How To Prevent Mold When Growing Cannabis

How to prevent mold when growing cannabis

Mold may very well be marijuana’s #2 enemy, topped only by the boys’ club of old reactionaries responsible for unfairly criminalizing it.

Few things are as frustrating as losing a promising plant or an entire grow-op to mold. Being ahead of the curve is key since the “enemy” here is unpredictable, invisible, and hardly noticeable until it’s too late. So how to prevent mold when growing weed, and which are the most important preventative measures you can take?

The Basics

Whether growing inside or out, step one is knowing which factors come together to create a “perfect” habitat for mold. A crucial advantage if you want to position yourself favorably and take action before impending issues become a real problem.

Mold enjoys a moist environment with high humidity and specific temperatures. You’ll often read how cooler temperatures are most favorable for mold growth, but this depends on the type of mold. Some like it hot, some not.

At any time during the growth cycle, mold can creep up on cannabis plants but the riskiest time is at the very end, with only a week or two to go. Of course, so close to that long-awaited harvest it is all the more tragic to have things go awry.

The increased risk near the end of the flowering state makes sense. This is when plants are full and densely packed with foliage and buds.

Poor airflow is another crucial component, regardless of whether this is surrounded by walls and roofing. Such a stagnant environment can be as large as the entire grow room/space, but it doesn’t have to be. Mold can find small spaces with very little airflow inside dense buds grown outside – a difference in scale, but the result is similar.

The unholy triad for mold growth:

  • sub-par airflow
  • a favorable temperature
  • high humidity

However, even though growing cannabis is definitely one of the cooler science projects for grown-ups, the mold threat isn’t always an exact science. Based on personal experience, mold can target plants when only two out of the three points are met.


What can you do to prevent mold from making its appearance in the first place? Next, we’ll cover mold-resistant cannabis strains, actionable steps you can take to prevent mold, and the importance of scrutinizing plants for any initial signs of mold.

Prevent mold when growing cannabis by being on top of temperature, humidity and air flow.

First off, there are more than just a few great strains that are specifically known to be more resistant to mold than others. If you gauge your growing location to be at risk for moldy disaster, consider going for one of the hardier, mold-resistant strains.

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Secondly, there are tricks and tools to engineer the environment to be as hostile as possible for any and all types of mold.

Last but not least, being on top of things can make a big difference. Keep a close watch and don’t let your guard down – especially near the end, during flowering – to take control of the situation at the first signs of an impending disaster.

1. Mold-Resistant Strains

Sometimes you just want to grow a particular strain and no other. That, we understand! However, there is a lot of sense in fine-tuning the type of strain to the climate, temperature, and humidity you’ll be growing in.

Some strains are bulkier and denser than others. Tightly packed and dense buds, gorged with water are a dream environment for mold spores to set up camp in. But it isn’t merely a matter of plump versus lean build.

Certain cannabis strains are genetically more resistant to mold. This includes classics like White Widow and Afghan Kush, but also many less well-known strains, for example, South African Kwazulu, Lemongrass, or Sishkaberry Kush (just to name a few).

Choosing an excellent mold-resistant strain is too easy of a head start not to take. Cannabis versus mold: 1-0.

2. Tools & Tricks to Make Sure Mold Does Not Feel Welcome

If you’ve had previous crops taken over by mold, it’s a bit of a different story because mold spores are notoriously hard to fully get rid of.

If on the other hand, you’re just concerned with mold being a potential risk, good! Here’s what to do and what to keep track of to make mold feel very unwelcome.

These things mainly apply to an enclosed space, a controlled environment. Outside, some of what’s mentioned below may be helpful but you obviously won’t be able to rule with an iron fist.

Mold thrives in obscure corners, so keep an eye out for areas where the plants’ foliage becomes dense. Strategically removing certain leaves to ensure the passage of air can be a good idea. Don’t go overboard though – the plants do need a healthy amount of their leaves to function properly and excessive defoliage causes other issues.

Scissors are pretty much a staple in every household. If they’re old and/or dull, this is a nice excuse to get a fancy new pair of trimming shears.

Mold loves stagnant air. So, wind it is! One of the most important actionable steps to avoid mold is to use a good ventilation system. A convenient way to start off on the right foot is with a complete grow tent kit. Or, if you don’t mind a bit of handiwork, it’s certainly possible to build and assemble the necessary set-up.

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A hygrometer is another important tool to keep tabs on how much humidity is in the atmosphere. In a grow room, the level of moisture in the air shouldn’t surpass 70% during vegetation. When plants are flowering, it shouldn’t be more than 40-50-%.

How to bring down humidity in the air

So when the humidity in your grow space rises to cringeworthy Turkish hammam levels, is it time to panic? Not quite yet. Assuming you have already trimmed foliage where possible as mentioned above, a few more things can be done to bring down the moisture in the air.

  • Could the plants do with less water? Overwatering plants is one thing that can contribute to or cause the grow room’s humidity to spike.
  • Look at the placement of fans to ensure airflow reaches all corners of the space. Sometimes an extra fan placed to blow air underneath larger, bushy plants can help.
  • Use a residential dehumidifier.
  • Evaluate the space’s ‘population density’. Is it possible (logistically and/or financially) to increase the space for your current number of plants? If not, remember that making certain sacrifices is better than losing an entire crop to mold.
  • Explore installing additional gear such as more exhaust fans and a temperature & humidity controller. The latter can regulate fan speed to attain the desired humidity level.

Keeping outdoor plants as dry as possible

Outside, the options are obviously more limited. If they get wet (due to rainfall, or even heavy dew in the morning) gently shake plants to get rid of excess water and prevent it from seeping into the buds. If you’re really on it, even better is to protect them from rainfall with a tent or cover.

3. Watch like a hawk

Even though something as nasty as bud rot can destroy buds from the inside out and may not be visible at first, don’t underestimate the importance of visually inspecting your plants.

Does anything look abnormal?

Mold can indeed take over the show awfully fast, but certain first signs of mold don’t look much like an established infection.

For example, powdery mildew can start off as small bumps on the leaves. If kolas suddenly develop strange coloring, that could be a warning sign they’re affected by bud rot from the inside.

Types of Mold

(White) Powdery Mildew

Where: Powdery mildew looks like a thin dusting covering the cannabis leaves, before gradually taking over the entire plant. White powdery mildew especially thrives in high humidity and loves a warm environment as well as stagnant air.

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Color / Texture: Indeed, it is white and powdery – with black spores if given the chance to mature to the point of reproduction. Cannabis leaves covered in the mold aren’t able to photosynthesize properly, which causes them to turn yellow/brown and die off.

Botrytis a.k.a Bud Rot

Where: Most often found at the base of stems and in or around the buds. Typically on mature marijuana plants, but not unheard of on younger plants.

Color / Texture: Botrytis can have different colors such as blue-green, grey, or white. As it matures and forms spores, these will show as small black specs. Cannabis flower infected with Botrytis turns either brittle or soggy, depending on the levels of moisture in the air. Bud rot is also called ‘grey mold’ and may appear white/light grey and fluffy at first. Later on, it attacks the inside of buds and causes discoloration of leaves or part of a kola.

Growing Mold-free Marijuana

Keeping in mind the steps above will help you avoid an all too common problem for new and seasoned growers alike.

So remember, you want to see those leaves dancing!

Indoors, adding an extra fan never hurts. The hygrometer is your best friend. While indoor growing allows you to craft the scene, outdoor growers are at the mercy of the weather gods. To stack the odds in your favor, select a breezy spot for your plants and be sure to stay one step ahead of the rain.

Even when taking all the precautionary steps, sometimes something can be missed and a random spore wreaks havoc on a perfectly good grow op. It can happen to the best of us. Don’t let it ruin your attempts or desire to become a successful grower!

Mold spores

By Felis Cannabis

Hi, Felis Cannabis here. This little corner of the web is my scratch pole. The legalize movement is growing, but not fast enough for me to give up the incognito status just yet. ;-) Let's keep at it! We should have the right to use any plant we choose.