Identifying and Preventing Mold in Harvested Buds

By Chris Bond
Published: October 11, 2022 | Last updated: October 11, 2022 07:49:56
Key Takeaways

If you find mold on your post-harvest buds, you can consider that batch and all of your work a write-off. To prevent this, follow Chris Bond’s advice on identifying and preventing mold in the growroom.

You’ve nurtured your prized plants from a seedling. You carefully harvested their bounty, packed it away and are ready to enjoy it. But what’s this fuzzy stuff all over? Your harvested buds have mold! Mold can undo all of your care and hard work through the harvest phase if you aren’t careful in the pre- and post-harvest phases, especially storage.

The two weeks leading up to harvest and the first few weeks afterwards are actually the most opportune time for mold to set up shop on your flowers. Unlike a nutrient deficiency or even a pest outbreak during the growing phases, once mold is on your buds, it ruins them, and the damage can’t be undone.

Smoking or otherwise consuming moldy buds can cause great harm. At a minimum, the inhalation of mold can cause allergic reactions; at worst, protracted illness can result from the inhalation of mold spores. To avoid this dangerous situation, it is very important to monitor the conditions leading up to and after harvest. Once you understand the conditions that favor mold and how to identify it, you will be in a better position to prevent it from occurring in the first place.


How Mold Forms

Mold spores exist nearly everywhere in the environment. If the conditions aren’t conducive for its growth and reproduction, then typically nothing happens, and no one is any the wiser. It is, however, extremely difficult to avoid these spores. They are in the air, they get tracked into growrooms inadvertently, and they are even already present sometimes in your grow media or on your plants.

Living, healthy plants, have natural defenses against these mold spores from taking root. That, coupled with air filtration will prevent most growing areas from having any serious issues with mold, but it is easy for the situation to turn. If the humidity levels get too high, mold will often follow, especially when there is another catalyst such as high temperatures (above 80°F/27°C) and decaying plant matter.

Large and dense buds are susceptible as are plants that are densely foliated. The more biomass of your cannabis plant, the more moisture it is releasing back into the air. Even if your growroom has good dehumidification, the air being released by your plants can still get trapped among the leaves and buds in plants that are too dense and hang around a bit longer before being extracted. Once there are these pockets of stagnant air, coupled with high humidity, mold is ready to set up shop.

grower handling ready-to-harvest cannabis flower with gloves


Identifying Common Molds of Cannabis

Unlike mold that can develop on roots and not be seen until it is too late, mold on buds is usually easy to spot. It may be any of several different colors depending on the type of mold present. Molds can be white, yellow, brown, gray, black, blue, green, even pink. It often appears fuzzy as well. The most common forms of mold that can develop onto your buds are Botrytis, powdery mildew, Aspergillus, Penicillium, and Rhizopus. Botrytis bud rot is one of the most common molds that will affect your buds.

Botrytis — Sometimes referred to as grey mold, Botrytis can affect more than just buds, it also affects leaves, stems, and roots too. Once it sets in, it moves quickly. On buds it will generally appear on the middle or sides, but it may have started in the middle of the bud. It could present itself as white, grey, or even have a bluish tint. These buds should be cut out immediately and destroyed. There is no salvaging them and trying to keep parts that don’t appear to have any damage could still have spores that will be dangerous to consume. This should happen as soon as it is discovered so any other plants not yet affected still have a chance to reach harvest. If discovered after harvest, then all buds in that batch should be considered a loss.

Powdery Mildew — Powdery mildew, sometimes called white powdery mildew, is another very common mold problem of cannabis leaves, stems, and buds. It often starts on older leaves first and is easily identified because it looks as though flour was sprinkled on leaves or buds. It forms in hot, humid conditions amongst stagnant air. Aspergillus, Penicillium, and Rhizopus can appear at almost any time and are among the most resilient mold pathogens.

Aspergillus, Penicillium, and Rhizopus — Aspergillus is a dark green to black mold. This can affect both greenhouse-grown and field-grown cannabis. Penicillium is primarily a concern for growroom- or greenhouse-produced buds. It is a lighter green to blue mold. It is also hazardous to animals if ingested. Rhizopus is a darker, grey to black mold. All three of these can cause infection of the lungs if ingested and should be destroyed immediately.

Identifying the specific type of mold on your buds is not as important as getting the affected ones removed and correcting the situation that caused a hospitable environment for mold to begin with. Any mold on your buds will result in a loss and they should all be considered dangerous to consume. Effort should be made to ensure mold will not be a problem for future crops as opposed to trying to mitigate it once it has been discovered.


Preventing Mold Pre-Harvest

The best way to prevent mold on harvested buds is first to make sure there is no mold present at the time of harvest and the weeks leading up to it. Several steps should be taken to ensure a mold outbreak is not right around the corner.


Temperature — Molds most often appear when temperatures are too hot (over 80°F/27°C), or too cool (below 68°F/ 20°C). A good temperature range to maintain is between 70-75°F (21-24°C).

Humidity — The humidity in the grow space should consistently be between 50 and 60 percent. This is high enough to keep your buds from drying out and low enough to prevent mold from developing.

Air Flow — Spores have a hard time landing and staying on buds where there is constant air flow. Fans should be placed around your plants so there is air moving under the foliage and over top of it. Ventilation working in tandem with fans is the best defense against mold.

Pruning — Air needs to be able to circulate around your plants so it may be necessary to do some selective pruning to achieve optimal circulation. This could include removing some leaves, or whole branches. Particular attention should be paid to the lower portions of plants where foliage tends to be the thickest.

Preventing Mold on Harvested Buds

Keeping mold off your harvested buds follows the same principles as keeping mold off your growing plants and intact flowers. Optimal temperature, humidity, and air circulation are critical. All through the drying, curing, and storage phases, the environment will need to be controlled to keep mold from appearing and ruining your hard-won harvest.

Just-harvested cannabis buds have a high moisture content. Ideal drying conditions include keeping the temperature to between 64-68°F (18-20°C) and capping humidity at 50 to 60 percent. They must also be in a dark place with good air circulation. Air conditioners can help to create these conditions if your drying area is too hot or too humid. Even with AC, fans should still be in the drying area to keep mold from settling in. Plan on this process taking about two weeks.

After drying the buds, you will want to properly cure them to avoid bud rot appearing. First, make sure your buds have been trimmed of all leaves and unnecessary plant material. Then, place them in sterile, airtight jars for at least three weeks. They should not be filled to more than three-quarters of their total capacity. As your buds cure, they will continue to release moisture. Being sealed up creates a favorable environment for mold to develop. To avoid this, it is important to burp your jars several times a day during the first week of curing. Starting at week two of curing, this can be reduced to one to two times daily. Additionally, humidity packs can be placed in the curing jars to help maintain ideal humidity. Once your buds have been cured, they can continue to be stored in the same jars with humidity packs and periodically opened to keep some amount of fresh air exchange.

Another great way to prevent mold in your harvested buds is to start with a cultivar that has known resistance to molds and mildews. While there are no strains of cannabis that are 100 percent resistant all of the time, there are a few that have demonstrated time and again they are less likely to develop these types of diseases. Four standout varieties that are mold resistant are Frisian Dew, Jamaican Pearl, Moby Dick, Spoetnik #1, and Sweet Tooth. In general, strains with shorter flowering periods are less likely to develop mold because they don’t spend as much time in the flowering phase where they are more susceptible.


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Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional

Profile Picture of Chris Bond

Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.

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