Growers Network was created as a resource for adults in the cannabis industry.

Please verify your age to enter.

Contaminated cannabis? Mold & Fungi solutions?

I’m sure many have seen this emerging story - is mold and fungi the next ‘pesticide’ scare? There is discussion of irradiation for legal supply here in Canada. What are some experiences from other areas?


Interesting - I have a mold allergy so that’s something that while not life-threatening, can make me miserable. Sounds like folks struggling with ailments should stick to concentrates.


Good point - have to know the provenance of what was used and have access to good testing. I know that @tasiakelle works on this aspect for her concentrates and extractions, since a lot of her clients are cancer patients.

It can be a concern - fungicides have been a big issue in the legalized medical market in Canada


I think this is a multifold problem:

  • Pesticides are regulated by the FDA. Since marijuana is not federally legal, most conventional pesticides cannot legally be used on cannabis. This can result in more problems with pest management, as it takes much more dedication and energy to control without conventional pesticides. People looking for shortcuts can make really rash choices.
  • Cannabis is usually grown indoors in more humid conditions, which favors mold. Mold and fungus generally prefer to avoid direct sunlight and like things wet. I’m not saying that outdoors is necessarily better (because then you have insect pests), but these problems are particular to indoor growing.
  • The process of making concentrates should (hopefully) wipe out fungicides. Solvents and high temperature variations should generally destroy those kinds of molecules. The only exception is something like RSO – which may concentrate the fungicides. Extraction methods matter!

The story in Canada just gets better… or worse…


They hid the banned pesticide above the ceiling tiles? This is like something out of a spy novel.


Or a ten year old shoving things under the rug.


Looks like this story is continuing to make the news:


It’s a concerning state of affairs. I am seeing increasingly that the big producers are looking towards irradiation. That would be a shame if that became the norm.

What are other solutions? I have seen suggestions of washing buds in a Hydrogen Peroxide solution or UV treatment. The reality is rapidly emerging that mold spores (and it appears now pesticides) are not so easy to keep out of your crop. Even at the highest level of capitalistic production.


How does the tobacco industry deal with these sorts of issues? I imagine they have similar concerns and ways of dealing with the problem.


The best cure is prevention of mold development. Controlling the presence of mold spores and their source of food is impractical/impossible. But, with proper wall materiel, (wiping them with bleach rags after each cycle), positive room pressure and humidity control the development of a mold problem can be virtually eliminated.

In addition, our recycling nutrient delivery systems, which pull the nutrients out of the room and back to the tank have reportedly eliminated this problem for some of the most challenging cultivation environments here in Southern California during recent hot/humid summers.

If you’re growing with ebb and flow or dwc with all that nutrient under the lights, well you may have issues in more challenging environmental conditions.

Here’s a good primer on mold and its prevention:


Tobacco has a couple things going for it.

  • Never been prohibited. It’s been grown outdoors for a very long time.
  • It evolved in the US South, a particularly hot and humid place. While fungus likes humidity, it doesn’t like heat.
  • Generally considered a weedy, ruderal plant in the first place. It doesn’t have a lot of natural pests.
  • Most tobacco farmers (at least in the early days) followed a slash-and-burn method of farming, where they simply moved their farm as soil quality diminished. That’s why a lot of states in the South are stretched horizontally.

I have heard from mold pro’s here that peroxide is a better solution, especially with more fibrous walls, also it is a cleaner and less destructive material when it gets ‘washed down the drain’. Peroxide breaks down better and has less of an environmental impact - which i think will become increasingly important as the scales of operations grow and our water tables become increasingly contaminated.


Bleach is less stable and therefore can only kill surface mold on porous surfaces as it degrades rapidly. Then again, it is less stable than H202 precisely because it is more oxidative, which is why water treatment and laundry get better results with chlorine.

While there are some peroxide advantages in other respects, we are talking about killing mold spores. Btw, we prefer stainless cladding for wall materiel anyway


Well yes. Stainless steel is awesome. But as part of your capex it will bite into your bottom line. Especially as the grows get larger.


Still have to point out that bleach in the water supply will come back to bite you in the ass. Although if you are on a city service or such i guess it’s not your problem. Personally I am inclined to think that we owe a certain responsibility to those that might want to use the water after us…

1 Like

We don’t find that to be a problem. It’s cost effective enough that most restaurants have it. Certainly not as much a problem as having to replace mold-laden walls or if you have a bloom 18 months later because it wasn’t cleaned effectively.

1 Like

Out of curiosity - do you lab test for mold and what are the results if so? Spores on plants may not show in visible mold colonies - and yet still have a health impact on those susceptible. As we are seeing from recent studies - even if you think you are mold free - you probably are not.

1 Like

Not as much as a highly concentrated/unstable peroxide! The carbon filter in your reverse osmosis system will neutralize the chlorine. That’s the reason it is there.

Btw, most municipalities have moved/are moving to chloramine disinfectant (i.e., chlorine mixed with ammonia) due to the carcinogenic byproduct called trihalomethane. Chloramine is about 5 times stable than chlorine and generally requires active carbon instead of the pressed carbon block (for the same size filter) to remove it.

If your source water is 50 ppm or more, you are either using reverse osmosis filtration or you need to be as the compounds in the water will compete with and change your nutrient mix. Also, stay away from pumps that have disparate metals exposed to the water as it will create a galvanic reaction, which will also affect your nutes.