March 2009

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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