If you're reading this post it means you've successfully managed your worm bin up to the point where you're ready to see the fruits of your labor - beautiful black gold! Congratulations!
Or, if you're not sure if your bin is ready to be harvested or not, go here.
But if you are ready, you need to know that in one way or another harvesting your castings is synonymous with separating your worms from your vermicompost.
This way, you can put the worms back into your bin, keeping your population growing, and also keep worms out of your finished product.
If neither of those are important to you, you can probably skip most of this.
But, if you do want to keep your worm colony growing and not have to worry about worms ending up in your worm castings/vermicompost, read on!
How to Separate Worms from your Vermicompost/Worm Castings
When you take out of the bottom tray of a stacking tray, or harvest out the bottom of a continuous flow through system like the urban worm bag, there will still be worms, cocoons, and possibly unprocessed material in your vermicompost/worm castings (The difference is explained here).
And of course in a plastic bin, the worms have no option but to stay in the "finished product" - all, or most, of the bin has to be fully processed to harvest it.
So when your system is ready for harvest, the problem remains: How do I separate worms from my vermicompost?
We will go over 4 methods, and the pros and cons of each to help you decide which one (or which combination) is best for you.
Method 1: Sifting
By using a hardware cloth as a sifter (usually ¼ inch, or ⅛ inch if your bin’s moisture level is lower) you can physically separate the worms from the castings. The small worm castings can fall through the hardware cloth, and leave behind worms and undigested foods.
Pros: The main benefit to this method is how much quicker it is than all the others. What could take hours, or even multiple days, with other methods can be done in 30 minutes by sifting. Sifting will also catch larger pieces of organic matter (like a banana stem, or leaf stems) that might not have been finished entirely.
Cons: If your vermicompost is wet, it is very difficult to properly sift because the material clumps together. You’ll likely need to use a ¼ inch hardware cloth. The ¼ inch gaps will catch most of the adult worms and unprocessed material to go back in the bin, but some smaller worms and cocoons will still fall through and remain in your finished product.
Method 2: Food Traps / Baiting
Baiting your worms out of your finished product is done by placing little “traps” through the finish product that will attract worms into the trap, which you can then remove along with the worms to place back into your bin.
This is done by filling a trap (something like a net pot or perforated cup - something that worms can crawl into, and you can pull up and out of the bin) with the most attractive and desirable worm foods you can find, and then placing your “trap” into your finished product. As long as the worms can fit into the trap, after a day or so you can remove it and and it should be full of hungry worms to be placed back into your bin.
You then replace the trap as many times as you’d like to keep removing the worms.
Pros: This method will, over time, remove all worms, and cocoons. It is much better than sifting if your primary goal is to get all of your worms back into your bin. Though the entire process is long, you only do about 5 minutes of work every couple days when you replace your traps.
Cons: Time. The baiting method takes the longest. Depending on how thorough you want to be, it could take a day, or it could take weeks. Though it doesn't take too long to empty out your traps and rebait them, you will have a second bin of your finished material that you are baiting worms out of that you will need to be maintaining as if it were a normal worm bin.
Method 3: Sunlight
The sunlight method is done by laying your finished product in the sun. Usually you’ll want to do this on a tarp of some sort. The idea is that worms will do all that they can to avoid the sun, and will usually stay at least half an inch (estimating) underneath the surface.
When you place your finished product in sunlight (worms work harder to avoid sunlight over electrical light), the sunlight forces the worms to go deeper - leaving the top half inch or so where the sunlight penetrates worm free and ready to be collected. Usually that is done by hand.
As quickly as your worms can evacuate exposure to the sun, the quicker you can harvest off of the top. Just to give a rough idea, because there are so many variables, every 15-30 minutes you should be able to scrape off the top half inch. Obviously if you are still seeing worms they need more time, or you need to be taking less off the top each time.
You repeat this until what remains is mostly worms huddled together, hiding from the sun. You can then say your apologies for endangering their lives (as they perceived it) and place them back into their bin.
To increase surface area, and thus exposure to the sun, many people will place their finished product into small (2-3 handfuls) pyramid/cone shapes (again, usually on a tarp) to increase how quickly they can collect worm free material.
But you understand the principle of using the sun to push worms down, and harvest worm free material off the top - use it how you see fit.
Pros: This one is going to be a little quicker at removing all of the worms than the baiting/trapping method. Not all of the worms will go for the food in a trap right away, but all worms will do everything they can to avoid the sunlight right away. If you harvest using the sunlight method you will spend more time on the day you decide to harvest than the baiting method, but the overall process is done quicker - just in one day rather than potentially weeks.
Cons: Sunlight harvesting does get all of the live worms, but it does not remove cocoons - though it would be pretty easy as you are going through it to spot them and remove them by hand. It is also best if done in the sunlight so it would need to be outdoors (maybe indoors by a window) and if outdoors it’d need to be worm-friendly weather. It will also require some space to lay out all of your vermicompost you plan on harvesting.
Method 4: By Hand
Going through your material and picking out worms and cocoons by hand is effective as much as you are thorough and committed. When I am harvesting by hand, I will take a handful and slowly let portions of material fall out of my hand onto a tarp. These portions are small enough that a worm couldn’t be covered by the material I drop, and small enough that I can quickly see any cocoons in the material. If that material that fell from my hand passes the worm/cocoon inspection, I move it to the side so I can focus on the next portion I am going to drop.
Simple enough - you repeat this until you have gone through all of your material.
Pros: Depending on how thorough you are, this can be a good way to make sure you get all your worms and cocoons. If you have a smaller worm bin and aren't harvesting many castings, this is might be a good method for you.
Cons: If you have larger amounts of worm castings to go through, this may take a long time. Picking worms and cocoons out by hand can get old pretty quick.
My favorite way to harvest includes a combination of sifting, and baiting, or sometimes the light method.
Since sifting is physically separating the smaller worm castings from larger things, it will also catch material that worms haven't eaten yet. Sometimes woodier material will take a little bit longer - even if everything else around it has been eaten already.
This is one advantage sifting offers that the others don't. So i like to sift, and then bait or use the light method to then make sure I get all my baby worms safely back home.
I suppose a really efficient method would be to sift, and then if worms and cocoons get though, use the baiting method for a few weeks (so that the baby worms you are baiting out do not lay more cocoons, and most of the cocoons hatch into live worms) then finish it off with the sunlight strategy to make sure you got all of the worms.
But there isn't really a best harvesting technique - it just depends on what you want and your situation. You know the different techniques, their pros and cons, so whatever you find works for you is the best one for you!