When you have a vermicomposting system, you periodically need to harvest your worm castings. This nutrient-rich “black gold” is the perfect fertilizer for your plants. Harvesting also helps keep the worms healthy. You will know it is time to harvest when most of your worm bedding has been turned into a rich, dark, soil-like substance of vermicompost and castings.
There are several good methods for harvesting worm castings for small home-based vermicomposting bins. Which method you choose will depend on the type of system you have set up, and on your personal taste.
There are single tray vermicomposting systems, stackable multi-tray systems and many do-it-yourself systems. Choose a method that efficiently extracts the casting while leaving enough worms behind to keep composting.
Method One: Encourage Worm Relocation
A very popular method for harvesting, the worm relocation method is based on the fact that worms will migrate towards food. In a single tray system, gather whatever uncomposted scraps remain in the castings and move them to one side of the tray, or to the center. Add fresh food only in the spot you want the worms. In a multi-tray vermicomposting system, put food in the tray above the one you want to harvest. In a matter of one to four weeks, depending on the size of the tray, most of the worms will naturally migrate to the food source. This will leave you with nearly worm-free castings to harvest. At this point, you can also fish out any remaining eggs, which are small and yellow, and will yield more worms in the future. This method is great for anyone who feels squeamish about touching worms.
Method Two: Relocation by Light
Worms will automatically move away from light, so applying artificial light or sunlight to your castings can also yield worm-free castings. Caution: Never expose your worms to excessive sunlight or heat, or they may dry up and die.
You can try one of two things:
- Gently dump out your worm tray on to a clean, stable surface and build mounds of castings. In about twenty minutes, the worms will burrow down to the bottom and away from the sides to escape the light. You can then scrape off the tops and sides of the pile, working in, until only small piles remains. Or,
- Gently put the worm castings into a temporary holding container. Add more food and bedding to the now-empty worm tray. Take a piece of burlap or other material with worm-sized holes and drape it over the worm tray. Spread a thin layer of castings and worms, 1-2 inches thick, over the cloth and wait twenty minutes. By then, the worms will have burrowed down into the worm tray. Put the worm-free castings into a storage container, and apply them to your plants’ soil.
Note: Instead of burlap, you can construct your own filtering trays using wire mesh. This method must be done in a timely fashion so that the worms can stay damp. Both sunlight and human hands can dry out worms, which can harm them.
Method Three: Hand Harvest
For those who are willing to do things at a slower, more intimate pace or who, only need a few handfuls of castings right at the moment, this method may be desirable. Simplest of all, it only requires gathering up handfuls of castings and sifting through them for worms which can then be placed back in the tray. You can dump out the whole tray at once and reload it with food bedding and sort through it all. Or you can choose just a few handfuls if it is not an appropriate moment for a full harvest. Children often enjoy the fun of this kind of sorting.
Method Four: Screen Compost
Sift the worm bedding through a screen and you will separate the worms, sticks, and undigested material from the finished compost easily. Screened compost is especially valued because it’s light and fluffy, and free of debris. You can buy or make a composting screen. See our instructions for screening compost.
After your worm castings are harvested, they can be stored in a sack, clean bucket or other container until you are ready to use them. They are teaming with healthy soil bacteria and nutrients that are perfect for adding to soil. Help your plants and lawn grow strong, and keep your worm population happy by harvesting worm castings regularly.
Note: If you need more composting worms, just order worms online from Uncle Jim’s Worm Farm. We have been in the worm business for more than 25 years, so we must be doing something right! They are guaranteed to arrive alive or we will replace them.