get to the grate floor of the tray, you will have very effectively separated out the vast majority of the worms that were left in the finished material - with the added advantage of also breaking up, and drying out your vermicompost.

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Screen Harvesting

The third major harvesting method involves the use of some form of screening device. This can be highly effective under the right circumstances - and may be the only realistic option for most large-scale vermicomposters and worm farmers - but it also comes with some important caveats.

Keep in mind:

• The equipment needed is often quite expensive, or quite complicated to make yourself.
  
  
• Physical harvesting with a screen is usually a lot more stressful for the worms than the methods we’ve outlined above.
  
  
• Screening requires your worm-rich material to be quite dry and granular to avoid clogging up the screens - something that is far less likely with materials coming from home-scale vermicomposting systems.

The three main categories of screen harvesters covered here are:

1. Hand Screeners,
2. Shaker Screen Harvesters, and
3. Rotating Screen Harvesters.

Hand Screeners

This category includes various screens and screen-bottomed containers that require manual shaking. On the positive side, these will be less expensive - or even something you can make yourself - and they can be effective for small-scale castings clean-up (separating out bigger chunks, leftover debris etc).

The downside, however, is that, in most cases, they won’t be an effective way to separate worms from the castings, or be a realistic option when attempting to screen much larger quantities of material. They are also time-consuming and sometimes physically demanding.

Pictured below, is the Turbo Sift by Worm Gear. It is designed to fit and roll on a plastic mortar mixing tray, helping to make the manual shaking - and castings-collection - process a lot easier. It’s important to note that material (intended for screening) will need to be fairly dry and crumbly, and you will be limited in terms of how much you can screen at once, but this system still offers a great ready-made solution for small-scale vermicomposters.

Shaker Screen Harvesters

These harvesters rely on flat screens that are vibrated or shaken back and forth horizontally. These machines are widely used for sifting soils, composts and other materials, so there are numerous models available. A popular mid-scale model among worm farmers is the Brockwood Worm Sh*fter (pictured below). It has a fairly gentle shaking action, which likely makes it a better choice (for worm-rich material) than some of the more heavy duty soil screening machines. Shaker screen harvesters can be less expensive than rotating screeners, but they often have a much smaller output capacity (so, slower harvesting). There is also some debate about whether or not they are more harmful for the worms.

Rotating Screen Harvesters

Also known as trommel harvesters, these are the most popular type of harvesters among professional vermicomposters. Typically, worm-rich material is fed into the upper end and then it passes over one or more screens - separating out most of the castings and unprocessed materials - as it makes its way down to the other end, where the worms are collected. It has been suggested that trommel harvesters are less harmful for the worms than shaker screen harvesters, and in many cases they can indeed process larger quantities of material more quickly. Once again, do keep in mind that these machines, on average, will tend to be very expensive. 

2420 harvester, manufactured by JetSpeed Compost, Carson City NV

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Our bottom-line recommendation for screeners is that unless you own an actual casting-production or worm farming operation, or are involved in some other type of project that requires screening a very large volume of material, buying (or attempting to build) a serious screening machine likely isn’t going to make sense.

We do highly recommend some form of screening as a final step for cleaning up your finished castings, though! This can be a great way to separate out bulky chunks of vermicast along with other materials that didn’t break down (which also means you end up with some nice “living material” that can be recycled back through your other vermicomposting systems). A basic, rectangular, wooden-frame screener - like the one shown at the beginning of this section - is quite easy (and inexpensive) to make, and can be surprisingly effective.

Summing Up

• Separating worms from their castings is a helpful - and highly-recommended - part of the harvesting process, in some cases even when using CFTs (prime example being stacking systems).
  
  
• The Light Harvesting Method (LHM) is a very popular choice for home-scale vermicomposters. It relies on the tendency of earthworms to move away from bright lights, vibrations, and airflow.
  
  
• The basic idea of LHM is that you dump out the contents of your system, often into another bin of some sort, and then you gradually scrape away the upper layers of castings, eventually ending up with mostly-worms separated from mostly-castings.
  
  
• Intensity of the light source, moisture content and temperature of worm habitat material, frequency of your activity, and how much air movement and air humidity there is can all greatly influence the speed effectiveness of the LHM process, as can the use of partner strategies such as migration harvesting.
  
  
• Migration harvesting relies on a worm's tendency to move away from less favourable environments into more favourable environments (similar to what happens in a CFT system). It is an approach that partners really well with light harvesting, since the light, vibration and airflow will encourage the worms to migrate that much more quickly.
  
  
• A simple and inexpensive - but still effective - migration harvesting system involves two identical plastic bins nested together, the upper bin with holes in the bottom. The lower bin (which only has holes in the sides) is set up like a typical worm bin, with food and habitat materials, to encourage the worms to move down from the upper bin, where the contents of your system get emptied.
  
  
• Mechanized screen harvesting - the use of shaking or rotating metal screeners to separate worms (and bulky materials) from the castings - is generally better suited for worm farming professionals than for the average hobbyist. Apart from likely being quite expensive, these types of machines can also stress or even harm the worms. There are some fairly inexpensive, manual screening options for home vermicomposters, that can be helpful for separating out bulky and unprocessed materials. Keep in mind that moisture content plays a very important role in the success of all types of screen harvesting.

If you haven’t already, be sure to check out Part I of this harvesting series: “Common Questions About Harvesting Worm Castings”.

Helpful Related Resources

Common Questions About Harvesting Worm Castings
Setting up and Managing Your Urbalive for Worm Composting Success!
How to Feed Your Worms to Get The Best Results From Your Worm Bin
Living Materials
Bedding - The Most Important Material in Your Worm Bin?

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