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10 Ways to Reduce Mold in Your Grow

By Ketch DeGabrielle
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Regardless of whether your grow is indoor or in a greenhouse, mold is a factor that all cultivators must consider.

Photo credit: Steep Hill- a petri dish of mold growth from tested cannabis

After weeks of careful tending, pruning and watering to encourage a strong harvest, all cultivators are looking to sell their crop for the highest market value. A high mold presence, measured through a total yeast and mold count (TYMC), can cause a change of plans by decreasing crop value. But it doesn’t have to.

There are simple steps that any cultivator can take that will greatly eliminate the risk of mold in a grow. Below are some basic best practices to incorporate into your operation to reduce contaminants and mold growth:

  1. Isolate dirty tasks. If you are cleaning pots, filling pots or scrubbing trimming scissors, keep these and other dirty tasks away from grow and process areas. Dirty tasks can contaminate the grow area and encourage mold growth. Set up a “dirty room” that does not share heating, ventilation and air conditioning with clean areas.
  2. Compartmentalize the grow space. Mold can launch spores at speeds up to 55 miles per hour up to eight feet away without any air current. For this reason, if mold growth begins, it can become a huge problem very quickly. Isolate or remove a problem as soon as it is discovered- better to toss a plant than to risk your crop.
  3. No drinks or food allowed. Any drinks or food, with the exception of water, are completely off limits in a grow space. If one of your employees drops a soda on the ground, the sugars in the soda provide food for mold and yeast to grow. You’d be surprised how much damage a capful of soda or the crust of a sandwich can do.
  4. Empty all trash daily. Limiting contaminants in turn limits the potential for issues. This is an easy way to keep your grow clean and sterile.
  5. Axe the brooms. While a broom may seem like the perfect way to clean the floor, it is one of the fastest ways to stir up dirt, dust, spores and contaminants, and spread them everywhere. Replace your brooms with hepa filter backpack vacuums, but be sure that they are always emptied outside at the end of the work day.
  6. No standing water or high humidity. Mold needs water to grow, therefore standing water or high humidity levels gives mold the sustenance to sporulate. Pests also proliferate with water. Remove standing water and keep the humidity level as low as possible without detriment to your plants.
  7. Require coveralls for all employees. Your employee may love his favorite jean jacket, but the odds are that it hasn’t been cleaned in months and is covered with mold spores. Clean clothing for your staff is a must. Provide coveralls that are washed at least once a week if not daily.
  8. Keep things clean. A clean and organized grow area will have a huge impact on mold growth. Clean pots with oxidate, mop floors with oxidate every week, keep the areas in front of air returns clean and clutter-free, and clean floor drains regularly. The entire grow and everything in it should be scrubbed top to bottom after each harvest.
  9. Keep it cool. Keep curing areas cool and storage areas cold where possible. The ideal temperature for a curing area is roughly 60 degrees and under 32 degrees for a storage area. Just like food, the lower the temperature, the better it keeps. High temperature increases all molecular and biological activity, which causes things to deteriorate faster than at cooler temperatures. However, curing temperature is a function of water activity more than anything.
  10. Be Careful With Beneficials. Beneficial insects certainly have their place in the grow environment. However, if you have a problem with mold on only a small percentage of plants, any insect can act as a carrier for spores and exacerbate the problem. By the same token, pests spread mold more effectively than beneficials because they produce rapidly, where beneficials die if there aren’t pests for them to eat. It is best to use beneficials early in the cycle and only when necessary.

Total Yeast & Mold Count: What Cultivators & Business Owners Need to Know

By Parastoo Yaghmaee, PhD
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Editor’s note: This article should serve as a foundation of knowledge for yeast and mold in cannabis. Beginning in January 2018, we will publish a series of articles focused entirely on yeast and mold, discussing topics such as TYMC testing, preventing yeast and mold in cultivation and treatment methods to reduce yeast and mold.


Cannabis stakeholders, including cultivators, extractors, brokers, distributors and consumers, have been active in the shadows for decades. With the legalization of recreational adult use in several states, and more on the way, safety of the distributed product is one of the main concerns for regulators and the public. Currently, Colorado1, Nevada and Canada2 require total yeast and mold count (TYMC) compliance testing to evaluate whether or not cannabis is safe for human consumption. As the cannabis industry matures, it is likely that TYMC or other stringent testing for yeast and mold will be adopted in the increasingly regulated medical and recreational markets.

The goal of this article is to provide general information on yeast and mold, and to explain why TYMC is an important indicator in determining cannabis safety.

Yeast & Mold

Photo credit: Steep Hill- a petri dish of mold growth from tested cannabis

Yeast and mold are members of the fungi family. Fungus, widespread in nature, can be found in the air, water, soil, vegetation and in decaying matter. The types of fungus found in different geographic regions vary based upon humidity, soil and other environmental conditions. In general, fungi can grow in a wide range of pH environments and temperatures, and can survive in harsh conditions that bacteria cannot. They are not able to produce their own food like plants, and survive by breaking down material from their surroundings into nutrients. Mold cannot thrive in an environment with limited oxygen, while yeast is able to grow with or without oxygen. Most molds, if grown for a long enough period, can be detected visually, while yeast growth is usually detected by off-flavor and fermentation.

Due to their versatility, it is rare to find a place or surface that is naturally free of fungi or their spores. Damp conditions, poor air quality and darker areas are inviting environments for yeast and mold growth.

Cannabis plants are grown in both indoor and outdoor conditions. Plants grown outdoors are exposed to wider ranges and larger populations of fungal species compared to indoor plants. However, factors such as improper watering, the type of soil and fertilizer and poor air circulation can all increase the chance of mold growth in indoor environments. Moreover, secondary contamination is a prevalent risk from human handling during harvest and trimming for both indoor and outdoor-grown cannabis. If humidity and temperature levels of drying and curing rooms are not carefully controlled, the final product could also easily develop fungi or their growth by-product.

 What is TYMC?

TYMC, or total yeast and mold count, is the number of colony forming units present per gram of product (CFU/g). A colony forming unit is the scientific means of counting and reporting the population of live bacteria or yeast and mold in a product. To determine the count, the cannabis sample is plated on a petri dish which is then incubated at a specific temperature for three to five days. During this time, the yeast and mold present will grow and reproduce. Each colony, which represents an individual or a group of yeast and mold, produces one spot on the petri dish. Each spot is considered one colony forming unit.

Why is TYMC Measured?

TYMC is an indicator of the overall cleanliness of the product’s life cycle: growing environment, processing conditions, material handling and storage facilities. Mold by itself is not considered “bad,” but having a high mold count, as measured by TYMC, is alarming and could be detrimental to both consumers and cultivators. 

Aspergillus species niger
Photo: Carlos de Paz, Flickr

The vast majority of mold and yeast present in the environment are indeed harmless, and even useful to humans. Some fungi are used commercially in production of fermented food, industrial alcohol, biodegradation of waste material and the production of antibiotics and enzymes, such as penicillin and proteases. However, certain fungi cause food spoilage and the production of mycotoxin, a fungal growth by-product that is toxic to humans and animals. Humans absorb mycotoxins through inhalation, skin contact and ingestion. Unfortunately, mycotoxins are very stable and withstand both freezing and cooking temperatures. One way to reduce mycotoxin levels in a product is to have a low TYMC.

Aspergillus flavus on culture.
Photo: Iqbal Osman, Flickr

Yeast and mold have been found to be prevalent in cannabis in both current and previous case studies. In a 2017 UC Davis study, 20 marijuana samples obtained from Northern California dispensaries were found to contain several yeast and mold species, including Cryptococcus, Mucor, Aspergillus fumigatus, Aspergillus niger, and Aspergillus flavus.3 The same results were reported in 1983, when marijuana samples collected from 14 cannabis smokers were analyzed. All of the above mold species in the 2017 study were present in 13 out of 14 marijuana samples.4

Aspergillus species niger, flavus, and fumigatus are known for aflatoxin production, a type of dangerous mycotoxin that can be lethal.5 Once a patient smokes and/or ingests cannabis with mold, the toxins and/or spores can thrive inside the lungs and body.6, 7 There are documented fatalities and complications in immunocompromised patients smoking cannabis with mold, including patients with HIV and other autoimmune diseases, as well as the elderly.8, 9, 10, 11

For this reason, regulations exist to limit the allowable TYMC counts for purposes of protecting consumer safety. At the time of writing this article, the acceptable limit for TYMC in cannabis plant material in Colorado, Nevada and Canada is ≤10,000 CFU/g. Washington state requires a mycotoxin test.12 California is looking into testing for specific Aspergillus species as a part of their requirement. As the cannabis industry continues to grow and advance, it is likely that additional states will adopt some form of TYMC testing into their regulatory testing requirements.

References:

  1. https://www.colorado.gov/pacific/sites/default/files/Complete%20Retail%20Marijuana%20Rules%20as%20of%20April%2014%202017.pdf
  2. http://laws-lois.justice.gc.ca/eng/acts/f-27/
  3. https://www.ucdmc.ucdavis.edu/publish/news/newsroom/11791
  4. Kagen SL, Kurup VP, Sohnle PG, Fink JN. 1983. Marijuana smoking and fungal sensitization. Journal of Allergy & Clinical Immunology. 71(4): 389-393.
  5. Centre for Disease control and prevention. 2004 Outbreak of Aflatoxin Poisoning – Eastern and central provinces, Kenya, Jan – July 2004. Morbidity and mortality weekly report.. Sep 3, 2004: 53(34): 790-793
  6. Cescon DW, Page AV, Richardson S, Moore MJ, Boerner S, Gold WL. 2008. Invasive pulmonary Aspergillosis associated with marijuana use in a man with colorectal cancer. Diagnosis in Oncology. 26(13): 2214-2215.
  7. Szyper-Kravits M, Lang R, Manor Y, Lahav M. 2001 Early invasive pulmonary aspergillosis in a leukemia patient linked to aspergillus contaminated marijuana smoking. Leukemia Lymphoma 42(6): 1433 – 1437.
  8. Verweii PE, Kerremans JJ, Voss A, F.G. Meis M. 2000. Fungal contamination of Tobacco and Marijuana. JAMA 2000 284(22): 2875.
  9. Ruchlemer R, Amit-Kohn M, Raveh D, Hanus L. 2015. Inhaled medicinal cannabis and the immunocompromised patient. Support Care Cancer. 23(3):819-822.
  10. McPartland JM, Pruitt PL. 1997. Medical Marijuana and its use by the immunocompromised. Alternative Therapies in Health and Medicine. 3 (3): 39-45.
  11. Hamadeh R, Ardehali A, Locksley RM, York MK. 1983. Fatal aspergillosis associated with smoking contaminated marijuana, in a marrow transplant recipient. Chest. 94(2): 432-433.
  12. http://apps.leg.wa.gov/wac/default.aspx?cite=314-55-102