My cannabis has mould… I need a solution!

After months of hard work growing your plants, the last step before enjoying your precious cannabis is the curing, a process that requires patience and precision to avoid problems such as the development of mould on the buds. Humidity, temperature control, and proper ventilation are essential factors to prevent the growth of these pathogens, which can completely ruin your harvest, affecting the potency, flavour, and even the safety of the final product. Therefore, showing you how to deal with these annoying guests is not only important but also absolutely essential!

Mould spores have been found everywhere, from the desert to Antarctica and even in the International Space Station. But the fact that spores are found everywhere doesn’t mean that all types of mould grow everywhere. Okay, it’s impossible to keep mould spores from entering the curing space, but just like other living things, moulds have requirements that dictate where they will grow. And this is where we as growers can protect our cannabis while curing it.

What types of mould can attack cannabis during the curing process?

The fungi prone to attack cannabis during the curing process vary in their features and level of aggressiveness. Among the most common is Botrytis cinerea, which is known as “bud rot” or “grey mould”. This fungus is especially destructive and is characterised by a greyish layer that quickly invades the flowers, causing decomposition in plant tissue and releasing spores into the air that can quickly contaminate other nearby buds.

When not controlled, this mould spreads from the inside to the outside of the buds, so in the early stages it is almost impossible to detect with the naked eye. Botrytis thrives in environments with high humidity and low ventilation, so it is essential to keep these variables at safe levels to prevent its occurrence.

Another type of fungus that can affect buds is Aspergillus, a mould that poses a serious health risk because its spores can cause lung infections and allergic reactions in sensitive people (especially in those who inhale it through combustion or vaporisation, since the Aspergillus spores can withstand high temperatures). This mould can appear as dark green or even black spots; and, just like Botrytis, it develops rapidly in warm and humid environments.

Another fungus is Penicillium. It is not as destructive as the previous ones but can still affect the quality of cannabis by degrading the taste and aroma of the buds and creating a pretty unpleasant texture that’s both spongy and moist. It usually manifests as a white or bluish powder on the surface of cannabis (in fact, it is used in the food industry for the production of blue cheese). Although it is less health threatening, it still needs to be removed completely before the product can be considered suitable for consumption.

How to identify mould in cannabis flowers?

Mould in marijuana is a threat that requires rapid and accurate identification to prevent its spread. In addition to the patterns of colour and appearance that we’ve discussed (and which are relatively easy to detect if you know what to look for), mould also affects the aroma and texture of the buds.
A common indicator of its presence is a humid or “mouldy” smell reminiscent of wet fabric or decaying vegetation – this is a clear sign, given that properly cured cannabis should have the earthy, fruity, or floral aroma that we so eagerly look for in each strain. And the texture can also be affected: mouldy buds may feel wet or sticky to the touch, rather than having the slightly crunchy texture that characterises properly cured cannabis flowers.

What causes cannabis buds to go mouldy?

The development of mould is a response to a combination of environmental factors and inappropriate practices during the curing process. One of the main culprits is excess moisture, as fungi need this to proliferate; and cannabis buds, being dense and compact, can easily retain water inside them if they haven’t been dried properly. If buds are stored or cured in an environment with humidity levels above 65%, the risk of mould significantly increases.

Lack of ventilation is another critical factor that contributes to the development of mould. Without proper airflow, moisture gets trapped inside the curing containers, creating a favourable environment for the development of fungi. This is especially important in the early stages of curing, when the buds still contain residual water that needs to evaporate gradually.

But heat also plays a key role, as temperatures above 21°C can accelerate the growth of fungi on the buds. Therefore, it is crucial for the curing spaces to remain cool.

How to prevent mould growth during the curing of cannabis

Therefore, to prevent the appearance of mould, it is essential to create and maintain a controlled environment in terms of humidity, ventilation, and temperature. And one of the first preventive measures is to ensure that the buds are dried well before the curing starts – otherwise they will retain internal moisture levels that are difficult to control.

The complete drying of the buds can take between 7 and 14 days, depending on the environmental conditions and the density of the flowers. During this time, a temperature of about 18 to 20°C and a relative humidity of 45 to 55% should be maintained to achieve gradual and constant dehydration of the flowers.

Once the curing starts, the most recommended option is the use of glass jars, as it allows for precise control of moisture levels and prevents static electricity build-up, which could attract dust particles, spores, and other contaminants. Jars should be filled to 75% of their capacity to allow adequate space for air circulation, which will help balance the internal moisture of the buds.

It is important to open the jars regularly (a process known as “burping cannabis”) to prevent moisture build-up inside the buds and allow dry air to enter the jars. During the first two weeks of curing, it is recommended to do this process two to four times a day, and then reduce the frequency as the percentage of relative humidity decreases.

This method requires daily attention and can create a large margin for error, given that it’s all done manually. If the buds are moist, it means that the humidity is above 70% – in that case, you must take the buds out and let them dry for about two hours before placing them back into the jars so they can cure properly. Also, if you live in an area with high humidity levels, burping the containers could be letting in air with a relative humidity that’s too high, which can cause your buds to become moist again.

Can I save my buds if they’ve gone mouldy?

While it is possible in the early stages to detect and separate buds that have been affected by mould, in most cases the contaminated cannabis cannot be completely saved as it is almost impossible to remove all the mould without compromising the product quality and safety.

In some cases, you can try to separate the affected part from the rest of the crop, as long as a thorough inspection is carried out and steps are taken to prevent further mould growth. Having said that, it’s important to remember that using cannabis with mould can have negative health consequences, especially for people with respiratory problems or weakened immune systems.

So, in a nutshell, protecting your buds from mould requires a lot of effort. But now that you have your delicious colas brimming with trichomes and free from rot, are you safe? Well, not yet, because there’s still one more step to take: storing them properly for the long term.

The best way to store buds to avoid mould

Proper storage is key to maintaining the quality and freshness of cannabis in the long run. Buds should be stored in airtight jars so that air and moisture from the outside environment cannot get in. Keeping the jars in a cool, dark place helps prevent degradation of cannabinoids and terpenes, which in turn helps preserve the potency and aroma of cannabis. Direct light and heat also accelerate the decomposition of active compounds and increase the risk of mould, so they should be avoided at all costs.

The use of humidity control sachets can also be beneficial during storage, as this helps maintain a stable humidity level in the safe range of 55% – 62%, which prevents cannabis from overdrying or developing mould due to excess moisture.

Preventing and avoiding mould in cannabis during curing and storage is essential to preserve its quality and safety, and it’s the final step before you can enjoy the fruits of all your hard work. No matter how beautiful, fragrant, and sticky your buds are when they’re done in the drying room – subjecting them to a quality curing process will always improve the product and prevent mould from ruining your dreams.

The only thing worse than failing outright when trying to grow cannabis is to have spectacular success and then see your entire crop ruined by mould. So, take your time, follow these tips, and enjoy!