Coffee Grounds and Mould
Up until this weekend, I had a shed floor covered in used coffee grounds, with another lot sitting in a cardboard box. The idea was simply to see what happens with it, in a cold and dry environment.
Yes these are the sorts of things that I find interesting – things growing in coffee grounds. Let’s not dwell on it too long 🙂
Here we have some 4 month old grounds, covered over with fungi. I have seen this happen without fail to damp grounds regardless of whether they are in a plastic bag, a bucket, or piled in a heap on the garden or around plants. As you can see from the photos, it has spread through the entire pile. So to answer one of the questions I’m asked about all the time – will mold grow on used coffee grounds? The answer is yes (if they are kept damp).
Coffee Cake anyone?
Inside the cake -more coffee!
Even older – The box of 5 month old coffee grounds.
It seems that much of the fungi (which we also refer to as mould) drops off once the grounds are handled. Not having done a analysis of the type of fungus, I turned to the Internet, just like a do when I need to have a medical condition diagnosed.
Is there a Problem with Mould Growing on Coffee Grounds?
The general consensus among the gardening community is that no harm will come to you when handling the type of fungi that make use of coffee grounds, and that it is just a natural step in the process of decomposition. There is an interesting thread here about fungus on coffee grounds, another post here about fungus on coffee grounds, and many many more if you care to Google it.
What I find works is by using the grounds before they visibly mold, avoid keeping them in a pile when damp, or letting them dry out and then used at a more appropriate time. It is not always easy getting hold of freshly made used coffee grounds, and I have had my fair share of collecting bags of moldy coffee grounds. And at the end of the day, I’m not overly fussed what they look like, because they will end up the same, breaking down in one or another into the garden.
If moldy grounds are an issue for you, see if you can strike a deal to collect coffee grounds with a local coffee shop when they are fresh (same day or day after). The Ground to Ground Google map might be helpful in finding an appropriate cafe to collect from.
Like with anything else in the garden, take some simple precautions with gloves and a mask if things are dry and dusty. And wash your hands afterwards. There are billions of little creatures swimming around in those coffee grounds, like any other organic material.
If you have had an experience with coffee ground fungi growth, let us know in the comments below.
2011 Summer Update – Some more photos of mold on coffee grounds
This series of photos are from coffee pucks taken out of the worm farm – were sitting in there for several weeks. See the variety of green to blueish to white mold growing on them?
It is as it should be.
Related Articles
- Effect of Used Coffee Grounds on Mosquitoes (groundtoground.org)
- Coffee Grounds and Pepino (groundtoground.org)
- pH of Soil from Coffee Grounds (groundtoground.org)
Ground to Ground 
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I use a 2 pound coffee can in which to save my used coffee grounds. They do not mold. If I discard them into the open waste can under the sink, they mold. Is this because in the coffee can which is sealed they get no oxygen; where as, in the open garbage can the grounds do get oxygen?
Also, It is easy to carry the 2 pound can of grounds to the garden once a month, or so to the gardens.
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Hi Shane, I’ve heard it said that the caffeine content of coffee can be bad for plant germination and growth (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1276408/ ), but that keeping coffee grounds and allowing them to grow fungus for about a month or so reduces the quantity of caffeine by almost a third ( https://www.mdpi.com/2571-8797/3/2/19/htm ). Should I start storing my coffee grounds to reduce their caffeine content before I compost it?
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I get fungal growth throughout a large Folgers coffee container in a week. I also add my smashed eggshells to the same container. I save my coffee pods in a gallon baggie until my coffee container is thoroughly moldy. I then chop it roughly and add to my garden. Empty the old pods back in the same container and repeat.
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Mine is in a McDonald’s tall cup, and turned white and orange like cotton candy
I’m afraid if the coffee from the pod was not good before I used it.
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Hey thanks for taking the time and sharing your pictures and thoughts. I let some coffee grounds dry out in a box and noticed they have the same fungus as in your pictures. Im planning on throwing some in the lawn tomorrow. I’ve read that the worms like it so that’s a plus.
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Thanks for writing in! Worms love the grounds. All the best 😊
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I read about the many uses for used coffee grinds. One of which was fertilizer for plants. I started to periodically add them to the soil of my house plants, inside and out. Over time I have noticed that mold would start to consume the soil.
Is this healthy for my plants ??
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Hi Mishka. Depends how much mould. Can you send a photo?
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Seriously, I tried to use coffee as mulch and mix in potting mix! And they grow so much mold! Of all colors!
Horrifying!
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What appears as “moldy” grounds are healthy mycelium colonizing the grounds. If the mold you are growing is a fungus, you might luck out and eventually get mushrooms!
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I absolutely love your blog! It’s so nice to find super extensive information about coffee grounds. I have only been using grounds in my garden for about a year and it’s going well. I throw it here and there as fertilizer, and put most of it in my compost piles to decompose. I’ve used some moldy stuff before and never had an issue with it. It’s just a natural process. 🙂
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Thanks heaps for the feedback, it is good to know it is making a difference out there. There was not really much info out there about coffee grounds besides the same rehashed articles by people that I doubt had ever gone to the trouble of collecting and using coffee grounds. Well, after 2.5 years of this there are over 100 posts about coffee grounds now and 10 tonnes of it in my place!
Hope to see you here again and thanks for writing in 🙂
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Hi Shane,
I love reading information on your site…it’s very useful. I recently put new grass seed down on my lawn and didn’t have any fertilizer handy. I have been using coffee grounds on my flowers and smaller plants for a while, but never thought about using them as lawn fertilizer. My flowers and plants look terrific, so I hope it helps the grass seed too.
Thanks for all the helpful tips.
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It is a pleasure Connie – glad to help, and thanks for writing in.
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I found this very interesting, thank you. However, as my coffee grounds dry, they are growing some sort of orange mould. Do you have any idea what that is? I have seen a series of random comments on online forums that make it sound like the orange thin mould looking layer may be everything from a mould to a bacteria. I do not have a way to have the orange substance tested, but was hoping to find information on whether it is harmful to either myself if accidentally touched or even inhaled (though I have evidently taken precautions against that) or for a compost heap. Thanks for any information should you have some. Again, thank you for your informative site all around!
Jennifer
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Hi there Jennifer. It sounds to me like you are referring to what is known as Slime Mold (in this case Fuligo septica or very close). There is some great reference material out there and one of the best is from the Carolina Biological Supply Company, which describes slime mold as:
Sounds like the same conditions you would find with used coffee grounds?
I found a nice photo of slime mold that you might find interesting.
And this article from Mississippi State University has a nice description of orange slime mold.
Final bit of info – it is thought to be safe for humans, although as with any organic material around the garden – try not to inhale it.
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I have mold growing on the inside of the compost bin. Does coffee have mold on it?
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Not at all Barb. Mould is just a natural part of the decomposition process and is a sign that things are working as required – turning used coffee grounds and other organic material back into soil.
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Instructive post – I enjoyed it very much! Won’t worry too much the next time I get mould on those coffee grinds.
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No Anna, don’t get too worried about the mold unless there is a large amount in the air. Keep the compost heap moist and take the same kind of precautions as you would with any other organic material.
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My husband and I own a coffeeshop. We have been composting our coffee grounds and veggie scraps off and on for almost 10 years…I am happy to say, we now compost 100% of all our used coffee grounds! My husband buys 55 gal plastic barrels that have screw on lids. These barrels are also recycled, they were originally used to store olives. He then fills them about half full of coffee, leaves, coffee filters, newspaper, kitchen scraps, etc. Once filled, he lays it down on the ground and drills holes the barrel. Then he just rolls them every few days. Soil looks beautiful after a few short months.
When I came across your blog the other day, I had to show my husband he wasn’t the only coffee composting guru. We are always promoting recycling coffee grounds and composting and I would love to help promote your Ground to Ground project. I instantly fell in love with your logo and I see that you allow a print to be made of your logo for in coffeeshop awareness. I wonder if I couldn’t place one of those on our coffee grounds recycling bin? You should also consider selling t-shirts, I would wear one.
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Wow. When I read your post Stephanie I felt like they do in those TV shows when they have a family reunion. We found each other after all this time!!
Just to answer that last bit first, yes you can use the logo on the recycling bin, or on the coffee machine (like in my post Ground to Ground in action), those uses are fine.
Love the idea about the t-shirts. I raised in with the wife and she agrees with you. Will check out some of the places that do that. Who knows, maybe even on a coffee cup??
Can I ask that you get some photos to me of how you are using the grounds so we can put them on the site. Getting about 2000 hits a month now and that is exactly the kind of practical advice we should be passing on to folks. It seems to be that people really want to make a difference and sometimes are not sure how to go about it.
So my long lost coffee gurus, now that we are family and all, please send me some snaps of the logo in action and some that show how you turn all your coffee and other organic material into soil.
Thank you for reaching out. Take care now.
Cheers,
Shane
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