First off, in my post on using coffee grounds in your garden I linked to the wrong article. The correct, and very useful publication by Linda Chalker-Scott, “Using Coffee Grounds in Gardens and Landscapes” can be found here.
There were a number of questions and emails about the pamphlet’s recommendation not to add coffee grounds to your worm bin. Why might coffee grounds not be good for worms? Chalker-Scott cites a study, “Evaluation of three composting systems for the management of spent coffee grounds” that looked at using worms to compost coffee waste. The study showed high worm fatality in spent coffee grounds due, the authors speculate, to the acidic pH of coffee and harmful organic compounds. The addition of cardboard reduced fatality. They go on to suggest pre-composting coffee grounds for three weeks before adding to a vermicomposting bin.
It should be noted that the study was looking at worm bins where the feedstock was entirely made up of spent coffee grounds. Adding a few coffee grounds to a home bin made up of a diversity of feedstocks is probably not going to kill the worms.
But, in a discussion thread on the Garden Professor’s Facebook group speculating about what percentage of coffee grounds would be safe to use, I found myself agreeing with Raymond Eckhart who says,
In the absence of peer reviewed literature as to what percentage is acceptable, the cautious approach is to avoid it altogether, is my takeaway. If and when someone studies the issue to determine a safe percentage, it would be unwise to recommend the practice, given the results of the referenced paper.
Coffee grounds also form large anaerobic clumps worms don’t like. Clearly, they prefer vegetable scraps and large amounts of fluffy carbon material like cardboard and wood shavings.
Now wouldn’t it be great if Elon Musk would fund local Extension Service home gardening research rather than trying to figure out ways to blast rich people into space? We need definitive worm bin advice!