The Coffee Grounds Conundrum

Worms Love Coffee Grounds...Or Do They??
These Red Worms seem to be loving the wet coffee grounds in their bed

Back in April, I wrote about the giant box of coffee grounds I received from a worm farming friend, and how I subsequently used some of the grounds for my ‘Coffee Grounds Worm Garden‘. As some of you may recall, I was really excited about the material, and quite impressed with the results of some initial testing (namely, feeding it to worms in my large ‘winter worm composting bed’).

Fast forward a month or so, and I was not nearly so impressed with coffee grounds as a vermicomposting ‘food’. My dad and I ended up with serious over-heating issues in the winter bed, and it seemed next to impossible to keep the material nice and moist. In my coffee grounds worm bed, the worms continued to avoid the material for what seemed like weeks. I ended up dumping aged horse manure over top and basically forgot about the whole thing. All my remaining grounds have simply been sitting in (partially open) tub in my backyard ever since.

Fast forward to the present, and I really just don’t know what to think about this material!

I was inspired to write this post after digging around in my coffee grounds garden yesterday and coming upon a zone that was absolutely writhing with Red Worms!

And the material they were in? Drum roll please…


I should explain that we’ve had a LOT of rain in the last few days, and the grounds were nicely saturated with water. When it comes down to it, that – along with at least some aging – seems to be the key to making this material appealing for worms.

Something I’ve observed many times with concentrations of wet coffee grounds simply left to sit outdoors, is that they dry out quite quickly and start to look like they’ve undergone some sort of combustion (see next image). My dad is in fact convinced that this is what is happening (either via hot composting processes, or spontaneous combustion). I disagree, and actually think the material is being rapidly colonized by some sort of mold, which gives it the lighter colored, powdery appearance.

Dry, Charred Appearance of Coffee Grounds Left To Sit
Once dark and moist, these grounds have dried out and taken on an almost burnt appearance

One thing is for sure – I’m definitely looking forward to finally putting the rest of those grounds to the test. Now that they’ve been sitting outside (and in the rain numerous times) for so long, I suspect they will be a lot more appealing to the worms…as long as I can keep them wet!

Something else I should mention – all grounds added to my indoor, enclosed plastic bins seem to stay nice and moist, and the worms have no reservations about moving into the material. Hmmm…

Anyway, I’ll be sure to keep everyone posted on my continuing trials and tribulations with coffee grounds!

Stay tuned.

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    • Ray
    • July 4, 2009

    hey bentley,I was going to have the local cafe save me grounds and the waitress asked me if it mattred if it was decafe or not?????will to say they lease,I did get some from them in the morning when I would eat breakfast,but there boss does’nt like them to do it,I just put it in with my beding as a treat,like osture floor I put in the bed with the manure,Ray from Calif.

  1. Somehow it seems to make sense that manure is better for outdoors and coffee grounds better indoors.

  2. Ther must be something to coffee grounds. Dunkin Donuts usecd to tell ther employees to give there coffee grounds away. Now they are not giving them away. What could they be doing with them?

  3. I have been using coffee grounds with my guys and they love them…I mix the grounds with tea bags and the soaking wet cardboard and they go nuts on them….I have found the bins with the grounds populated a lot better than ones with peelings and cardboard mix with no grounds. Still working on the best combination here for my guys.

    Thinking on puree the food down to a point of almost like baby food and having paper shredded through my shredder and no cardboard bigger than a quarter. Having the paper and cardboard sit in a bin for a week outside with at least one rain and 2 1/2 gallon bucket of lechtate poured into it and keeping it moist in there. Now this does get stinky so dont do it beside your house..(your girlfriend/wife will not appreciate this) TRUST ME 😉 This has increased the amount of food they are eating each week by almost double though. Great if looking for more casting production

    Just what I found so far

    • Sherry
    • July 7, 2009

    Everyone seems to recommend coffee grounds for their lads. I’ve always saved my grounds, and I can honestly say that my guys can take or leave them. Mostly leave them.
    I’ve mixed it in the food, and also put some in a spot in my bin. The spot in my bin stays untouched. I keep the lid on so it stays moist. So I haven’t been feeding mine coffee grounds the last few weeks. No since bothering if they could care less.

    I’ve also read that it encourages mites. Now that I don’t need…..MORE mites.

    • Duff in VT
    • July 8, 2009

    I gave up on coffee grounds, too. My worms did not seem to care for them. We drink lots of coffee, but my blueberry plants love the grounds for mulch. I also put them in my barrel composter and they heat up the mix nicely.

    • Louis
    • July 8, 2009

    Hi Bentley,

    I agree with you that it is some sort of mould. I have noticed that the mould only appears when I keep the coffee grounds in a sealed container.

    I puree my kitchen waste & let is sit in a plastic container for a week or so. I have my bins in my house and sometimes, after a week, the mix smells a bit. I lay the mix on top of my beds & sprinkle the coffee grounds quite generously over the top before covering it with wet newspaper. No more smell!

    • Zeb
    • July 12, 2009

    I’m an indoor vermiculture novice, but here’s my theory which I’m 99% positive on:

    Coffee grounds are almost surgically sterile after being brewed with scalding water. I mean how do you disinfect something? Boil it in water. This is the problem because as we all know: worms thrive on the gunk eating the food, not the food.

    So you need to let the coffee re-populate a biotic colony before giving it to your worms. Plenty of ways to do this, but you must plan ahead a little. Mix in a bit of finished compost and let the coffee sit a while to get nice and icky. One mans “icky” is another worms “delicious”. 🙂 Coffee is much slower to rot than normal food stuffs which is why a little pre-treatment will probably do the trick.

  4. Interesting thoughts everyone – thanks for sharing!
    ZEB – I think your ‘theory’ definitely holds water, unlike coffee grounds (haha). If you can keep the grounds wet AND let them age, the worms should feed on them. Right?
    I am putting this to the test right now. I’ve left a bunch of grounds to sit for a couple months now out in my backyard. Today I totally soaked a tub full of the material and made little ‘coffee pockets’ in various locations in one of my vermi-trenches. I’ll be interested to see how quickly the worms start feeding on the grounds.

  5. There must be something in coffee grounds a lot of people use it for a top dressing. But it would seem to me that most of the nutrients would be gone after being boiled and diluted in water. When you boil potatoes are vegetables do you save the water for your worms or garden. I would imagine most of the nutrients are gone then.

    • Zeb
    • July 13, 2009

    I never knew worm-talk could be so fun. (:

    Well I think everyone will do well to consider coffee-grinds to be basically the same as finely ground wood chips sterilized by hot water. Coffee beans have a tough cellulose content which on it’s own isn’t very nutritious to anyone but (drum-roll) the fungus among us. Basic biology 101 taught me that without the fungi of the world we would be elbow deep in cellulose today – since only they posses all the right tricks to take fairly inert stuff like wood fiber and gracefully turn it back into the sugar from whence it came.

    So like leaves and wood chips – once you get some decay started, yum! Worm food! But it’s not fast as with vegetables and their low cellulose / high sugar and water content.

    Let me know if this idea pans out. If coffee behaves similarly to sawdust in your worm bins then I think we’ll have a good working theory! I know my outdoor compost bins LOVE wood chips and coffee and get piping hot each time I mound up another heap of mixed grass clippings and chipped tree branches (plus a good watering). I’ve then observed weeks later my normal earth worms moving up into that mass as they go to town. I live in the German countryside now as an ex-pat, but I was quite surprised to discover my first monster Euro-worms when I originally got here! They are like.. Snake sized! 😀

    But regardless of size, they only move in after the molds and fungi have done the ground work. Must be the same with coffee. I’m not quite ready yet, but I can’t wait for the day when I start my first red-worm bin. 🙂 Thanks to everyone for the wealth of info!

    • Louis
    • July 14, 2009

    Is it then a good thing if the coffee grounds become mouldy? I stand by my previous statement that it goes mouldy when I close it up in an airtight container. I have even noticed that it goes mouldy when under newspaper & plastic covering in my outside bins.

    • Zeb
    • July 14, 2009

    I would say yes! If the molds have moved in that means that the cellulose is being attacked, and it will soon be worm food. (-: Remember mold is a fungi too.
    (There’s a pun there somewhere, but I’ll leave that to the more witty and fun guys out there in the crowd.)

    • Bentley
    • July 17, 2009

    ZEB – I definitely agree that coffee grounds are rich in cellulose, but I definitely wouldn’t say they are similar to sawdust / wood chips – since the C:N ratio of the latter material is generally MUCH higher. Grounds are actually considered a ‘green’ (N-rich) waste for the most part.
    I think you are bang-on with the idea that it’s fungi doing most of the decomposition though.
    What amazes me about this material is that it seems to be able to heat up a LOT even when concentrated in very small amounts. I think this could be very useful for our big winter bed once the weather gets cold again!

    The little pockets I added to my trenches the other day (mentioned in a previous comment) were very warm when I went back to check on them (much warmer than the surrounding manure) – and they were already starting to dry out. Also – surprise surprise – there were no worms in them yet.

    Anyway – definitely an interesting topic, and something I’m going to be looking into a lot more in coming weeks.

    • Don H
    • July 18, 2009

    Your site has been a great help to me in my first six months of growing red worms. I have been getting bags of coffee grounds from Starbucks, (locations in Eastern Canada and New England offer them to gardeners) and experienced drying and a crust on the top. I have solved this by making sure wet waste is always put on top of them and the layer is sprinkled no more than 1 inch thick. I harvested my bin today for the first time and was amazed with the worm clusters mostly in the wet coffree grounds. Lots of cacoons as well which I will separate and add to the new bin. Next time I will use the bin harvesting method. Thanks for keeping up your site! Don H

    • ms
    • July 29, 2009

    I was told by the lady I work for that coffee ground is bad for the red worms, it will stop reproduction, any proof to this?

    • Bentley
    • July 30, 2009

    DON – I am definitely finding the same thing (importance of keeping wet wastes on top)
    MS – I definitely haven’t seen any evidence of this. In fact, the wet, well-aged zones of grounds seems to almost serve as a little nursery for the worms. I always find loads of babies.

    • Gareth
    • May 16, 2013

    Has anyone experienced their worms acting strangely on coffee grounds? I have found that more worms than usual are making their way up the sides and on the lid of their stackable worm bins?

    What percentage are people using coffee grounds in their farms? I’ve just topped off a bin by the last 30% and added another bin shortly after.

    The bin is warm-ish. But only slightly more than usual and not really hot at all, I have also noticed my worms are noticeably
    More red after feeding on coffee grounds, anyone else found this?

    I think letting them age or even composting them to a certain extent might be better, but if just adding them straight to a worm bin, is there any fear or adding too much of this material? Is it likely to heat the bin up to any great extent? It is winter here so I suppose I have added them at just the right time.

    • Barrie
    • May 2, 2015

    My worms love this stuff. They really cant get enough of coffee grounds. I have a supplier that gives me a large bag(several Kgs) per week.Mixed in with the rest of the compost this provides the worms with an extra boost.

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