Mold, bud rot, powdery mildew – these are words that no cannabis farmer or consumer wants to hear. Between pests, nutrient deficiencies, and all the new obstacles associated with compliance, the last thing any grower wants to deal with is some type of mold. Unfortunately, these issues do arise. Mold is especially worrisome because it can annihilate your entire crop just weeks before harvesting (when your plants are most vulnerable). No cannabis user wants to smoke fuzzy white or fuzzy black buds – it simply isn’t safe to do so, and the terpenes suffer drastically.

Prevention is crucial!

We’re going to talk about the best ways to set up your grow to maximize airflow and reduce the chance of having to fight any attacks of bud rot or white fuzzy mold.

Setting Up Your Greenhouse or Indoor Room

Mold prevention begins far before you cut your first clone or plant your first seed. As you start setting up your operation, begin thinking about climate, weather conditions and sun exposure. Whether you’re growing in a sunlit greenhouse or a controlled indoor space, there are steps you can take at the beginning and throughout the grow cycle to help keep your facility mold-free.

For Light Deprivation Greenhouse Cultivation

Greenhouses work by simulating, well, the “greenhouse effect,” which means that sunshine – when contained – is converted into heat. Today’s top-of-the-line greenhouses have a number of automated features to control any number of features – from lighting to heating to pressure. But these greenhouses can cost tens, even hundreds of thousands of dollars depending on your square footage needs. Older greenhouses and more basic models that suck in unfiltered air run the risk of mold and contamination.

A south-facing greenhouse will soak up more direct sunlight early in the morning. This light will help dry morning dew as quickly as possible and reduce the chance of mold developing due to a build-up of moisture being trapped inside the buds. But south-facing alone won’t prevent mold and mildew. Without proper ventilation, that trapped heat – combined with the carbon dioxide that your cannabis plants produce – will create a humid, moist environment that ultimately hinders plant health and yield.

Mechanical ventilation, or the use of exhaust fans, is ideal for medium-to-larger-scale facilities to move air efficiently and control circulation. At Garden Rebels, we recommend wall-mount cabinet exhaust fans from Fantech. These fans are great at reducing moisture to help your greenhouse maintain a steady and healthy temperature. Keep in mind that some kind of air movement is necessary for all scale growing operations.

We tell our clients that the goal is a complete air exchange in the structure every 1.5 to 2 minutes. To accomplish this, you want to match the cubic foot per minute (CFM) of the fan you choose to the CFM of your structure. Be sure to strategically place circulation fans around the space to ensure the air is constantly in motion – don’t leave stagnant air in any place under your canopy. Maintaining a clean canopy so air can flow freely through the plants will also help keep the air flowing. Remember: Stagnant air is prime real estate for mold.

While passive ventilation alone (creating a system of vents) can work for smaller hobby grows, we recommend incorporating fans into your ventilation regimen to reduce your chance of mold. Using ridge vents at the top of your greenhouse and roll-up sidewalls at the bottom will create cross ventilation – where the fresh cool air from outside enters your greenhouse and flows toward the upper vents where it exits. This technique allows heat to escape while enabling a consistent supply of carbon dioxide. A good rule of thumb is to measure the area of your vents and make sure they are either equal to or greater than one-fifth of the area of your greenhouse floor. If they fall short, consider adding additional vents to ensure sufficient airflow.

Don’t make the mistake of thinking that a ventilation system isn’t necessary for your light-deprivation greenhouse! Even if you’re using a breathable greenhouse cover, proper ventilation is crucial to help eliminate overheating and prevent the build-up of moisture – whether your growing inland or on the coast; at sea level or in the hills.

For Indoor Cultivation

Even though indoor grows offer the ultimate control, with a much lower potential for mold, it still happens. But prevention is key. From the beginning, control humidity by having a dehumidification system.  At Garden Rebels, we recommend Fantech’s EPD 250CR dehumidifier, one of the most durable and versatile devices, to pull moisture out of the air. When your plants are flowering, a relative humidity (RH) level of around 45 is optimal.

Make sure you have constant air circulation, preferably utilizing an under-canopy duct system that blow air up through your canopy to ensure dust and other particles don’t land on your plants. If you’re growing on the ground, make sure your canopy air is well-circulated; this prevents big colas from sitting in stale, stagnant air. Also, be mindful of your leaf volume – the greater number of leaves, the more water your cannabis plants drink, and the more moisture is being added to the air. A higher leaf mass, therefore, releases a greater amount of water vapor into your space.

Invest in a temperature and humidity controller – there are many to choose from. We recommend a total environment controller that will monitor and control your temperature, humidity (although most dehumidifiers have a digital control on them the analog ones can plug into a controller), and carbon dioxide (CO2). These controllers can turn off the CO2 injection while the fans are running.

If CO2 is not a factor, you can use the Fantech Aeolus EC temperature controller that allows you to:

  • Exhaust on two legs
  • Exhaust on one leg and open a damper on the other
  • Exhaust on one and use the other side as a room scrubber

You can also daisy chain multiple fans together to all be controlled on a single controller.

Be sure to check this monitor each time you check on your ladies. You never want your plants to exist in an environment that is too hot or too cool. While white powdery mildew likes warm stagnant temps (around 80°F), fuzzy white mold and bud rot prefer cooler temperatures of 68°F or so. If you live in a humid climate, aim to keep your cannabis crop in a comfortable room temperature – around 75°F. Another tip for an indoor grow is to defoliate, or remove some of the leaves on big, leafy plants. When light can’t get through, it’s difficult for air to pass through as well – so consider cleaning up the fan leaves and un-developing buds near the bottom of your plants. Be careful not to over-defoliate, as this can compromise your yield – but the less vegetative matter in an enclosed space, the less humidity your plants will be subject to.

To ensure that mold and other contaminants are kept out of your facility, you’ll want to make sure your environment is clean. Change the HVAC filters multiple times a month and consider installing HEPA filters and UV lights in your HVAC system to reduce the threat of contamination. Another simpler tip is to refrain from sweeping the floors of your grow room – this will only stir up contaminants. Instead, use a filter backpack vacuum or even think about installing a central vacuum system for larger operations.

Final Tips for Preventing Mold, Thanks to Fantech Products

Here are some tips to help reduce the chance of mold in your greenhouse or indoor grow:

  • Prevention is key; be sure to maintain a proper environment with complete air circulation and exhaust system.
  • Keep a clean grow room at all times – never walk into another grow without changing your clothes first. Have employees and guests suit up in a Tyvek suit or coveralls, and wash their hands.
  • Using a bleach mat upon entering the facility or shoe booties so contaminates do not enter from shoes.
  • Keep a clean canopy; pre-treat or quarantine every plant that comes to a facility with products like Plant Therapy as well as a bi-weekly foliar application through the veg stage and first 2 to 3 weeks of flower.


If you have questions or aren’t sure where to start, contact the Garden Rebels team. We offer on-site consulting and will help you set up your grow and troubleshoot issues so that you can maximize your yield and revenue stream.