We recently had the privilege to participate in a Q&A on Ed Rosenthal’s Instagram. We took to the comment section to answer growers’ questions about HVAC, humidity control, and mold and powdery mildew (PM) prevention.

For those who aren’t familiar with who Ed Rosenthal is, he’s a cannabis activist, author, and entrepreneur. He is best known for his work in advocating for the legalization of marijuana and developing cannabis-based products. Rosenthal has written several books on the subject of cannabis, including The Marijuana Grower’s Handbook (2009), Beyond Buds: Marijuana Extracts, Hash, Vaping, Dabbing, Edibles and Medicines (2015) and most recently, Cannabis Grower’s Handbook: The Complete Guide to Marijuana and Hemp Cultivation (2021). 

Ed is a strong proponent of the benefits of cannabis, and he continues to work to make it more available to people who can benefit from its use.

What Are the Main Causes of Powdery Mildew?

Powdery mildew is one of the most common plant diseases. It can affect both indoor and outdoor plants, and it commonly appears as a white or gray powder on the leaves and stems.

Having high humidity or bringing in an infected strain will speed up the outbreak of PM in your facility. PM spores are all around us and are most likely brought into the facility via infected clones, on employees’ clothing, or from unfiltered air that is brought into the facility. It’s when the environment is unstable, or your plants become stressed that PM spores are able to germinate and grow on the plants.

Preventing Mold Outdoors  

Anyone who has done any gardening knows that mold can be a serious problem. A user asked, “What precautions can I take to prevent mold in an outdoor grow?”

In response, we noted that to help prevent mold from growing in your outdoor production, take these simple precautions: 

1. Proper plant spacing is important to limit leaf wetness. By having plants that are spaced properly, you can prevent leaves from becoming too wet. This will help your plants stay healthy and avoid problems caused by excess moisture.

2. Limit leaf wetness by utilizing drip irrigation, or you could even grow under a high tunnel. This eliminates wasteful runoff and keeps the leaves dry, which reduces the risk of mold problems.

Preventing mold and PM outdoors can be more difficult than in a well-designed sealed environment. This is simply because indoor growing gives you total control over your climate and because you are not bringing in outside air along with its contaminants.

Positive Pressure Or Negative Pressure? Benefits and Drawbacks of Each

A sealed room will provide a more precise and stable environment, without being as influenced by the outside conditions as a room cooled with outside air. We recommend sealed environments for commercial grows with the ability to bring in outside air on demand. 

The main benefit to using outside air for cooling and dehumidifying a small grow room is it can be simpler and cheaper than ACs. They size cooling and dehumidifiers for heat and moisture load, not necessarily aiming for a specific ACH, and use fans or air scrubbers if more air movement or filtration is needed.

In addition, positive pressure can push contaminates out of the room to other spaces as well as deplete injected CO₂. Negative pressure will also deplete injected CO₂ and potentially bring in contaminates from uncontrolled infiltration.

CO₂ Safety for Indoor Cannabis Growers

If you’re not supplementing with bottled CO₂ you have to ventilate. (Although there are a lot of benefits to sealing it up and supplementing with CO₂ instead). Your plants will deplete the CO₂ faster than you think! It’s important to understand you’re exposing your plants to whatever is outside, good or bad. Implementing an air sanitization system (like Air Sniper UVC technology) can help you get in front of potential risks.

On the topic of CO₂, another user asked if a carbon filter is all that is needed for his intake or if he needs anything else.

“A carbon filter is a great particle and odor filter, but it doesn’t sterilize or filter out fungus or pathogens. Inline HEPA filtration or UV would be a great addition to your intake to protect that crop.”

Carbon filters can’t remove all contaminants, but there are other filtration and sanitization options available as well. For example, HEPA filters are designed to capture very small particles, making them an essential part of any comprehensive filtration system. Be sure to regularly replace your filters to maintain optimal effectiveness.

Furthermore, UVC bulbs sanitize the air by killing pathogens rather than filtering them out. You should pay attention to the dwell times that different brands advertise, though. Insufficient dwell times will not allow the pathogens sufficient time to be exposed to the UV lights to be effective in killing them. 

What to Do About White Dust From the Humidifier?

One user commented they had noticed a white, powdery substance on the surfaces of his humidifier. We explained that it is most likely mineral deposits from the water used in his humidifier, and while it’s not harmful, it can be unsightly. 

Fortunately, reverse osmosis (RO) water should help, but RO does not remove all the minerals. We commented that it seems some are still in this user’s humidification water.

Another option is to look into a steam humidifier such as the Surna by Anden S35FP which can use tap water and will not leave white dust on surfaces like an ultrasonic humidifier. 

AC Keeps Freezing up During the Winter Causing Humidity to Spike

Another user noted that their air conditioner (AC) keeps freezing up during the winter and causing humidity problems. Our response was that it’s best to keep their system tuned and cleaned to prevent these problems:

“It sounds like your refrigerant pressures are dropping too low with the low ambient temperatures, it makes sense that this would happen primarily during veg when you are using it for a longer period of time and into the cooler parts of the early morning/night.”

Traditional ACs are meant for comfort cooling and don’t do well when it gets cold. Even those with low ambient operation have limitations. It is recommended to have your refrigerant pressures checked to make sure you have a good charge, and it’s a good idea to ensure your unit is outfitted with a low ambient kit for cooling. You can also contact us and we can help you find a unit that’s meant for operation in winter.