mold cannabis

Cannabis Mold Prevention

Few sights are as disappointing as watching your cannabis plants or weed stash succumb to mold. Mold fungal infections can affect the plants as they grow and flower as well as a poorly stored harvest stash. You can prevent these major disappointments, though. This blog tells you more about the various types of fungi that can affect cannabis as well as how to prevent such mold from ruining your grow – or your lungs!

How Can You Tell If There’s Mold On Your Cannabis?

Mold on cannabis plants or on harvested buds can really ruin any grower’s day. Moreover, it can be a real health hazard when inhaled. That makes it extremely important to regularly check whether your weed is clean and safe for consumption. You can verify by looking closely and by using your nose. The following signals are cannabis mold red flags:

  • Tiny black specks on the buds. These are usually the early signs of fungus infection;
  • Yellow, brown, or grey fluffy patches mean a mold infestation is well underway;
  • Unpleasant odour: plants affected by mold tend to stink, and that’s why your nose is an important detector. If cannabis suddenly starts to smell like sweat or urine, it is time to ditch your crop or your stash.

Visually checking cannabis for mold works best in a well-lit area. Natural daylight is not optimal; try using the flashlight on your mobile phone instead, or use a brightly lit (grow) room. Make sure that all colours are visible and inspect the buds from all angles. Using a magnifying lens is recommended to make sure you don’t miss a spot.

If you discover that your weed is infested with fungi, the best course of action is usually to just get rid of the plant, pronto. You may wonder whether that is really necessary – well, yes, it is. Smoking moldy cannabis can damage your lungs and cause serious health problems. So don’t try to wipe off the mold or remove only the affected parts of a cannabis plant. Once you notice mold on a plant, it will have spread throughout the entire plant even if you can’t see it. Don’t go risking your health!

mold bud rot cannabis
Bud rot or botrytis

Types Of Cannabis Mold

Three types of mold or fungus are commonly found on cannabis.

Bud Rot (Botrytis cynerea)

Bud rot is the most common cannabis mold problem, which tends to appear on flowering plants rather than on harvested colas. You can recognise bud rot by buds that turn grey, black, or brown. This process happens from the inside-out, so you’re likely to discover it only once it is too late to save your plant.


Aspergillus, by contrast, is the most common fungus infection to look out for after harvesting. Airborne spores can become a real problem for people with weakened immune systems. This fungus can cause aspergilosis that can spread to the lungs and the brain. Even though this is a rare condition, it is still the main reason never to smoke moldy weed. Ever.

Powdery Mildew

Mildew is a fast-spreading fungus that can start infesting any part of a plant, including its leaves or stems. Any part affected by mildew will perish. Affected plants look as though they are sprinkled with talcum powder, leaving white, lumpy patches. If you detect powdery mildew early, you may still save your plants from this mold menace, but if not, your plant and its neighbours are probably doomed.

mildew cannabis mold

What Causes Mold On Cannabis?

If you want to prevent cannabis from being affected by mold, you should work with diligence throughout the growth, flowering, and harvest phases, while talking extra care while drying and curing the buds. Fungi love warm, humid, stagnant air. This basic information is your best weapons in the fight to keep plants and colas safe.

Indoor Mold Prevention

Indoor growers need to keep a close eye on air humidity and ventilation. You may want to install a hygrometer to measure air humidity. The following humidity ranges are a general guideline for indoor cannabis growing without mold:

Vegetative phase: 50-75% (aim for 60%);

Flowering phase: 45-60% (aim for 50%)

Make sure you have enough air circulation throughout the grow. Using fans is very helpful here, as is directing your plant’s growth using supercropping or similar techniques.

The denser the foliage canopy of a plant, the greater the risk of high humidity and reduced circulation. That is why strategic pruning and careful defoliation can go a long way towards keeping cannabis free from mold issues. Additionally, feeding your plants the right nutrients can help them become the strong, healthy individuals required to resist fungi throughout the grow.

Next is your method of drying, curing, and stashing your harvest. Here too, high air humidity, temperature, and air circulation are the main factors to consider.

air circulation mold indoor cannabis growing

Overall, though, hygiene is the most important measure in mold prevention. Always try to work with clean equipment, clothes, and fingers, and be aware of how easy it is to bring fungi into the grow environment from outside. Having multiple tents or grow rooms only increases the need to prevent cross-contamination. Fungal spores are invisible, so you’ll want to take every precaution when avoiding mold on your cannabis!

Be sure to thoroughly clean your grow space after each grow, and refresh the air before starting a new run. Good hygiene is often a matter of discipline, so try to make a habit out of clean and neat techniques to avoid most problems before they start.

Avoiding Mold On Outdoor Cannabis

When growing cannabis outdoors, moisture is your main enemy when it comes to mold, usually in the form of rain (drizzle), fog, or mist. The riskiest bit of the grow is the last stage of the flowering phase. By that time, the outdoor season has usually progressed into late summer or early autumn, depending on that year’s weather conditions. Bigger, denser buds make a perfect fungal habitat at this stage. Read this blog to learn how you can protect your harvest from fungi in the last stages of your outdoor cannabis grow.

water ventilation outdoor grow
Moisture is always a risk.

The Best Way To Store Cannabis

If you dried your cannabis harvest correctly, curing and storing your buds now becomes your primary concern. Here too, mold can ruin a cannabis stash. You can store cannabis in several ways, but not all of them are ideal for keeping mold at bay.

Plastic Bags

We strongly advise against storing cannabis in plastic (zip lock) bags, since they are poor protection against heat, cold, or moisture. These baggies are more permeable than you might think. High temperatures and humidity render your buds vulberable to fungi. Moreover, the quality of your nugs will deteriorate rapidly as the cannabinoids soon start to degrade.

Stash Boxes

You can get specially designed stash boxes to give your weed a airtight, waterproof, and UV-resistant accommodation. They’re a great option for protecting harvested cannabis against mold.

Cannabis fungus prevention
Good old mason jars – just make sure to burp them regularly!

Glass Mason Jars

Glass mason jars are a classic alternative. You could go for a sterilised marmite jar or similar glass container of course, but a mason jar is a time-tested way to provide a sealed, dry, clean environment. Glass provides an intrinsically hostile environment that fungi tend to hate. Whatever jar you use, be sure to clean and then dry all surfaces thoroughly before use.

Glass has the added benefit of inertia: it does not release any chemicals over time in addition to the air- and moisture tight closure of a mason jar. If you find a good cool, dark, dry and secure spot to store those jars, you have a good chance to keep that cannabis harvest free from mold for months or more. Keep temperatures below 23°C and air humidity between 50-60% and you should be good. If you want, you can add moisture detectors and absorbents for added safety.  While you store your buds, they cure for better taste and potency. Just make sure to ‘burp’ the jars regularly to let air out, as we explain in our drying and curing blog.

The Best Cannabis Genetics Against Mold

Mold is a true scourge for all cannabis growers and lovers. Luckily, you now know how to protect your grow, harvest, and stash from fungal disaster.

cannabis seeds nu mold

Of course, there is one other crucial factor in mold prevention: using premium cannabis genetics! In recent decades, expert breeders have been working their buds off to create more mold-resistant strains.

Use that expertise by choosing a seeds supplier with their own selection of high-quality strains, and read the product descriptions to find out which ones are most resistant to mold and disease. As you have probably guessed, ordering your cannabis seeds at Amsterdam Genetics is an excellent first step on your way to avoiding mold issues from square one!

Pick your mold-resistant strain

Ultimately, though, the struggle against mold will remain part and parcel of growing cannabis. We’re working with nature here, after all; and fungi are an ancient, respectable, and inevitable part of any ecosystem, including your grow tent. At least the tips provided here arm you with the knowledge you need to protect yourself and your plants, so let’s pick those seeds and start growing!

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