Should I Put Coffee Grounds in a Worm Bin?

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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┬áspeculating 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!

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  1. Since we had limited success with backyard compost piles and food scrap-fed worm bins, but still want worm castings for our potted plants, several years ago we moved to an all-coffee-grounds diet for our worm bin and it’s still going strong.

    We add a little clay and coir, but other than that, it’s all coffee grounds (from our home super-auto espresso machine), and while the worms do sometimes turn a little yellow, they’re thriving. And the output is superb – fine and rich. It really helps round out the wood-heavy potting mixes that seem to be all that’s on the market.

  2. My impressions aren’t science, but I’ve found that worms are very adaptable. Over the years we’ve included coffee grounds as a small percentage of the worms’ diet and they seem fine. Maybe those coffee pockets were irritating them all along–I don’t know. But if so, they worked around it. Mostly I’d say that folks shouldn’t get “analysis paralysis” — just feed your worms a diverse diet and give them lots of bedding, and they’ll be fine.

  3. I use coffee grounds by the bucket load in my outdoor compost, usually to get the heap to heat up to kill weed seeds,there is always a mixture of fruit/veg waste in there as well. By the time it is just cooling down, the worms are all through it and happily living in the claggy stinky clumps of coffee. I think generally, in a smaller system, the odd handful thrown in acts as grit for them.

    • I was wondering about that.. I include a very small amount of coffee grounds in my compost heap (not a wormery) but there are barely any worms in mine. I just turned it over and while there are many other types of small insects, i only found three (!) worms.

      I suspect its because of the citrus fruits rather than the coffee though… the compost heap actually smells of oranges.

  4. Thanks for the followup on the topic! I’m still trying to establish my worm bin, and coffee grounds will probably be a regular addition.

  5. As desert (Phoenix area) gardeners, we were genuinely surprised to find worms in our cold compost bin! We have zillions in a 3’x3′ bin! We add coffee grounds and kitchen waste of course! We just stated adding shredded paper which they seem to like. The worms survive our hot (110 and up) summers, warm nights (above 90) and cold winters (down to 18 at night). Not sure what the magic is, but we love the castings and compost.

    • That’s great–it sounds like your compost bin has turned into a big worm bin–which is fantastic! If you want to keep it that way, just keep adding the paper, because they do like that, and don’t go and get ambitious and turn your pile, because that will heat it up and kill them off. Just keep doing what you’re doing, because you’re doing it right!

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