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Dealing With Cannabis Mold

It’s a sight every cultivator dreads – you’re inspecting your cannabis crops, and the much-dreaded mold catches your eye. That familiar, powdery-looking substance on the fan leaves means one thing – act fast. But what do you do? First of all, don’t panic. Secondly, read this blog, and we’ll tell you all you need to know about dealing with moldy weed.

What Is Cannabis Mold?

There are a few different types of mold that can affect cannabis plants. Grey mold, or to give it its fancy Latin name, Botrytis Cinerea, is a fungal pathogen that all cannabis growers must guard against. It’s a destructive force that can attack any number of crops, including soft fruits, vegetables, flowers and shrubs. Many cultivators contend that mold is a cannabis farmer’s biggest threat, chiefly because it can ravage your entire crop in a matter of days due to its fast-acting, fast-spreading properties.

The worst thing about mold is that nobody is safe from the threat, and the danger continues even after you’ve harvested your weed. If you don’t practice good habits and good hygiene with cannabis cultivation, mold can quickly settle in after harvest. When it does, it will attack your buds, causing them to decompose, rendering your bounty utterly worthless. Sounds like a nightmare? It is.

Even though certain types of mold look like they could easily wipe clean, this is not an option you should consider. When mold appears on your plant, you have to remove the affected areas altogether, or if your plant is too far gone, destroy it completely. Mold spores are highly toxic to humans, and ingesting mold spores can cause serious health problems.

What Does Cannabis Mold Look Like?

The only good thing about cannabis mold is that it’s easily identified. Keep a close eye on your buds late in the flowering stage, and be wary of grey webbing, white powder, or black powder appearing on the leaves. If you see that, you’ve got mold. These may be fungi, but they’re not fun, guys. That grey webbing contains fuzzy spores that are activated and released, quickly spreading and causing damage to your cannabis plants. Look out for grey or brown colouring on your buds and dry, lifeless, wilting leaves. That’s grey mold.

Then there’s powdery mildew. Look out for white patches appearing on your plants’ fan leaves. It resembles baking flour in appearance and can lead to stunted plant growth. It can easily be mistaken for glistening trichomes by beginners, but you’ll soon realize that it’s not when it spreads and starts damaging your plant. Inspect closely to make sure.

Sooty mold is dark and can appear on fan leaves in the same way powdery mildew does. It’s linked to pest infestations, as the mold feeds on excrement left behind by the pests. Sooty moud disrupts photosynthesis and can stunt plant growth, but it can be treated relatively easily.

Fusarium – aka root rot – is a far more devastating type of mold. Once root rot sets in, little can be done, and it can lay your crop to waste in a matter of days. Prevention is the best action you can take to guard against root rot, because once you see symptoms – namely wilting, falling leaves, it’s underway and the damage is pretty much irreversible.

What Causes Mold to Grow on Cannabis?

It depends on the type of mold, of course, but if you spot mold on your cannabis, it’s a safe bet that there’s an issue with your growing environment. Mold loves humidity, so if your growing space has a high temperature or moisture levels, chances are pretty good that you’ve invited mold to the dance.

This is why keeping these levels in check throughout your operation is essential. Indoor growers will find it easier to take control of ambient temperature; outdoor growers are a little more at the mercy of the sun. You can do things to mitigate the damage caused by the rise in mercury, and we’ll discuss these later.

Dealing with Cannabis Mold – Prevention

If you’ve struggled with mold in previous grows, or if you’re new to cultivation and want to reduce the risk, there are preventative measures worth knowing. You can start by choosing a strain with mold-resistant properties; that way, you have a layer of protection from the outset. Nowadays, a great selection of cannabis strains are bred to offer good resistance to mold and other pathogens, and you’ll find a great selection on

Managing your environmental factors is a critical consideration when looking to prevent the chances of mold. Aim to keep your temperature under control, as too much heat is an open invitation to mold, one it’s only too happy to accept. Make sure you have a thermometer in your grow area and check the temperature regularly. If it gets out of hand, knock the temperature back a bit on the thermostat to give your plants a chance to cool off a bit.

Or, you could add an extra fan or two. This will allow adequate cooling and air circulation around your plants and effectively stop mold from appearing in the first place. You want your temperature around 24 degrees C if possible.

Outdoor growers have it a little more complicated when it comes to controlling the temperature, but there are steps you can take, and some involve preparation upfront.

For example, if you plant in portable containers, you can move your plants quickly if the weather becomes problematic. Keep your plants spaced well apart, so they’re not touching; this will help manage airflow better. You can use temporary tarpaulins to protect your plants during periods of heavy rain.

Manage and Control Humidity Levels

Keeping the humidity at the correct number is integral to stopping mold from appearing on your marijuana plants. High humidity is a good way to guarantee moldy cannabis.

Under 60% relative humidity is a safe zone for plants to thrive and bloom without the risk of mold appearing. Invest in a hygrometer, and use it in your growing space. Ensure your indoor grow has adequate fans and an exhaust system to keep the air moving. If those humid conditions creep up, take action fast.

What Action Should you Take?

If you find your humidity levels nudging up into the danger zone, you’ll have to take steps to bring it down as quickly as you notice it rising. Adding a dehumidifier to your grow area is relatively easy to manage. It’s naturally going to be a bit easier to install one of these in an indoor setting or greenhouse than for crops grown outdoors. If you’ve never used a dehumidifier before, they’re available in different forms and in different price ranges.

These devices will pull a surprising amount of moisture out of the air; thankfully, they don’t necessarily use a lot of electricity.

Pruning is Important

As plant growth progresses, there’s likely to be a reduction in space between the branches. This can lead to problems with airflow through the plants, so it’s a good idea to monitor the growth inside the plant between the leaves and buds. This enables you to see just how densely-packed things can get, and all that close-knit foliage can cause problems for the plant’s ability to breathe.

The result is an increased potential for mold, so if your plants are getting overly bushy, grab your pruning scissors and get to work removing excessive foliage from the inner regions of the plant’s growth. Always sterilize your pruning equipment before and after each use, whether dealing with mold or not.

What to do When you see Mold

If you’ve taken all the necessary precautions, but it’s all been in vain and that pesky mold pops its head up in your grow room, get to work, pronto! How you should react depends on the degree of decay. If it’s minor, you can take steps to remove the mold, but if it’s significant and you’ve got grey mushy buds? You’ll have to remove the infected plants, post haste, to save unaffected plants from succumbing to the spread.

Ramp the humidity level down further, as this may help slow the spread of the mold. Sanitise your pruning equipment before each use (and after), don your protective gear (gloves, mask, and apron if you wish), and remove the mold immediately. Prune off any infected parts of the plant immediately, and take great care to avoid contacting unaffected, healthy areas of the plant. That means ensuring no infected parts touch uninfected parts and keeping your tools away from the healthy areas.

Don’t let the moldy party parts fall to the ground and lay there, either. Instead, manually remove infected cuttings from the plant, and dispose of them safely and away from your plants. Doing it any other way could spread the infection to otherwise healthy buds and leaves, rendering your pruning operation ineffective.

Avoid Using Chemicals

It may be tempting to employ certain mold sprays and fungicides you see in gardening stores, which promise to stop mold in its tracks. However, it would be best if you resisted this, as although a few of these can put paid to fungus, they can also damage your plants. Not only that, they contain chemicals you don’t want to ingest, and if you or anyone else is planning on smoking that weed, you don’t want chemicals or pesticides on your crops. If you do go down the route of a topical treatment, do your research beforehand and be absolutely certain that it’s safe for use on cannabis. Neem oil is considered a safe product, but it will only offer benefits in certain instances.

Stay Vigilant

If your cannabis plants have mold, removing the first wave is only half the battle. You’ll have to remain vigilant in your checks and diligent in your practices, just in case the mold returns. Monitor daily, keep a close eye on the temperature and humidity levels in the grow room, and keep up your defoliation. Prevention is better than cure, they say, so make sure you keep your grow area optimised to avoid moldy cannabis the rest of the way.

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

Duncan Mathers