Common Questions About Harvesting Worm Castings

Common Questions About Harvesting Worm Castings

This is Part I of a 2-part harvesting vermicast series. Be sure to also check out How to Separate Worms from Their Castings

Let’s face it, one of the most exciting things about vermicomposting is actually reaping the rewards of the process! Worm castings (also known as “vermicompost” and “vermicast”) are one of the most highly regarded forms of compost on the planet.

This is a material known to:

  • Increase the water-holding capacity, porosity and microbial biomass of soils.

  • Significantly boost the growth of plants, even when all the plant’s nutritional needs are already being met.

  • Offer protection against multiple plant pathogens, and even some arthropod pests.

And one of the best perks of all?

Castings can have a major positive impact even when relatively small quantities are used, unlike most other types of compost!

Ok, so it’s no secret that castings are pretty amazing. The big question is how do we get our hands on some? This article starts the ball rolling by addressing some of the most common (and important) topics related to harvesting this greatly-revered soil amendment.

Harvesting Castings - Q&A

Worm castings

The term “harvesting” here simply refers to the removal of castings from a vermicomposting system and any additional steps taken to prepare the material for use (e.g. separating out the worms, screening etc).

Some of the most common questions relating to harvesting include:

  • How Soon Can I Start Harvesting?
  • How Do I Know When My Castings are Ready to Harvest?
  • How Much Can I Harvest at Once?
  • What is the Best Way to Separate the Worms from the Castings?
  • How Do I Store the Castings I Harvest?
  • How Do I Deal With Really Wet Castings?
  • What About Worms and Cocoons Left in My Castings?
Let’s now look at each of these in more detail!

How Soon Can I Start Harvesting?

This depends on a wide range of factors, including temperature, moisture content, airflow, types of materials being added, worm densities, the type of system being used, and the experience of the vermicomposter.

When you are just starting out with a new system, a reasonable expectation is to be able to perform your first (major) harvest within 2-4 months. This assumes temperatures fairly close to “ideal” (approx. 16-26 C / 61-80 F range), and an effective system that’s cared for properly.

NOTE: We understand how eager a lot of newcomers will be to start putting their castings to use, so we often encourage people to actually start removing smaller quantities a fair bit earlier (as soon as 3-4 weeks in, once they start seeing decent accumulations of castings in the system). This sampling works well when it coincides with new bedding/food additions, since it reduces the number of times the system is disturbed, and helps to keep the overall volume of material in the bin balanced.

How Do I Know When My Castings are Ready to Harvest?

There are several important factors to consider here.


As touched on, you should expect to wait at least a month, even if you just want to remove a small amount to test out, and at least 2-4 months before doing a larger harvest. As more time passes, the quantity of castings that has accumulated in the system will naturally increase as well. Just keep in mind that the effect of time depends a great deal on conditions (e.g. when temperatures are cool or cold, the process can slow down significantly, meaning food/bedding can take much longer to get converted into castings).


Appearance of worm castings

The exact appearance of the castings can vary, depending on the types of food and bedding materials you are using (along with other factors), but you should be seeing darker, finer material that looks a lot more like really rich soil or compost than your starting materials. For more substantial harvests (in 2-4 months, or longer), you should notice that the castings make up a significant portion of your total habitat volume.


Castings should have a pleasant, earthy smell to them. Like rich garden soil after a rain. If the material has any sort of unpleasant odour, it will, bare minimum, need some more time in a highly-oxygenated environment in order to be ready for use. NOTE: in the lower reaches of some plastic bin systems (without good drainage), conditions can get quite anaerobic by the time you have a good accumulation of vermicast. This isn’t something to panic about; refer to our section on wet castings further down.

NOTE: Once you determine that your system is ready for a more substantial harvest, it’s time to…wait. We highly recommend stopping your feeding, and leaving your system alone for 1-3 weeks prior to a major harvest. This encourages the worms to finish processing the remaining food, and likely even a fair bit of bedding, leaving you with even more castings that are better stabilized.

How Much Can I Harvest at Once?

This will depend on the overall timeline, and on the particular system you are using. If you are simply collecting some castings to test out fairly early on, it’s best to limit this to 2 or 3 cups (500 - 750 mL) of material per harvest, and make sure to space out these harvests by at least a few weeks.

If you are performing a more substantial harvest - multiple months after starting a system - you can remove a larger amount, or even all of it, in the case of a basic tub system or a finished tray in a stacking system.

What is the Best Way to Separate the Worms from the Castings?

This is highly dependent on the type of system you are using. The two main categories of harvesting are:

  1. Batch Harvesting and
  2. Continuous Harvesting.

Batch Harvesting

Unlike the term “batch composting” - which refers to the type of composting where the materials are all composted at once - batch harvesting can be done with a system that receives wastes on an ongoing basis, such as a typical worm bin. It simply means that the vermicast is harvested all at once, and then the system is (usually) restarted. Most basic tub systems are harvested this way, but really, any system you are ready to completely clean out and start over can be a candidate for this approach.

Continuous Harvesting

Urban worm bag

This is the type of harvesting that involves periodically removing castings from an active system. It could be small amounts removed from a system that will ultimately be batch harvested or, more typically, involve some type of continuous-flow-through (CFT) system. These can include stacking systems (like the Urbalive Worm Farm), single compartment flow-through bins/beds (such as the Urban Worm Bag), or more low tech approaches like walking windrows.

IMPORTANT NOTE: Of the two main harvesting categories, batch harvesting is the one most likely to require some extra effort in order to get the worms effectively separated from the castings, but even material harvested from CFT systems can sometimes benefit from additional worm separation efforts.

The 3 most commonly used separation methods are:

  1. Light Harvesting,
  2. Migration Harvesting, and
  3. Screen Harvesting.

Each is covered in more detail in Part II of this harvesting series: “How to Separate Worms from Their Castings”. 

Split-Harvesting - A Hybrid Approach

For those of you who want to enjoy a somewhat more continuous supply of castings while still using a simple tub (or other container) system, one option involves “split harvesting”. The basic idea is that you will set up the system like a typical worm bin, and then remove the vermicompost from only one half of the system - ideally after 2-4 months. The empty space can then be immediately filled with new habitat and food materials, and your ongoing feeding should be on that side as well.

After another 4-6 weeks, harvest the other side - and then just continue back and forth, harvesting the more mature half every 4-6 weeks (but keep in mind, each half is actually operating for 8-12 weeks before being harvested).

Apart from having castings available more regularly (than if using a batch approach), another perk of this approach is that you are also improving the quality of the habitat for the worms more regularly. It also means you won’t ever really need to perform a major system overhaul (which can be fairly time consuming). On the flip side, you will of course end up with a smaller quantity of castings per harvest.


How Do I Store the Castings I Harvest?

Assuming you can’t (or don’t want to) use your vermicast right away, you should store it in a way that preserves its potency for as long as possible. It’s important to remember that the microbiology of the material plays a key role in its value as a soil amendment.

So, as much as possible, you should keep your castings:

  • Stored at cool to moderate temperatures (10-20 C, ideally)
  • Moist - but not overly wet, or dry
  • Protected from the elements, including precipitation and sunlight
  • Aerobic
How to store vermicast

A very simple (yet effective) smaller-scale storage strategy involves keeping your finished castings in bins very similar to a typical plastic tub worm bin. If you happen to have an empty tub previously used as a system that should work, but making one (or more) is very easy as well.

Rubbermaid, HDX or any comparable plastic bins should work just fine. Similar to worm bins, it’s best to avoid clear bins (less important than with a worm bin, but still helpful). All you really need to do is drill 10-20 small holes in the lid ( ⅛” should work well), and that should provide you with enough airflow to keep the material oxygenated, but not so much that it will dry it out.

Breathable bags - such as sand bags and old feed bags - can work well for more temporary castings storage. Just keep in mind that the material will likely dry out a lot faster in them than in tubs.

IMPORTANT: Castings should only be stored once they are mature enough, and once you’ve established the right moisture level. Earlier we covered the guidelines for how to know if your castings are finished. As for moisture level, they should be somewhat moist - and ideally at least somewhat crumbly - NOT wet/muddy. We’ll look at different ways to deal with wet castings next.

How Do I Deal With Really Wet Castings?

With many plastic worm bins, you can end up with really wet material down in the bottom by the time the system is ready for a serious castings harvest. Sometimes it can even be really swampy and anaerobic (with a foul smell).

The good news is that there are definitely ways to get everything back on track and ready for use!

In the case of overly damp, muddy castings, simply dumping the material into an open tub/tray and leaving it to sit can help, especially if you add some dry, absorbent bedding down in the bottom of the receiving container (eg. shredded cardboard, coco coir).

If the material is literally immersed in liquid (this will likely only happen in tub systems without any drainage), the first step will be to drain the excess liquid away. Here are a couple of different ways you can accomplish this:

Super Simple Drainage

An easy option involves transferring the contents of your bin into some type of fine mesh bag, burlap sack, or even an old pillow case that is sitting in another tub/tray, or outside in the grass (or garden). If you are using a tub, we recommend adding a thick layer of absorbent bedding in the bottom (such as coarsely ripped up brown cardboard) to help soak up the liquid that drains out of the bag. As an added bonus - this damp bedding will then be a great habitat material for your worm bins!

Double-Bin Drainage Method

Double Bin Worm Castings Drainage Method

A more involved - but also more hands-free - approach is to use two stacked bins (identical size and model). The upper bin will have ¼” drainage holes in the bottom, the lower bin won’t. Fill the lower bin about ½ or ¾ the way up with dry, absorbent bedding, such as shredded cardboard, before placing the other tub (one with holes in bottom) on top. Next, simply empty the wet contents of your system into the upper tub.

What’s great about this approach, versus the super simple drainage method mentioned above, is that you can tweak things a bit (eg add some food materials in with the bedding in the lower bin) and then proceed with a full castings harvest (and worm separation), once the material in the upper bin has dried out sufficiently. Be sure to check out Part II of this harvesting series for a more detailed look at the “Double-Bin Migration Method”.

What About Worms and Cocoons Left in My Castings?

Regardless of how effective your harvesting strategies are, there will always be at least some worms and (especially) cocoons left in your castings.This is perfectly normal, and not something you should be overly concerned about - but it’s definitely a topic worth taking a closer look at!

Are Worms and Cocoons Left in Your Castings a “Problem”?

Overall, the loss of worms and “future worms” (in the cocoons) shouldn’t be all that significant in comparison to the serious concentration of worms you will likely be separating out. Depending on the end use for your castings, these worms will almost certainly survive - if not thrive - in whatever environment they end up in. For example, let’s say you are using the castings in your garden. While the soil environment itself often isn’t ideal for composting worms, they should still be able to survive (especially if you have rich soils). If you happen to have any composting systems located nearby, the worms will likely move into them and do even better!

Some may worry about the potential risks of releasing non-native earthworms into “the wild”. The good news is that because of their need for really rich organic matter, it is highly unlikely composting worms - unlike certain soil species - will outcompete local earthworms or alter natural environments in any sort of harmful way. Be sure to check out our “Can I Add Red Wigglers to My Garden” article for more information about this important topic!

A Simple Method for Separating Out Your Stragglers

If you really don’t want to lose any worms (including the babies in the cocoons), and don’t mind waiting a little longer, a fairly easy approach is to simply put your castings in a bin and add a small amount of tempting food on the surface.

How to separate your red wiggler composting worms from the worm castings when you harvest your vermicast

A typical plastic tub worm bin, or castings-storage tub (described earlier) should both work well for this task. Given the lack of food in the castings, it’s hard to go wrong in terms of food selection, but you can definitely kick things up a notch by adding worm favourites, such as various types of melon, or some aged horse manure.

It's important to keep in mind that, while the existing worms should come up to the surface within a day or two, the cocoons themselves may take awhile to hatch - likely 1-4 weeks, depending on how recently they were produced. You may also notice other worm bin organisms, such as springtails or mites showing up on or near the food deposits. This is perfectly normal, and they can be scooped out along with the leftover food and worms when you are ready to do so.

Summing Up

  • With most systems, you should expect to wait at least 2-4 months two before major harvesting, and at least 3 to 4 weeks for small sample harvests.

  • Important indicators of castings maturity include: the length of time the system has been running, the appearance (dark, compost-like), along with the smell (rich, earthy) of the material.

  • It's a good idea to leave your system alone (no feeding) for 1-2 weeks prior to performing a big harvest. This helps the worms finish a lot of remaining materials and for the castings to mature.

  • With small (sample) harvests earlier on, be sure to limit the amount removed to 2 or 3 cups (500-750 ml) and space your harvests out by a few weeks or more.

  • The two major categories of harvesting are "batch harvesting" (more material - all at once) and "continuous harvesting" (less material - more regularly), each with their own perks.

  • Split Harvesting is a hybrid approach (well suited for plastic tub systems) that offers the best of both worlds.

  • Worm separation is typically an important part of batch harvesting, with the 3 most common methods being light harvesting, migration harvesting, and screen harvesting.

  • Storing castings properly is important for maintaining quality. Using a breathable, but moisture-retaining, container (eg old plastic worm bin), storing at somewhat cooler temps, and protecting the material from the elements are all beneficial.

  • A great remedy for really wet castings is the use of some form of drainage and/or moisture-wicking system. This can even partner nicely with worm separation and new system prep.

  • Even if your harvesting methods are highly effective, you will likely end up with some worms and cocoons in your finished material. In many cases, there is no issue at all with leaving them be, especially if you are using the castings in a spot where there are composting systems nearby.

  • If you don’t want to lose these worms, an easy method involves adding some tempting food on top of the castings and simply waiting for the stragglers to come up (which may take 1-4 weeks in the case of the cocoons, since they will need time to hatch). These worms, the remaining food, and any new organisms that appear can all be scooped out quite easily and relocated to a more favorable composting environment. 

As a follow-up to this article, we highly recommend you also check out “How to Separate Worms from Their Castings”.

Helpful Related Resources

How to Separate Worms from Their 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?

Recommended Products

Red Wigglers
Worm Starter Kits
Worm Rake
Urban Worm Bag
Urbalive Worm Farm



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