If you are grinding and brewing your own coffee beans, you might find yourself with a surplus of used-up coffee grounds after you’ve cleaned out your coffee maker. These grounds are far from useless, as they can be used as a fantastic fertilizer for garden and house plants.
The problem arises when these coffee grounds become moldy. Are they now useless, and could they damage the plants or even your own health?
In this article, we will be covering why your coffee grounds may become moldy in the first place, whether or not they can still be used, the different types of molds that you can expect to find, and a whole bunch more, so keep reading.
Mold will grow on coffee grounds if they are not completely dry before being stored. In most cases, what you are seeing as mold are harmless fungi that are in fact, beneficial to the plants you plan to spread the grounds on. Care should still be taken when handling mold strains you are uncertain of.
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Why do used coffee grounds go moldy?
Once you’ve got everything you can out of your ground-up coffee beans, you may simply put them to one side, ready to either throw out at a later date or to use as fertilizer for your plants.
The reason that mold will begin to grow on these grounds is that you may well have given mold the perfect environment in which to thrive.
Mold needs only a few things in order to grow, moisture, nutrients, an ambient temperature, and a lack of natural sunlight.
The grounds themselves provide plenty of nutrients, and if not fully dried, the moisture level will be high enough to support the mold as it grows. They are also often left in what’s called the “knockoff box”, (a container for the used-up coffee grounds). If this container is left for even 24-48 hours in an ambient temperature between 60-80 Fahrenheit (the temperature ranges most American thermostats are set to), mold will certainly begin to grow.
Is the mold growing on your coffee grounds dangerous?
As a general rule, the “mold”, that you see forming on your used coffee grounds is not dangerous, and can still be used on your plants as a fertilizer.
The reason for this is that most often, what you see as “mold” may in fact not be at all. What you are most likely seeing, is a type of fungus called Trichoderma. This can come in color variations of blue and green and is in fact, quite beneficial to the plants you will be spreading them on.
A word of caution*
Despite the message above, you should always be very cautious of anything that resembles mold. Whilst it may be Trichoderma growing in your coffee grounds, it could also be any one of a number of mold strains that can be hazardous to human health.
For this reason, if you are in any way unsure of what type of mold or fungus you are dealing with, it is best to wear a breathing mask, rubber gloves, and eye protection if you plan on removing it or using them in your garden.
Can you use moldy coffee grounds?
Absolutely, coffee grounds are an excellent source of nitrogen, calcium, potassium, iron, phosphorus, magnesium, and chromium. Even if the grounds have begun to form a layer of Trichoderma, this will not harm the plants in any way. In fact, this fungi is a completely natural part of the decomposition process.
The obvious exception to this rule would be if you were referring to “using” moldy coffee grounds in the sense of making another brew with them. In this case, the answer would be a resounding no.
Consuming moldy coffee grounds could potentially cause gastrointestinal discomfort, and it certainly would affect the taste considerably, so any mold (or fungi), found growing on your old coffee grounds means it’s time to either throw them away or sprinkle them on your plants.
Colour variations in coffee ground molds:
Orange
Orange mold found growing on old coffee grounds is most likely the sitophila strain. This is also commonly found growing on, bread, vegetation, and other food items.
This form of mold can be hazardous to human health in those sensitive to its spores.
Yellow
Aspergillus strains can sometimes appear with a yellow tone, and if it is indeed this strain, it can cause several symptoms in both humans and animals, including breathing difficulties, headaches, gastrointestinal discomfort (if ingested), and skin irritation on contact.
Green
There are three strains of mold that can all present with a green color, these are Cladosporium, Aspergillus, and Penicillium, which are some of the most common molds found within the home.
There are many variations of these molds, and despite some of them causing little to no Ill effect, if disturbed or threatened, there is the possibility of these strains producing mycotoxins, a metabolite that can cause both Illness and potentially risky even death in those particularly sensitive to them.
How do you remove mold from coffee grounds?
In the majority of cases, there is no need to remove any mold found growing on coffee grounds before spreading them onto plants, as is it more likely that what you are seeing is harmless Trichoderma.
Even if mold is seen on the grounds, as long as you are wearing a mask, it would be safe to spread these on outdoor plants. If being used indoors, it would not be recommended to use them as fertilizer, as any mold that was present could spread spores throughout the property.
How to prevent mold growing on old coffee grounds
If you are concerned about mold growing on the coffee grounds that you plan to use for fertilizer, you can prevent this easily by keeping them as dry as possible.
You can dry them out under direct sunlight, as the sun’s UV rays and heat will kill any mold spores that may land on the grounds whilst dehydrating them to the point where they lack the moisture mold requires.
You can also mix them with dry leaves or grasses to help draw out any excess moisture, as this will have much the same effect.
Conclusion
Coffee grounds can get moldy, but in most cases what you are seeing is harmless fungi rather than mold as such. You should still treat this situation with caution, as it is often hard to tell what you are dealing with at first glance.
If you are remotely uncertain of what you are dealing with, it’s important to remember to wear protective equipment when attempting to use or remove it.
Sources:
https://en.wikipedia.org/wiki/Trichoderma
https://www.gardeningknowhow.com/composting/ingredients/coffee-grounds-gardening.htm
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/neurospora-sitophila
