By Dr. Daniela Vergara
Yeast and mold are types of fungi found everywhere— in the air, soil, water, and even on plants. Some fungi are useful, but others can be harmful.
What Are Fungi?
Fungi are living organisms that include molds, yeasts, and mushrooms. They are different from plants because they don’t make their own food. Instead, they absorb nutrients from their surroundings.
- Helpful fungi: Yeast is used to make bread and beer, and certain molds help create cheese and antibiotics like penicillin.
- Harmful fungi: Some molds produce toxins that can make people sick, cause food to spoil, or infect plants and animals.
How Do Yeast and Mold Grow?
- Yeast: Tiny, single-celled fungi that reproduce quickly by forming small buds. Yeast is often used in baking and brewing [1].
- Mold: Made of many tiny cells that grow in long, thread-like structures called hyphae. Molds spread by growing outward, forming fuzzy patches on surfaces. Some molds are harmless, but others can release toxins dangerous to people [1].
Why Is Yeast and Mold Testing Important?
The Total Yeast and Mold Count (TYMC) measures how much yeast and mold are in a cannabis sample. This test is important because:
- Too much yeast and mold can cause health problems – This is especially risky for people with weak immune systems.
- Some molds produce harmful toxins – These can cause allergic reactions or lung infections if inhaled.
- If the count is too high, the cannabis may not be safe – It could be contaminated and unfit for sale or use.
Many regions in North America have strict rules on how much yeast and mold can be in dried cannabis. Limits vary by location, ranging from 1,000 to 100,000 colony-forming units per gram (cfu/g).
Overview of Punja et al. 2023
Punja and collaborators [2] analyzed over 2,000 cannabis samples from a greenhouse between 2019 and 2022 to find out what influences yeast and mold levels. They identified different types of fungi and yeasts using lab tests. The most common genera were:
- Penicillium
- Aspergillus
- Cladosporium
- Fusarium
However, they found in total 21 species of fungi in their samples. Some of these fungi are harmless, but others can produce toxins or lead to infections.
What Increases Yeast and Mold Levels in Cannabis?
The study found that certain factors made mold levels worse:
1. Some Cannabis Strains Are More Prone to Mold
Not all strains are created equal, and some have more risk of mold contamination. For example, strains with dense leaves trapped moisture, making them more likely to develop mold.
In their study, the strains “Watermelon Kush” and “Powdered Donuts” had more mold levels, and the strains “Jack Herer” and “Death Bubba” had lower levels. However, strain names may not mean much and definitely doesn’t tell you about the relationship between strains, but that’s an entire different story!
2. Greenhouse Conditions Contribute to Mold Growth
- Leaf litter on the greenhouse floor provided a breeding ground for fungi.
- Workers moving through crops during harvest spread fungi from plant to plant.
- High temperature and humidity near the buds created perfect conditions for mold.
3. Drying Methods Affect Mold Growth
- Wet trimming and rack drying led to higher mold levels because the buds stayed damp longer.
- Hang-drying whole branches helped reduce mold growth by allowing better air circulation.
What Helps Reduce Yeast and Mold in Cannabis?
Punja and collaborators also give some suggestions that may help prevent mold and yeast growth:
- Choosing strains with fewer leaves – Less moisture gets trapped, lowering the risk of mold.
- Using fans to improve air circulation – Reduces temperature and humidity, making it harder for mold to grow.
- Harvesting in cooler months (November–April) – Lower temperatures mean less mold.
- Hang-drying whole branches instead of wet trimming – Allows buds to dry more evenly.
- Drying buds to 12–14% moisture (water activity of 0.65–0.7) – Keeps mold levels low while preserving quality.
Why Does This Matter for Cannabis Safety?
While some fungi in C. sativa are harmless, others such as Aspergillus ochraceus and Aspergillus niger, can produce toxins that cause serious health risks, but the disease they produce, Aspergillosis, is uncommon in healthy individuals but can affect immunocompromised people [3].
The study also found inconsistencies in how commercial labs test for yeast and mold, suggesting the need for a standardized testing method across the industry.
How Can Cannabis Growers Improve Safety?
By understanding what increases or decreases yeast and mold levels, growers can take steps to produce safer cannabis.
- Selecting mold-resistant strains
- Improving airflow in greenhouses
- Cleaning up plant debris
- Using proper drying techniques
- Storing cannabis at safe moisture levels
Following these best practices can help ensure clean, high-quality cannabis products for consumers.
For NYS, this is a very brief summary of the TYMC according to the OCM: (from this website visited on March 26, 2025 https://cannabis.ny.gov/system/files/documents/2025/03/ocm-testing-limits-final-2-26-25.pdf)
Cannabis products must be tested for Total Yeast and Mold Count (TYMC) to make sure they are safe to use. The TYMC measures how many yeast and mold cells are in a sample, reported as colony-forming units (CFU) per gram (cfu/g) or per milliliter (cfu/mL).
Who Needs to Test for Yeast and Mold?
- All cannabis products must be tested for yeast and mold before they can be sold.
- Labs must report the TYMC results using cfu/g or cfu/mL.
Limits for Medical Cannabis
For medical cannabis, there are strict limits on yeast and mold levels:
- Unextracted cannabis (flower, ground cannabis, etc.): Must have 10,000 cfu/g or less (10⁴ cfu/mL).
- Extracted or infused cannabis (oils, edibles, tinctures, etc.): Must have 1,000 cfu/g or less (10³ cfu/mL).
Limits for Adult-Use (Recreational) Cannabis
For adult-use cannabis, the rules are different:
- Unextracted cannabis (flower, ground cannabis, etc.): There is no official limit. Instead, labs must report the results, and regulators may require extra testing if there are safety concerns.
- Extracted or infused cannabis (oils, edibles, tinctures, etc.): Must have 1,000 cfu/g or less (10³ cfu/mL).
Recent Rule Changes
- In January 2023, New York changed the rule for adult-use cannabis flower. Instead of setting a limit, they now only require labs to report the results.
- Before April 2022, yeast and mold were classified as contaminants, meaning products with high levels were not allowed.
In conclusion, these regulations support overall quality control and are designed to promote product safety, particularly for consumers. TYMC provides a general measure of total yeast and mold in samples, even though it does not differentiate between harmful and harmless fungi. Because of this, more targeted methods such as mycotoxin testing, are also being used to provide additional safety insights.
1. McGinnis, M. and S. Tyring, Introduction to Mycology. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston;. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8125/
Page visited on March 26, 2025. 1996.
2. Punja, Z.K., et al., Total yeast and mold levels in high THC-containing cannabis (Cannabis sativa L.) inflorescences are influenced by genotype, environment, and pre-and post-harvest handling practices. Frontiers in Microbiology, 2023. 14: p. 1192035.
3. Brophy, S., et al., Aspergillus sp. testing in the emerging Cannabis sativa industry in New York State. 2023.
