Harvesting vermicompost is potentially one of the most frustrating parts of the process. Unlike regular composting, you have the worms to consider. If you really wanted to, you COULD just dump out the contents of your worm bin and treat the material like regular compost, but of course that would be the end of your vermicomposting efforts (assuming you didn’t purchase/obtain new worms).
To add insult to injury, the material that comes out of many worm composting bins (plastic enclosed bins, that is) is often heavy with moisture – and looks more like sludge than good quality compost. Generally speaking, you are probably going to want to dump the contents of your bin out and let everything dry for awhile. A shallow, open tub or even some sheets of corrugated cardboard on your basement floor can serve as a good location. Just MAKE SURE you break the material up repeatedly during the drying period! Otherwise, you may end up with a solid block of vermi-concrete (you’d be amazed how hard this stuff can be)!
In case you are wondering WHEN all this takes place, my recommendation is start thinking about harvesting around the 3-6 month mark (after setting it up). By this point, a high proportion of the material in the bin should look like dark compost.
Before you actually harvest, it’s not a bad idea to stop feeding for a week or two (the above-mentioned “drying period”, if necessary, would serve the same purpose) – this allows the worms time to process a lot of the unfinished material. This period also serves as a good time to set up (and age) a brand new bin for the worms to get transferred into.
As for the actual harvesting itself, my favorite low-tech approach is something I refer to as the “turbo light harvesting method”. The traditional “light harvesting method” involves dumping out the contents of your bin (on a tarp etc), shining a light directly over top, and gradually scraping away the compost – eventually leaving you with a gob of worms at the bottom (in theory). The principles involved are sound – worms DO indeed readily move away from light – but this tends to be back-breaking, and very slow work. My variation of this approach involves using shallow trays under bright lights (or even better – outside on a sunny day). Check out the video below for a demo of the process. Just so you know, similar to the regular light harvesting method, this approach won’t work all that well with really wet vermicompost. You will almost certainly need to follow my drying advice in order for it to work well for you (assuming you are using an enclosed, moisture-retaining system – the material coming from well ventilated or completely open systems should work great as-is)
If you want to learn more about various harvesting methods, be sure to check out the harvesting section of the Red Worm Composting “HOT TOPICS” page – you’ll find links to various relevant posts on the RWC blog.