When people ask about using grass clippings (and green yard waste in general) in their vermicomposting systems, I tend to offer them fairly cautious advice. This is definitely NOT a material I consider to be an ideal worm food for the beginner vermicomposter. That being said, it’s potential as a worm food in general need not be ignored.
First, let’s examine why grass clippings can be a challenging material to work with.
For starters, this material has a low C:N ratio (approx 20:1 or so) and breaks down quite quickly – interestingly enough, these characteristics actually contribute to making this a valuable materials as well (we’ll talk more about that in a minute).
The potential issue here is that rapid breakdown of low C:N wastes can lead to the release of ammonia gas, which is very toxic for worms. Rotting grass clippings also tend to get matted together, becoming a bit of a slimy mess.
Another issue with this material is that it doesn’t hold moisture very well – at least not until it is well rotted. Since worms thrive in very moist conditions, the ‘habitat’ value of rotting grass (on its own) is very low.
Add to this the fact that grass can have pesticide and inorganic fertilizer residues on it, both potentially hazardous to worms. This is why I wouldn’t likely ever use grass coming from unknown sources (ie the stuff collected by landscapers).
Moving on to the positive aspects of grass clippings…
As mentioned above, the fact that grass breaks down quickly and has high N content can actually be a good thing, since this makes the material an ideal ‘microbe food’ (and we know how much worms love a diverse microbial community).
So how do we compensate for the low C:N and the poor moisture holding issues?
By mixing it with an absorbent C-rich material, of course!
Those materials I typically refer to as “bedding” are great for improving the food and habitat value of grass clippings. Bulky materials like shredded cardboard and shredded newsprint would have the added benefit of greatly increasing air flow as well.
My recommendation would be to simply mix your clippings with your bedding in a 1:1 ratio, soak everything (ideally in a location where excess moisture can drain away), then leave the mix to sit for a week or so.
You can also mix grass with other ‘brown’ wastes, such as straw and fall leaves, but you may need to let them rot longer in order to improve the moisture holding properties of the mix.
It is important to mention that this method is certainly not the ONLY way grass clippings can be successfully used as a worm food. In all honesty, my typical approach involves adding clippings in very thin layers to the top of my outdoor vermicomposting systems (such as my trenches). Since these systems are already well-established, there is no chance the clippings are going to create any issues – they can simply rot on top (and help to keep moisture in), and the worms can come up to feed on the material once it’s ‘ready’.
I haven’t really used grass clippings very much in indoor systems – this is definitely a bit trickier, especially when using enclosed bins. I do have plans to test this out though. I am planning to set up a system and use ONLY grass clippings as a food source (while it is available anyway), once the main ‘habitat’ is set up. I’m curious to see how well worms grow with a solely grass diet.