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How Cannabis Molds, and Why That’s a Bad Thing

Cannabis may be a wonder plant but it’s also prone to its own unique challenges. One of those challenges is the omnipresent threat of mold. 

Mold is a type of a fungus that thrives in warm, humid climates — or in other words, the type of environments that are perfect for growing cannabis. It can thrive on cannabis plants that haven’t been properly grown, cured or stored.

Consuming moldy marijuana can make you ill, particularly if you are a medical cannabis patient and have a compromised immune system. Here’s how you can learn to spot mold and what you can do for your plants if that’s the case.

The mold problem

Inspecting moldy cannabis. (Shutterstock)

The presence of mold in cannabis is a lot more common than you might hope. In 2017 researchers from the University of California, Davis Medical Center teamed up with Steep Hill Labs to test 20 cannabis samples from dispensaries across the state.

Dr. Land’s team found evidence of pathogenic bacteria and fungi including Aspergillus mold, Mucor and Botryotini fungus. 

“We were a little bit startled that 90% of those samples had something on them, some DNA of some pathogen,” Steep Hill Labs’ Dr. Donald Land PhD said in an interview at the time.

Conditions that lead to mold

Mold loves to thrive in warm, humid conditions. It also loves the damp areas that can occur when cannabis is stored in the dark, isn’t properly dried, or ventilated as a lack of air circulation can help spores thrive. 

How to identify moldy cannabis

The best way to spot mold is to make yourself familiar with the most common types of mold that affect cannabis plants. These include:

White powdery mildew

This type of mold looks like a white powder that often dusts the leaves of cannabis plants. It can look like a glimmering of trichomes to the untrained eye, which is why you’ll want to get your hands on a magnifying glass or microscope to confirm. 

To the untrained eye, some mold can appear similar to glimmering trichomes.

Botrytis (gray or brown mold)

Another common type of mold expresses itself as a discoloration of the cannabis plant, typically in the cola. This type of mold is usually grey or brown in color. If you notice a healthy plant with a drooping cola this is a good indication there’s mold present. 

Web-like fungi

You may also encounter a cobweb like structure forming on your plants, generally brown, white or grey in color. 

Mold seen on a marijuana flower. (Shutterstock)

Slimy mold

Some mold can also take on a slimy appearance that makes the plant look rotted. It may also create dark spots, generally brown or green, in your plant.

Other signs include yellowing/drooping leaves or discolored roots. You can also spot moldy marijuana by smelling it as mold can smell like a harsh, ammonia-like odor. Mold can also be spotted with a UV-A blacklight.

What to do when you spot mold

It may be hard to hear but the best thing to do when you spot mold is to throw out your plants. You’ll also want to make sure the infection hasn’t spread to nearby plants. 

You can always opt to cut the moldy section away and hope for the best, but given the potential health risks associated with mold it’s best to chalk it up to a loss and move on.

Health risks of mold

Some mold can make the plant look rotted. (Shutterstock)

Coming into contact with mold can have a wide range of effects on your health. These include:

  • Cough or asthma-like symptoms
  • Fatigue
  • Weakness
  • Chest pain
  • Brain fog/memory difficulties
  • Dizziness
  • Sinus and lung infections in people with compromised immune systems 

Preventing mold

The best way to deal with mold is to prevent it. This can be done by ensuring you have:

  • A high quality air filtration system set in place
  • Dehumidifiers 
  • Fans
  • A temperature/humidity monitor to maintain even temperatures
  • Spacing plants out to avoid crowding
  • Watering appropriately 
  • Pruning plants as necessary (defoliation)
  • Ensuring cannabis is fully cured before sealing it in airtight jars
Always ensure your cannabis is dried and cured before storing it in a jar. (Shutterstock)

You can learn more about how to properly store cannabis here.

Testing for mold

Testing for mold varies depending on your location. Some states including Maine, Nevada, and Colorado rely on yeast and mold count testing (TYMC). This process determines a maximum yeast and mold count threshold that must be met for the sample to pass. California also tests for species-specific strains of Aspergillus mold by running cannabis DNA samples through PCR (polymerase chain reaction testing). 

Cannabis horticulture authority Ed Rosenthal recommends that labs implement DNA based microbial testing methods such as PathogenDx, qPCR or DNA microbiome sequencing to “…provide the necessary scientific rigor to ensure compliance with pharmaceutical standards.”

In other words, if you grow your cannabis in a well-ventilated environment and cure it well, there’s little for you to worry about. Want to be 100% safe? If you’re not growing your own, buy from a licensed dispensary and consult your strain’s certificate of analysis to view its microbial testing. 


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