Lots of Mold in My Worm Bin!

I have decided to add a new category to the blog – “general questions”. Unlike our “reader questions”, these are not exact questions I find in my inbox (and quote directly on the blog) – but they will cover topics that people just generally seem to be curious about.

Today’s topic is a prime example of one that seems to come up a lot (especially lately, for some reason) – mold growth in worm bins. i.e. ‘Should I be concerned?’; ‘How much is too much?’ etc

As many people have discovered, setting up a worm bin in the manner I typically recommend – that is, mixing bedding with food waste then letting it sit for a week or so before adding worms – can (and in fact, likely will) lead to some obvious fungal growth. This is to be expected, and is definitely nothing to worry about. Excess mold growth in a bin containing worms on the other hand may be an indication of something potentially more serious.

Let’s deal with each of these scenarios separately. First we have the case of various fungal species taking hold in the moist, warm (usually), nutrient-rich environment of a bin that’s just been set up. This should really come as no surprise – you’ve basically created the ultimate, low competition microbial buffet (and habitat) – kinda like a five star resort for microbes.

Add to that the fact that fungi tend to thrive in somewhat acidic conditions (typical of rotting food wastes), and it should almost be surprising if they DIDN’T appear!

As mentioned above, this situation is generally not something you need to worry about. What I would recommend you do when you see this growth is simply mix up the contents of the bin (again, we are talking here about a situation where there are no worms). Mixing the contents of your new bin once or twice before adding the worms is actually a really good idea in general. If, aside from the mold growth, things seem a little too wet, you may want to add some new dry bedding as well. Similarly, if after mixing, there still seems to be a lot of dry bedding you may also want to spray everything with some water.

This mixing will break up the fungal mycelia (the hair-like growth typically associated with fungi), thus impeding further growth. Once the worms (and associated ecosystem) are added, the fungi should be kept in check via the movement, and direct grazing of the worms (and other critters).

This is why excess fungal growth when worms are present in the bin can be an indication of a problem – typically one of two things (often closely related to one another). The most common issue will likely be overfeeding. Plain and simple – if you add a lot more waste materials than the worms can consume, or if you add it in a form that is not particularly worm-friendly (i.e. you don’t do anything to assist the process), other organisms are going to take advantage of these food resources, often including various types of fungi.

Overfeeding can also be closely linked to the other main cause of fungal growth – the dreaded ‘sour bin’! This basically occurs when excess acidity builds up in a worm bin, most often as a result of too much food being added, or simply too much of a particular type of food being added. As some of you may recall, this is exactly what happened to me when I added too much food waste from one of my bokashi buckets to my European Nightcrawler bin (see “Symptoms of a ‘Sour’ Worm Bin“).

As I mentioned above, acidic conditions tend to favor the growth of fungi. Composting worms are actually quite tolerant of acidic conditions, so some drop in pH generally won’t be an issue, but obviously there is a limit to their tolerance.

Rather than waiting for the appearance of ‘mold’ in your system to let you know your bin is going sour, I would recommend being proactive in your efforts to keep things balanced. Slow-release pH buffers like crushed egg shells can help to prevent these conditions from developing in the first place.

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    • Danielle
    • July 22, 2009

    Thanks for this insightful and reassuring post. I am just starting my bin and I have run into the first mold scenario listed above and I feel much more reassured. I’m very thankful that there is such an abundance of information on your site to help guide me through this composting experience. This is all very new to me but I’m excited for this to play out and see the developing stages of composting with my new red worm buddies. Thanks again 🙂

  1. Ok, so ;I read the above about mold, which I just found a royal flush (no pun intended) of in my bin. It probably was from overfeeding just lately, (cause I’ve had a successful bin for over 3 years). It has been very hot here AND I put in a lot of stuff twice in the last 10 days. I took out the top 3″ or so (with most of the obvious mold) yesterday, and did stir up the top after that.
    My question now is about altering the pH. You mentioned egg shells, but I’ve heard that they are too sharp for the little critters, and I can believe that. I know I could grind them up, but have not wanted to get my grinder all funky with dried egg whites (even if the egg whites are good and dried). I will look into a way to get some of them ground to use, but I’m wondering if you have any other ideas about mechanizms of changing the pH.
    AND, even tho I do not see dead worm YET in my bin, do I have to worry now that the bin has GONE SOUR, and do something drastic to change it SOON!!!

    • dave
    • December 19, 2009

    Thanks for a helpful post! I have some kind of yellow-green fungus or mold that seems to be spreading a bit; I will try your tips above and see how they work!

    • Ashley
    • August 18, 2010

    Thanks for the post, I have some crazy weird mold in my worm bin. Since I already have worms in there, what can I do? I can add some crushed egg shells, fresh shredded paper leaves. Is there anything else? Should I get more worms to eat up all the fruit waste that is attracting the mold? Help is MUCH appreciated! I don’t want this mold to kill my poor worms!!!

    • Bentley
    • August 19, 2010

    Hi Ashley,
    I wouldn’t worry too much about the mold killing your worms, but overfeeding in general can certainly lead to this. It sounds like you have the right idea. Fungi love acidic conditions, so adding egg shells is a good idea. I would definitely also add a lot more bedding material – mixing it in well (breaking up the fungal mycelia while you are at it).
    Removing excess food is probably not a bad idea either.

    • jen
    • September 24, 2010

    One idea I’ll throw out there, even though I haven’t even tried it yet (but am planning on it) is to grind the eggshells with a mortar and pestle. If you don’t have one in your kitchen, you’ll find it’s useful for a lot of things that don’t even have to do with worms!

    • Mike
    • November 7, 2010

    ya umm i just started my worm bin like acouple of weeks ago and there are fuzzy white mold around should i be worried ?

  2. Hi Mike,
    I wouldn’t worry too much about this – it is very common to see fungal growth in a worm bin – especially early on. One thing to keep in mind though – it CAN be an indication of the fact that you are adding too much waste and/or have not added enough bedding materials (especially if there are already worms in the system)

    • Ben
    • December 8, 2010

    Hey Mike,

    Breaking up the mycelia won’t diminish it’s growth in such a nutrient rich medium, in fact it may even speed growth as the new location each bundle of mycelia finds it’s self in could be “virgin” at least in mycological terms. Eradicating mold in such an environment is more than unlikely even with sterile conditions for any real length of time. Better to fill the niche the mold has found, kind of like vermi-aikido.
    I’m going to be looking through my books (I’ve been a bit bemushroomed as of late) and find an edible mushroom likely to do okay but not great in my bin with some attention to the types of food I’m putting in. Rather than using a heavy innoculation (the goal is to repress unbeneficial molds not so much raise mushrooms) I’ll go easy and any mushrooms that DO pop up? Bonus! That said make sure the mushroom you’re putting on your pizza is a fungal friend not frenemy.
    I’ll keep you informed.

    Great GREAT website,

  3. Thanks you! I needed to know what was going on with the mold

    • Nanette
    • April 22, 2011

    I woke up to some yellow goo growing out of my worm box. I also have a question about how to separate the worms from the castings. Thanks so much for your articles.

    • Bentley
    • April 22, 2011

    Hi Nanette,
    Yellow goo and weird, bright colored fungal growths in general are very often “slime molds”, which actually are not fungi at all. They seem to appear very quickly and often disappear at the same rate (sometimes leaving a crusty, dry lump o stuff (one variety is appropriately referred to as “Dog Vomit” by some).
    As for harvesting, be sure to check out the harvesting section of the HOT TOPICS page.



    • RoseAnn
    • May 10, 2011

    I never had mold in my bin but since I put the cobs in a couple of days ago I now have mold.

    • Nanette
    • May 10, 2011

    Thanks for the info on the yellow fungus. It has dried up to a brown mass, similar to the worm poop(?) My next question is why to the worms try to crawl out? I find them in the lid edges and also in the tea tray.I still need to find out how to separate the worms from the good stuff. I am sure I am not using “worm talk”,but hope you know what I am talking about.nwm

    • Derek
    • May 12, 2011

    Thanks for the info. Recently, I’ve actually had an explosion of green mold on the sides and top of my worm bin (it’s made from wood) AND lots of reddish mites running around. I don’t even want to go near the bin! I’m new to this, and I suspect that I overfed them. Is there anything I should do? If I leave it alone for a while (I haven’t fed them for about 2 weeks now) will the bin reach “equilibrium,” or will the worms die? I would appreciate any advice!

    • Bridgette
    • June 25, 2011

    I just started my bin 1 weeks ago. It happened that I suddenly got a can of worms from a friend so I wasn’t really prepared to start the set up a week prior to getting them. I have 1 (rubbermaid) bin that I keep them in and not the 2 bin set up (but I may change that today). Anyway, it seems that every time I open the bin there is a layer of “cotten-y” mold on top. I’ve been just mixing the top layer up a bit to get rid of it but after reading this I’m worried that it’s more of a problem. I don’t have much newsprint in the bin and started out with a bunch of shredded junk mail and brown cardboard. What do you think?

    • Fred allen
    • January 8, 2014

    The only comment I will make is, when I added “KALE” bingo, white fungus appeared? Never a problem before Kale.

    • Brad Y
    • April 5, 2014

    I do not have any mold in my bins which have been running for about a year. But I do have mold from time to time in my waste pail. I usually feed the worms once a week. Is it recommended to remove the mold before feeding or can I just mix it in?

  4. I have a worm farm set up. The kind u purchase. It was given to me 2nd hand. I’m doing fine with it. No problems. Only a couple of questions: #1. I have a spigot at the bottom for draining liquid. I don’t know how I should dilute this to use as liquid fertilizer. #2. I’m giving some finished compost to a friend. But because there has been mold on the scraps (not a lot. Reg old bluegrreen kind that grows on orange peels, she is worried she will be importing mold spores to her yard even though the.mold itself has already been digested by the worms. I never thought of that. Any thoughts?

  5. I have had a worm farm for about 3 yrs. Haven’t had any problems. But mine is the 3 layer kind that u just keep moving around as your compost finished. But it also has a spigot in the supportive framework where all the liquid collects. I got it free with no directions, so have been winging it. I assume that the “tea” u get from the spigot is good plant fertilizer. But I have no idea what its nutritive composition and PH is. What would be the dilution ratio with water. I would appreciate some feedback as I almost killed a couple of plants. Lol .

    • Ponicman
    • April 19, 2014

    The liquid fertilizer you get from worm compost has a lot of ammonia(which is a weak base), and also other macronutrients, but it´s probably not going to affect the pH that much. Common dilution rates are something like 1:20-40, depending the plant you are growing. It works perfectly in soil cultivation but if you intent to use it in hydroponics, you are going need a biofilter for nitrification to occur.

  6. So if there are bananas are grape stems etc with mould on them, that’s okay for the worms? I’t not hairy mould at this stage.. I have a 60 x 50cm worm bin, and am not sure how much food is too much. I don’t even have one total layer of food, so I’m guessing I ‘m not putting in too much..?
    Also, are there particular foods other than citrus which they don’t eat?

    • Shaun
    • July 25, 2015

    Hey your website is awesome,

    I have a can ‘o’ worms I set up about 4 weeks ago with 1kg or tiger worms.

    It’s been going really well and it’s starting to make casting but this morning I noticed a lot of white fluffy like mould/fungi the beetroot peelings. I mixes it up and added some shredded card board will this be ok or do I need to do anything else

  7. wish this had more answers. My compost has been moldy from day one. How can it not be? table scraps are wet, coffee grounds are wet. There were no instructions to tell you to dry it out first. do you lay it out in the sun to gather flies?? How do you dry it? I think I killed all the worms. I have never had any juice. it rained a lot, yet half my plants died from no water. Now I know to water it through all the holes too. My 4 ft tower is a failure I think. Could have got much better plants in the ground. Only have harvested some leaf lettuce & a few green beans so far, still waiting. the season is almost over. Usually I have all sorts of produce by now! Anyone have any advice how to make this work????

    • Bentley
    • August 7, 2015

    Apologies for the delays responding here, guys!

    There are various factors that lead to moldy scraps. It is almost always related to adding too much waste that hasn’t been optimized enough.

    Shaun – mixing it up with new bedding is a good approach. My guess is that will be all you need to do.

    Holly – you definitely do NOT need to dry out scraps, nor would I ever suggest you do so. The key is to chop them up really well and to mix with something absorbent (like shredded cardboard) and to feed in moderation. If the worms are healthy, and they don’t have too much waste to deal with, they should be able to keep the mold in check pretty easily.

    Burying wastes is also important.

    If anyone has further questions – please feel free to email me directly!

    • Linda
    • October 8, 2015

    I got mold in my bin will it hurt my wormes pleas let me now what I should do pleas thank you

    • Shaun
    • October 9, 2015

    Hi Bentley didn’t know where to put this but I was reading this and thought I would just add it here. Sorry! I moved my stackable bin to a wheelie bin flow through wormery I made and there isn’t much material in the bin. As its about 240ltr now. It’s coming to winter here on the uk and it will be out side so should I be adding a big bag of well rotted farmyard manure and mixing it in with the small layer of food/bedding so they have somewhere to move too if it gets too cold?

    • Evelyn Dahlberg
    • September 11, 2017

    Your web site would be most helpful as I am just starting the worm factory and have mold, and some fruit flies and worry about the health factor of having this indoors. Thanks

    • Bentley
    • January 12, 2018

    Late comments but for the benefits of others…

    Mold – will not harm the worms. But if it is getting out of hand it likely means you are over-feeding (normally worms keep it in check). From a health perspective, if anyone in your house has a mold allergy, you might be best to keep the system outside – especially if you are fairly inexperienced. Lots of ways to keep fungal growth to a minimum (and certainly avoid having fruiting bodies that are releasing spores) but sometimes it is better to err on the safe side.

    SHAUN – Any small system exposed to sub-freezing temperatures (without any sort of artificial heating) is a worm death-trap waiting to happen. It is amazingly easy to keep Red Worms alive in the cold (low-lying systems covered with lots of bedding materials) but smaller above-ground systems simply have no ability to ward off serious winter freezing. And even if the worms DO survive, the process itself slows to a stand still as temps drop. Sounds like you are talking about a somewhat bigger bin and you may be in a location where temps are more moderate than here in Canada, though, so you may do OK! When in doubt, try to keep a small insurance bin indoors as well.

    • Billy
    • May 12, 2018

    Thank you for the reassuring post. I have just started my worm bin and saw mold growing on an apple. It scared me half to death, but now I understand that it wont harm the worms.

    • Ome
    • April 24, 2019

    So we are supposed to mix the paper with the food?

    • Bentley
    • April 29, 2019

    Ome – mixing some bedding in with your feedings isn’t a bad idea, but a really easy way to make sure that happens is simply to always keep a thick layer of cover bedding up top. This can then gradually get mixed in with the food you are adding. The cover bedding is a good way to suppress mold growth somewhat as well.

    • Diana
    • September 7, 2019

    What do you mean by cover bedding. I have a bought ?WormCafe. Following instructions I put wetted cardboard wetted coir peat, worms, food, wet newspaper on top. It has a black lid but that’s a few inches above the worms. Today the wet newspaper and thin cardboard is all white mould. Should I remove the mouldy paper and/or mix the whole lot up
    I don’t understand all the worm farm lingo.

    • Bentley
    • October 8, 2019

    Hi Diana – that’s one of the challenges with stacking systems. Not a lot of room in the trays. Cover bedding is just something like shredded newsprint or shredded cardboard place over top of the main composting zone. With a tub system it is easy to make it nice and thick. Don’t stress about fungal growth – definitely part of the process – although if you are seeing a lot on food it could mean you are adding more food than the worms can keep up with.

    • Jamie
    • May 22, 2020

    I was starting a new layer and guess I put too much food in to entice the worms up. Woke up to a solid yellow bin. Man, that stuff is fast growing!

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