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How To Prevent Mold On Cannabis Plants

Created by
JoshuaHolt JoshuaHolt
Added 29 September 2020

White or Grey Web Can Develop On Moldy Cannabis Plants

As a grower, discovering mold on your cannabis plants is one of the worst feelings ever. Once an infestation hits your garden it can spread very quickly, destroying everything you have worked for. The best way to stop this from happening is through prevention. This article has got some useful tips so you never have to experience mold consuming your beautiful crop.

Mold on Cannabis Plants

The problem with mold is that it can arrive at any moment. Mold spores are always in the air and will reproduce when the conditions are in their favour. It can also be quite hard to detect in it's early stages, meaning it could be too late to do much about it by the time you realise there is a problem. 

Preventing mold from entering your grow space mainly comes down to keeping the correct environmental conditions. Whether you are growing indoors or outdoors, there are a few types of mold which can affect cannabis plants. The 2 most common types are powdery mildew and botrytis, also know as bud rot.

Other molds sometimes found in cannabis are fusarium, sooty mold, and mold in soil. Let's take a quick look at some of these to know what we're dealing with.

Powdery Mildew - Powdery mildew looks like a thin layer of white powder which initially grows on foliage but then spreads over the whole plant as it gets worse. Leaves start to turn yellow and brown, withering until they die.

Plants badly infected with powdery mildew develop black spots, meaning the reproduction and transfer of spores is accelerated. Powdery mildew eventually creates a coating over leaves that prevents cannabis plants from photosynthesising properly, cutting off one of its main resources.

Powdery Mildew on a Cannabis Plant

Bud Rot: Botrytis - Bud rot grows first around the stems and branches of a cannabis plant. This makes it quite hard to detect unless you are inspecting your plants closely every day. Botrytis is more of a risk during flowering but can affect cannabis plants at any stage during their life.

Bud rot usually appears as a fluffy, white or grey web but also grows in shades of blue or green. It consumes plant matter from the inside out, leaving behind a broken down mush. Botrytis normally affects top buds first. On the outside of the plant, it can be noticed through sudden changes in the colour of the small leaves, which tend to turn yellow and dry out.

Botrytis on a Cannabis Plant

Mold Type




Powdery Mildew

White powder/floury/dust


    • Pruning
    • Neem oil


White, grey, or coloured web


    • Low Humidity
    • UV Light


Brown roots with white fluff


    • Clean/disinfect area
    • Trichoderma supplements

Mold in Soil

White/grey fluff

Top Soil

    • Water less
    • Amendments to aerate soil such as perlite

Sooty Mold

Black soot


    • Check for pests
    • Proper ventilation

Why do Cannabis Plants go Moldy?

Mold can thrive in most organic matter with the right conditions, so it is no surprise cannabis can also be a victim. Mold usually appears because there is too much moisture in the air. In other words, the high humidity is probably the likely cause.

Cannabis plants are prone to mold in the flowering stage because of the increased levels of transpiration and enzyme activity. Air gets trapped in pockets around the bud, creating the perfect environment for fungus to develop. 

The curing period after harvesting is another time where you need to pay special attention to the environmental conditions to avoid buds developing mold problems. High humidity in curing jars creates a risk, so it is sensible to dry and cure properly and check the moisture content regularly with a hygrometer.

Drying and Curing Conditions Need To Be Accurate To Avoid Mold

How to Prevent Mold?

You might not have ever had issues with mold but we don't want you to have to learn the hard way. Preventing mold makes the difference between a successful or failed harvest so be strict with your practice and you will hopefully never have to deal with this annoying problem.

It sounds simple but the best way to stop mold from invading your garden is by providing ventilation and fresh air to your plants. Mold thrives in humid conditions so there is less chance of it settling when the air is constantly moving.

Indoor tips to prevent mold

  • Fresh Air - Setting up the correct intake and exhaust fans replenishes the air in a grow space multiple times over an hour. Usually measured in CFM (cubic feet per metre), fans should be filling the grow space with fresh air every 5 minutes, at the most.
  • Ventilation - Having a few oscillating fans around the grow room prevents air from becoming stagnant. Fans help to circulate the air, reducing the possibility of mold spores landing on your plants. 
  • Low Humidity - Using a hygrometer to measure humidity gives you an indication of whether your cannabis might be at risk of a fungal infestation. Consider installing a dehumidifier if humidity levels are reaching over 55 - 60%.
  • Temperature - Grow room temperatures should sit between 20 - 24°C. Mold thrives in colder conditions, so make sure temperatures do not drop too much, especially during the night.
  • Keep Grow Room Clean - Regularly clean up dead leaves and other organic waste. Dying plant matter is another perfect way for fungus to establish itself in your garden.
  • Pruning - Trimming leaves off your cannabis plants to improve airflow and reduce pockets of humidity can be a beneficial way to prevent mold.

Prevent Mold By Setting Up Your Environment Correctly

Note: Open wounds are not good if you are trying to prevent mold, so take care when you are performing high stress techniques such as defoliation or topping. Keep the process as hygienic as possible. The same goes for the outdoors.

Outdoor tips to prevent mold

  • Grow in a Greenhouse - A greenhouse provides a protected environment to work in, giving you more control over the conditions. Fans and heaters can be set up.
  • Choose Resistant Strains - There are plenty of strains on the market nowadays that are highly resistant to pests and mold. Choosing the right strains for the outdoors simply takes a bit of research.
  • Plant in Pots - Cultivating in containers gives you the possibility to move plants around to give them space or more exposure to light. Plants in pots can also be moved to cover if bad weather arrives.
  • Sunlight - Placing plants in the sun for as many hours as possible helps to keep temperatures up. Remember, fungus loves cold, humid conditions.
  • Cover - If you are not able to move your plants, try to cover them in the event of a storm. Heavy rains often bring sudden drops in temperature. Combined with the moisture it can be a problem.
  • Shake Water After Rains - You can shake plants that get drenched from a heavy rain to remove droplets sitting on leaves and buds.
  • Treat Soil - Beneficial trichoderma and fungi can be added to your soil mixture to improve defences, preventing soil or root molds such as fusarium from developing.

Mold Can Grow on Top Soil From Overwatering or Poor Drainage

What to do if your Cannabis Plant goes Moldy?

There is no real cure for cannabis than has gone completely moldy. It depends a lot of which stage of growth you are at. Symptoms can be treated but any fungicides applied to plants will likely affect production, taste and smell, especially if used in the flowering stage.

If buds have started growing mold, they aren't really worth saving and should not be smoked for health reasons.

Most of the time you just have to cut your losses, accept, and learn for your next grow.

Firstly, always ensure you protect yourself when dealing with moldy plants or buds. Wear gloves and a face mask to avoid spores spreading. Breathing mold spores is also potentially dangerous for one's health.

Always Wear a Mask and Gloves When Dealing With Moldy Cannabis Plants

It is recommended that you remove any infected plants from your grow space, being careful not to spread the spores. Check plants thoroughly for other possible areas of infestation. If the problem is severe, consider discarding your plants.

Depending on the type of mold, you can try to treat plants using fungicides but it is recommended you keep them in a separate room away from unaffected plants. Do not tend to moldy plants and then go back into your healthy grow space. You can try to remove and discard the affected areas to stop it spreading, however there is still a risk of the infection surfacing again. 

Proper ventilation can be set up in greenhouses. Video by AutoCrazy from GrowDiaries.


Look after your cannabis plants with these precautions and you should be able to stop mold from finding a home in your grow room. Take care when treating fungal infestations but most importantly, keep the humidity down and give plants lots of fresh air to stop molds from even having a chance.

External References

Biogeography In The Air: Fungal Diversity Over Land And Oceans. Biogeosciences. - Fröhlich-Nowoisky, Janine & Burrows, Susannah & Xie, Z & Engling, Guenter & Solomon, Paul & Fraser, M & Mayol-Bracero, Olga & Artaxo, Paulo & Begerow, Dominik & Conrad, R & Andreae, Meinrat & Despres, Viviane & Pöschl, Ulrich. (2012)

This article was updated September 2020



Happy I seen this an my email. Was nice just to get a notice... Guess not everyone liked that LOL... Thanks for the read and sharing. Happy growing all!


Top stuff unfortunately had mold due to high humidity on my last harvest and lost about a third of my final weight, nothin worse than that sinking feeling!


@SinbadtheSailor, sorry to hear that! losing bud to mold sucks big time. Keep that humidity down next time round ;)


Thanks for this article guys, PM sure is a problem!

I grow outdoors and I deal with this just about every grow. I've tried most of the home remedies (h202, bicarb, milk, soap etc) and although they work in veg, just about all of those do damage in flower.

Recently I discovered something that could possibly be a solution, an organic fungicide that is ascorbic acid based! I was worried about it damaging my buds so I tried it on a few larfy ones and there was no shrivelling/drying of pistils. After looking it up, I discovered an article on Ed Rosenthal's site that talks about finishing ingredients:

He mentions ascorbic acid (vitamin c) as being a great finishing ingredient for cannabis! So, this probably needs more verification, but this could be an answer for a lot of folks. Note - when you use the stuff, it doesn't kill the appearance of the mold, but it definitely stops the spreading and forming on any new growth. I applied it once a week. In future I might proactively spray this at pre-flower to prevent rather than cure.

Since this article is focused more on prevention - I'd like to add that I'm trying 2 new preventative methods on my next grows. Trichoderma (a beneficial fungus) and silicon. I'll be using these on my up and coming grows and will report back.

As always, if I've ever come across any PM during a grow, I bud wash with h202, then bicarb/lemon, then 2x water baths before hanging to dry.

Here's a link to the products (South Africa)

Hope this helps a fellow grower out there!


@JHGD420, hope there's something useful there for someone ;)


@Epokwan, Interesting info, thank you for sharing with the community!


Haha Thank you @growdiaries I just got hit with bud rot for first time and didn’t know I had it somehow I transferred it to another tent and lost some serious buds thankfully not everything. Great information thank you again :+1::seedling:


@Lionsshare, Glad to head you managed to save some of it! Good luck for your next grow :)


I had a bud hang appx 200lbs of harvest in a 12’x12’ garden shed with no ventilation... lost at least 2/3 of their crop due to Mold almost overnight! Be wary my friends, nothing worse than spreading it within your harvest!


@Mykooh, That's a big loss, sorry to hear that! Thank you for sharing and hope you never have to deal with mold again :)


Most of the time you don't see the mold! If you really want to make sure that you don't lose your hole plant than you have to check the density of your buds. When you feel a soft spot at a bud you should look closer. Most of the growers say "don't touch your plants" But sometimes with gloves or washed hands it is okay and will safe your harvest.


Good luck everyone!! Take heed and have a bountiful harvest