Mould on CBD flower: the signs and their limits
Mouldy CBD flower sometimes shows itself through brown, decaying bracts, grey fuzz or a musty smell, and any one of these signs means you should not use it. A healthy-looking flower, though, can still carry invisible spores. Proper drying is the real safeguard, as it brings the flower's water activity below 0.70, and only a microbiological analysis of the batch measures contamination.
Key takeaways
- Over 100 fungal species have been linked to hemp and cannabis, some of them in genera able to produce toxins.
- Most fungi stop growing below a water activity of 0.70. For dried flower, the ASTM D8197 standard aims for 0.55 to 0.65.
- Mould fuzz and trichomes can look alike; separating them reliably takes a microscope or a laboratory culture.
- Ochratoxin A turned up in one third of 142 illicit cannabis samples and in none of the more than 9,000 samples from regulated legal markets.
The signs that should make you reject a flower
Certain signs justify rejecting a product with no analysis at all. In work published in 2019 in Frontiers in Plant Science, Zamir Punja and his team describe grey mould caused by Botrytis cinerea: the bracts brown and decay, and at advanced stages the whole head is destroyed. The same authors describe powdery mildew as a white felt of mycelium that makes leaves look powdery, and Penicillium colonies as blue-green.
| What you see or smell | What it may indicate | What to do |
|---|---|---|
| Brown, soft bracts that fall apart | Grey mould (Botrytis) | Do not use, tell the seller |
| Grey or grey-brown fuzz in the centre of the bud | Botrytis mycelium | Do not use |
| Blue-green spots | Possible Penicillium colonies | Do not use |
| Powdery white film on the leaves | Possible powdery mildew | Reject if in doubt |
| Musty, cellar or wet-hay smell | Excess moisture, fermentation | Do not use |
| Spongy flower that does not snap between the fingers | Insufficient drying | Do not store in a closed jar |
Two further points. The outside of a dense bud does not show everything, so break it open through the middle before deciding. And the grey-brown fuzz described by Punja and his colleagues appeared after the tissues had spent 48 hours incubating in a closed plastic bag. That is a laboratory method, which nobody can reproduce at a shop counter.
White fuzz or trichomes: why the eye gets it wrong
Trichomes are tiny mushroom-shaped glands covering the bracts, where resin is produced. Seen with the naked eye, they make the flower look frosted, white or silvery. Powdery mildew in its early stage leaves a white film too. We found no study validating a visual method that would let a consumer distinguish them.
A jeweller's loupe is of some help. Trichomes have a clear stalk and a round head and grow mostly on the bracts. Mycelium felt looks more like a web of tangled filaments and also spreads onto the leaves. Reliable identification still needs a microscope or a culture on growth medium. In the 2019 work, researchers placed 2 mm fragments on agar and looked at what grew within seven days. The shape of trichomes at different stages is shown in our article on trichomes.
Keep the reverse rule in mind: the absence of visible signs proves nothing, since a flower that looks perfect can carry spores.
The threshold that matters: water activity
Water activity, noted aw, measures the "free" water in a product, meaning the water available to micro-organisms. The scale goes from 0 (dry) to 1 (pure water). Multiplying it by 100 gives the relative humidity of the air in equilibrium with the flower, so an aw of 0.62 matches 62% relative humidity in a closed jar.
Kimberly Gwinn, Zamir Punja and two co-authors state the rule in a 2023 review in Frontiers in Microbiology: sufficient drying brings water activity below 0.70, and most fungi cannot grow under that level. The review adds a caveat straight away, since Penicillium and Aspergillus can survive between 0.62 and 0.70. For dried flower, the ASTM D8197 standard recommends 0.55 to 0.65, leaving a margin under the threshold.
Three habits follow from this threshold. Store flower in an airtight container away from heat. If you use a humidity-control pack, pick one rated between 58 and 62%, which falls inside the ASTM range. Never return a flower to its jar while it is still damp. Our article on storing CBD flower covers temperatures and containers. The decisive step comes earlier in the chain and is the subject of our article on drying and curing.
What is known about the health risk
The 2023 review lists more than 100 fungal species linked to hemp and cannabis, including pathogens, epiphytes and endophytes. Among toxin producers it names Aspergillus, Penicillium, Fusarium and Mucor first, and Penicillium is the toxigenic fungus most frequently recovered from the tissues analysed.
Ochratoxin A, a mycotoxin made by some Aspergillus and Penicillium species, gives the clearest figure. The same review reports it in one third of 142 illicit cannabis samples and in none of more than 9,000 flower and extract samples from about 300 producers on regulated legal markets. As a reference point, the EFSA panel on contaminants concluded in 2020 that the tolerable weekly intake of 120 ng per kilo of body weight set in 2006 was no longer valid. That opinion applies to food and does not cover cannabis.
Inhaled spores are the other concern. Using 2016 insurance records, a CDC study found that people who used cannabis were 3.5 times more likely to have a fungal infection than others (confidence interval 2.6 to 4.8). The authors report an association and do not claim a proven cause. Among the patients, cannabis users were more often immunocompromised (43% vs 21%) and more often smoked tobacco, and the route of use was unknown. People with weakened immune defences are the first exposed to this risk.
What a batch analysis should show
No French text sets a microbiological limit specific to CBD flower, so quality depends on the tests the producer and the seller choose to run. On a certificate of analysis, look past the cannabinoids for:
- a total count of yeasts and moulds, given in CFU (colony-forming units) per gram;
- a search for named pathogens such as Aspergillus or Salmonella;
- a mycotoxin screen covering aflatoxins and ochratoxin A, with its limit of quantification;
- a date of analysis and a batch number identical to the one on the pouch.
If a certificate only lists CBD and THC, it tells you nothing about mould. Each line is explained in our guide to reading a certificate of analysis, and the laboratory page explains the measurement methods. Pesticide residues, the other contaminant to watch for on flower, are the subject of our article on MRLs.
Frequently asked questions
Discard any flower showing brown, soft bracts, grey fuzz, blue-green spots or a musty smell. Open dense buds through the middle, as the outside can hide what is inside. Even a healthy-looking flower may carry spores, and only an analysis can check for them.
Yes. Both trichomes and early powdery mildew make a flower look frosted. Through a loupe, a trichome shows a stalk and a round head, whereas mycelium forms tangled filaments. When in doubt, do not use the product.
What counts is the flower's water activity, which differs from the humidity of the room. Most fungi stop growing below 0.70, although Penicillium and Aspergillus can survive between 0.62 and 0.70. ASTM D8197 recommends 0.55 to 0.65, which corresponds to 55 to 65% relative humidity in a closed container.
No. Cutting off the visible part leaves nearby spores and any mycotoxins in place, and heat is no solution either. According to the 2023 review, aflatoxin B1 is destroyed at 180 °C under experimental conditions, but ochratoxin A loses only 25 to 44% at 200 °C in wheat. Some breakdown products remain just as toxic. Throw the product away and give the seller the batch number.
Written by the Phytogrammes team
Each journal article draws on cited primary sources: ANSM, EFSA, EUR-Lex and peer-reviewed publications. Product profiles are distinguished from documented laboratory results. Our approach.
Article published on . CBD is not a medicine.