BOM Coffee Grounds Update

I just wanted to post a quick update on the coffee grounds vermicomposting front. As you can probably tell from the picture, I’ve decided to discontinue the experiment.

Shortly after adding everything to my BOM bin, I reached the conclusion that the worms were not thriving in the grounds-cardboard system. Aside from looking rather unhealthy (thin and off-color), most of them seemed to be congregating in zones where there was still some of the compost material that came with them when they were introduced to the stacking bin.

I won’t say that I am 100% convinced that Red Worms can’t thrive in a coffee grounds & bedding system, but I’m certainly not 100% convinced that they CAN either. It seems that I’m making very little progress in terms of getting to the bottom of the “coffee grounds conundrum”. This is a strange, unpredictable material indeed!

Anyway, I’ve started adding food waste to the system and the worms seem to be doing better so I think I should be able to re-establish a certain degree of “balance” before long.

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  1. For over a year I fed coffee grounds to my worms exclusively. I precomposted it to the point where it no longer heated and then fed it to the worms. I was picking up nearly 100 lb/day from a local convenience store that does a huge coffee business. The worms thrived on grounds only.

    • Paulo Silva
    • November 17, 2010

    Hi Bentley, worms don’t like acidic environments that’s why your experiment failed, can you please try adding crushed egg shells to those coffee grounds?

    I’m sure that a good mix of coffee grounds and crushed egg shells would have almost neutral PH and we all know how much they like both substances.

    Good luck with your experiments 🙂

    • Kator
    • November 18, 2010

    Hi Paulo: Acidity .. that’s what I thought as well until recently. After researching the subject and verifying the finding myself, we find that during the percolating process a great deal of acidity is leached out of the grounds resulting in a neutral or almost completely neutral pH. The worm issue is a little more complex than first thought and it appears that we a missing something. We’re still experimenting. I agree with you .. adding finely ground egg shell is good for all systems.

    • John in Huntington Beach
    • November 18, 2010

    Hi Bentley…
    First of all, congratulations on your new bambino. The world seems a brand new place when you hold a newborne and contemplate the miracle.
    Try this as an experiment. Get a nursery flat tray that small potted plants come in. put some 1/4″ hardware cloth on the bottom. Put the tray on top of one of your large worm bins where it will not have to be moved during other maintenance of the bin. Fill the tray with nothing but coffee grounds. Wait a month and observe under the tray. If your result is anything like mine, there will be swarms of worms under the tray and hundreds burrowed into the pure coffee grounds. But the grounds will take a long time to be fully consumed.
    I don’t know if the little darlings are just making a little visit to suck up some caffeine (which is mostly extracted in the brewing process as Kator observed about acidity), or if it is some function of coolness and moisture. But I think any casual observer would conclude that they absolutely LOVE coffee grounds.

  2. Hi everyone! I have a big clay vase. I played around and made it a bin. I put in some vermicast I bought from a store. I added coffee grounds and mixed in some shredded paper. And since I’ve read that coffee grounds are quite acidic, I did add some crushed eggshells (hand crushed so I’m not sure if that’s fine enough).

    I put in a couple of worms and added moisture. A few weeks later, I found that the bin seems very attractive to ants and they’re attacking my worms. So I’ve resorted to regularly flooding the bin with just an island of coffee grounds above water so the worms will have a safe haven.

    Still, I find the results not very exciting so I’m also thinking of discontinuing this experiment.

    • Bentley
    • November 18, 2010

    MIKE – Rub my nose in it why don’t you!!! hahaha
    Seriously though – thanks for sharing that. Doesn’t surprise me in the least (given the wide range of results I myself have seen with grounds). Can you tell us a bit more about the system(s) you were using? How was it set up initially (were worms literally just tossed in a heap of grounds and left to do their thing or was there some other steps involved?)
    PAULO – Red Worms are quite tolerant of a wide pH range (according to Dr. Clive Edwards it’s something like 5-9), and I was actually adding egg shells. Apart from that, in other systems I’ve literally had bunches of worms seemingly doing very well in pure coffee grounds. It’s the inconsistency among my different trials with grounds that has me puzzled.
    JOHN – thanks for the congrats! Like yourself, I’ve actually found worms thriving in grounds (in other systems). It seemed as though the only limiting factor was the moisture content (grounds seem to dry out really easily in outdoor systems). Perhaps in this case it was an air flow issue or me just not being patient enough.
    Cianoy – flooding a coffee grounds bin sounds like a risky venture to me. Are you sure the ants are actually attacking the worms, and not just in there grabbing scraps? If this is a small bin you might try propping it up on blocks in a tub of water (forcing ants to swim to bin if they want access)

  3. Bentley: Sorry about being so blunt. Always in a rush.

    Here’s a little clarification. I precompost the grounds in a Mantis Dual Bin Composter. I watch the temperature and when the heating cycle is on the downward trend the grounds are ready to feed to the worms. In summer this usually takes about 3 weeks.

    The grounds are fed to the worm in one half of the bin so the worms have a choice. Usually within a couple of days they have invaded the grounds. When the grounds are nearly consumed I feed the other side of the bin.

    I use a number of types of bins both indoors and outdoors. The surface of the grounds dry out rapidly outdoors. Daily misting in summer is necessary outdoors to keep the grounds moist. For all of my bins I use old carpeting to cover them. Indoors I seldom have problems with drying out of the bin contents.

    • Kator
    • November 18, 2010

    Wow .. great information. Really boosted my interest in introducing grounds as a supplement feed.

    I was thinking that precomposting might be the answer. My daily supply is constant but small in volume (5-10 lbs). I’m inoculating my accumulated aging cache (45 lbs) with VC laden microbes and allowing it to age for a few weeks before feeding – first aged batch into the bins today. I’m inclined to agree that aging/precomposting is important. I’d really like to know what precomposting does aside from breaking down the graules. Is there a chemical change to make the product more inviting to microbes?

    Mike, I’m also keeping the grounds separate from the main feed bins. I want to monitor the interest .. degree of attraction.

    • Larry D.
    • November 19, 2010

    I literally will pre compost coffee,or quit using it altogether.It seems a flowthru in my case,you have a month or so to get them to dive in.It appears when the coffee literally goes through a change they suddenly become readily interested.Surely different people use different amounts.I’m talking loads from a coffee shop.Even my BSFL are not interested until it goes through a microbial change in my case.My worms just plain would rather starve to death than get into two week old coffee!

    • Larry D.
    • November 19, 2010

    Kator,i forgot to mention.If you hot compost manure you get a powdery looking substance that people mistake for mold.The literature i read said it is actually microbes.Coffee can get an ashy look to it,that is microbial activity.If you find that,give a little to the wormies! They love hay that has it on it too!

    • Jorge
    • November 19, 2010

    Just wanted to mention an issue I’ve been having with coffee grounds.

    The worms in my bin have been raised mostly on grits and coffee (and eggshells). We started with ~8kg in a 2m^2 bin and after three months the population has slightly more than doubled. The only issue I have with the grounds is that there seems to be a certain amount of charred-looking particles left over that do not break down easily and it’s been difficult to separate them from the worms during harvests.

    Otherwise, it’s my understanding that the worms will react differently to different types of coffee and that precomposting is a favored way to feed a vermiculture unit in general. Our grounds come from an on-campus coffee collective that sells fairtrade organic stuff (but I wouldn’t know if that has anything to do with the worms thriving on it) and we’ve been feeding the worms roughly 5-8kg of grounds a week.

    I remember reading that spent coffee is great for compost b/c of it’s buffering properties and high N content – does anyone have any lit. on this?

    • Kator
    • November 20, 2010

    Larry: Your post was interesting, timely and helpful. Thanks 🙂

    I’ve been salvaging grounds periodically in their original filters since late August, placed in large Ziplocs which were frozen, thawed, aired and then stored. I didn’t accumulate much (45 lbs) as, until recently, I had concerns about their value and risk to the system. Shortly after posting yesterday, I went to my cache and picked out a couple of aged grounds packed filters to introduce for the first time. What I immediately observed was a noticeable change in the complexion of the product. Your term, “ashy,” appearance is a perfect description of what I found – definitely important as it will be my future “ready,” marker. And your caution that aging the grounds beyond a few weeks was a flag.

    So I’m going to continue my original routine except that when the containers are aired after thawing, the grounds will be VC inoculated and stored at 70F. I’m thinking that this might speed up the precompost process and I’ll know by observation when they’re ready for the Reds.

    The more research material that I read, the more convinced I am that coffee grounds, alone or combined with other conventional feed, can play a very important role in producing high quality VC. I’ll post further after I assess worm reaction.

    • Larry D.
    • November 20, 2010

    I should also state that coffee also supports fungal growth.And people that value what vermicompost tea does swear by using coffee in their vc. I’m still learning.But i will vouch for what tea does,as seen with my own eyes.And i haven’t even mastered it yet.All i know is it grows my veggies better than the best fertilizer i used to buy.And it can be used as often as you want without harm.And i make the tea for only a few cents.
    Coffee has been giving me fits since i started messing with it.Sometimes it is not good to get in a hurry.Worms eat any thing once it gets to where they like it.And the coffee i get takes time.But it is worth the time!

    • Kator
    • November 20, 2010

    You’re talking to a very patient guy 🙂

    • Michael from Roanoke
    • November 21, 2010

    Dear Bentley,

    I have also had a lot of difficulty with coffee grounds as an exclusive food-source added to cardboard bedding. My best success has been in bins where half the bedding was already converted to vermicompost. If I buried coffee in the vermicompost rather than just the cardboard, the worms seemed to move into it quickly.
    I also remember reading a monograph a while ago which detailed coffee grounds being used very successfully as a food-source. They used aged horse manure mixed with straw as a bedding rather than cardboard and I wonder if that had an impact, with the horse manure probably being more microbe-laden to begin with than cardboard? I seem to recall you posting that you had ready access to horse manure and it might be worth a trial? I’m always excited to hear the results of your experiments 🙂

    Yours Truly,


  4. re: comments by Jorge
    coffee grounds ARE very high in nitrogen. A couple of years ago I used coffee grounds as a fertilizer. I mixed it with my soiless mix at various rates. The higher rates stunted the plants. At low rates the plants went nuts. They were the greenest strawberry plants I had ever grown.

    Literature suggests that coffee grounds work as a fertilizer on lawns. I haven’t actually tested it in side-by-side tests but have worked it into the garden soil without precomposting or feeding to the worms. One bed where it was spread heavily (2″ on the top) and then worked in produced the largest tomato crop I had ever seen.

    Anybody with similar experience?

    • Bentley
    • November 23, 2010

    Wow – this has really turned into a lively discussion. Sorry for the delay responding (and in advance, for not adding anything further at this time). Just wanted to send a big thanks to Mike W. for sharing your pre-composting approach – that’s fascinating!

    I came across a beautiful compost tumbler this fall that was being offered for a very low price ($90 vs the usual $300 or so) – I’m really kicking myself for not grabbing it when I had the chance. There are LOTS of precomposting mixes I would love to test out!
    Hopefully I can find an inexpensive tumbler before spring, or make something myself.

    • Kator
    • December 1, 2010

    Larry, referencing yours of November 20th:
    It’s been 10 days since I added the aged and VC inoculated grounds to two of my bins, a wooden flow-through and a Rubbermaid. The grounds are well separated from the feed troughs. The first sign of interest was in the flow-through, noted yesterday .. a handful of worms in the grounds. I’m a little surprised as the alternate feed in this bin is pumpkin .. which the lil guys dearly love.

    I’m seeing no interest in the other bin. The food competition there is blended food scraps and oats .. which has accelerated propagation beyond expectation.

    This certainly confirms your observation that’s it’s an acquired taste. This is a great experiment and I have confidence that it will eventually work. Once that I see evidence that grounds are attracting greater numbers of wigglers, coffee will be added to my blended feed in good portions. Wish I had an indoor hot compost system.

    • Angie
    • December 3, 2010

    I have used spent coffee grounds in small quantities in my wormbin for years. Never had any problems, but then I add them along with other veg/fruit waste and cardboard. The worms seem to love them, but only once they’ve begun to decompose. I give the bin the odd sprinkling of ground egg shells too.
    I read long ago that material in the wormbin has to be decomposing and rich in microbes before the worms can take it up. So I think it might be a good idea, if you have a lot of coffee grounds, to pre-compost them first. I find they go mouldy quite fast when damp, so this should start the process off and make the material more appetising to them.
    When I used to get large quantities to compost, from a cafe, I found the grounds tended to go into a solid mass, and the worms avoided this for some time. Once I broke it up, and mixed it with other food waste, they partied!
    I put surplus in my regular compost bin, and also on the garden. Great source of nutrients, and it keeps slugs at bay.

    • Larry D.
    • December 3, 2010

    I started adding it in smaller doses,and in the shape of a triangle.I figured it would give them a small taste,before they got into a big pile of it.I find some coffee has a fibery look to it.Darned if i know why?I thought i had put some coir in there by mistake.Does any body else notice this? I am talking about after it is composted.I don’t know about all the blends.Maybe some flavored coffees have something else ground with it?

    • Kator
    • December 3, 2010

    Angie, I like the info on slugs. Was not aware that grounds repelled those lil pests and I know just where to scatter a batch next Spring.

    Larry, my older batch of grounds is growing a fungus, but it was not pre-composted, so no fibrous appearance.

    We’re into winter in this part of North America so all my wiggler operations will be indoors for a good many months. I’m really interested in building a small indoor hot compost system, in particular to speed up preparing coffee grounds. I know that these can be bought (they’re not cheap). I’ve seen a cutout diagram of a commercial unit. Doesn’t look too complicated .. a fan and a small source of heat to supplement that generated by decomposition. Has anyone built one?

    • Angie
    • December 3, 2010

    I put a ring of coffee grounds round slug-susceptable plants, with crushed eggshells too, and it seems to work. But you have to make sure there aren’t any overhanging leaves for them to climb over it! Am sure the slugs over here have learnt how to jump! hahaha! Copper rings work well too. Gives them a minor electric shock.

    Yeah-it’s winter in the UK tot. Temperatures down to -20C in the North-a meter of snow in places-started early, in November this year, so it looks like it’s going to be a long old winter. More snow forecast, and temperatures plummeting-not been above freezing for!
    So, I’m a bit concerned about my wormery. Too heavy for me to move inside, so I have wrapped them up as best I can with fleece. They are in a sheltered spot, under a tree, but with night temps well below freezing, and in the day as well, I may well lose quite a few this year. 🙁 Have packed it with scrunched up paper and leaves, so fingers crossed.

    Never had any fibrous bits in the coffee grounds here, so can’t help with that I’m afraid Larry. The fungus will decompose the grounds quite well. Be careful not to inhale any spores though-not good for the lungs, and some people may have a severe allergic reaction to them as well.

    Getting dark soon, so must go and check my worms now.

    • Kator
    • December 3, 2010

    Good point Angie. We should be cognizant of risks associated with inhaling fungus spores.

    I keep my blended feed in sealed containers and I always wait until a light coating of fungus developes before I place the feed in the bin troughs and cover it liberally it with VC. Fungus plays an emportant role in the compost process and I’m counting on it to break down the grounds. I’d really like to have a small indoor hot compost sytem. I have a great untapped source for grounds.

    • Kator
    • December 4, 2010

    I should explain that my blended feed storage tub containers have small air holes (I only keep two active at a time), are kept moist, not watery (balanced by oats), and are contained in a 5gal sealed pail (to contain any fungus spores from wandering yet provide access to a greater volume of air), in a cool dry environment.

    I’d like to share another observation that I’ve made concerning fungus. A few months ago I observed a few dead wigglers on the surface of two of my bins – one or two worms in each on three occasions within a ten day period. It was a first for me and had me puzzled. There may have been more subsurface deaths but I found none.

    I took my problem to Bentley and he suggested that small pockets of ammonia (or hydrogen sulfide [sulphide]) may be responsible (both deadly) and also warned of the potential of a bin meltdown if not corrected. I had a look at the batch of blended feed that I was using and found that particular mix was becoming unusually watery and was developing a pungent (anaerobic) odour. What I also noted was a lack of fungus .. and that should have tipped me off that the feed was bad .. lack of oxygen = anaerobic = poison. Thanks to Bentley I saved both bins, became acutely aware of a potential feed issue, which resulted in a healthy environment leading to a greatly increased production of wigglers :))

    • Katie
    • December 7, 2010

    I am a graduate student working on a sustainability project looking for someome to take used coffee grounds from a large local business. Internet research linked me to vericomposting, something I have been interested in for years but have not done yet. I offered the grounds to a local worm farmer but he was not interested stating the coffee grounds are toxic to red worms due to a growth inhibitor unless the grounds are first hot composted. After reading some of the postings, the type of preparation of the grounds may be why some of the wigglers thrive and some don’t on the grounds…Just a thought; you are the experts, certainly not me.

    Thank you for your most interesting posts. Now I am more anxious to get started with vc. If you are in FL and want some coffee grounds or know of any other uses for them, please let me know. Thank you and happy red wiggling.

    • Larry D.
    • December 7, 2010

    I’m no expert! But i am in Florida.If it doesn’t matter the location,maybe you can post,or get with Bentley.I can use several hundred pounds for sure.I prefer huge loads,so i don’t have to counter act by burning a lot of gas to pick it up.Kind of makes me feel like i am defeating the purpose!But i do choose routes wisely,and make sure a starbucks is on the way!

    • Kator
    • December 17, 2010

    It’s been 25 days since I added coffee grounds to two of my bins – a Rubbermaid and a wooden flow-through. This morning I discovered a number of wigglers in the grounds in the flow-through, which is promising. There were none there three days ago. It’s interesting since the alternative is pumpkin located in a separate area of the bin. I have about 25 lbs of grounds waiting for a hot compost system. The stash is being aired and tumbled but I’m seeing no increase in temperature.

    • Kator
    • December 23, 2010

    Thirty days in and seeing more interest in grounds planted in the flow-through. This is encouraging.

    Larry, I envy your climate. Too late in the year to hot compose outside here, but I’m hopeful that I can build a small indoor composter that will work for the grounds. I’m trying to find someone with a working model that I can copy.

    • Iva
    • January 22, 2011

    Just read this article. I feed my worms almost exclusively the “homemade manure” mix that Bentley suggested. I keep my covered bucket in my kitchen and stir it every day, and whenever I have coffee grounds (make a 12 cup pot about 7x a wk) I throw them in there along with the filter. I let the mix sit and brew for about 2-3 weeks (though the coffee grounds I keep adding don’t brew that long) and my worms are chomping on it practically the minute I spread it in my bin. They LOVE the mix, and that’s with a decent amount of grounds, uncomposted, in it. We also use a huge amount of eggs every week (about 5 dozen? sometimes more?) so I keep busy drying and grinding up eggshells. Every time I feed, every time, I sprinkle on a layer of eggshell powder. So far, I’ve not had a problem with a sour bin so I am thankful.

    • Larry D.
    • January 22, 2011

    Just be careful about breathing that much powdered eggshells.From what i have seen,lime and eggshell dust should not be inhaled.I crush egg shells,but i quit grinding them.If they come out in the vc,i sift and throw it back in the bin.

    • John Donaldson
    • April 20, 2013

    Thanks for all of this discussion on coffee grounds and VC. It’s helping me a lot!

    • Korey K.
    • June 10, 2013

    I added probably a full 5-gallon bucket to my existing worm bin. (3 months old, originally peat moss/aged manure). When I added the coffee grounds to the worm bin, the next day I felt it with my hands and it was really hot. Is there anything I should do? It is already getting hot here in Texas. (around 93 degrees) Should I just let them be? Add water? Take it out and replace it with other bedding? Thanks guys!

    • Bentley
    • June 13, 2013

    KOREY – Are you saying you have a bin outside in Texas, and added coffee grounds to it? Yikes.
    I would probably mix in more bedding and (definitely) rotate multiple frozen water bottles (or something similar) between the bin and your freezer (but definitely DON’T add frozen food waste – this will only create more trouble)

    • Elle
    • November 8, 2015

    There is a difference between the grounds based on the roast. The variables in the coffee grounds could have an affect on the experiment.

    • asaf
    • May 24, 2017

    my understanding is that red wigglers do not directly feed on the feedstock, but on the microorganisms that grow on in.
    This would explain why they like pre-composted coffee grounds, not fresh.
    Fresh coffee grounds are pasteurized by the brewing process and are therefore void of edible microbes for the worms.

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