Tips for Preventing Mold Growth in the Grow Room

By Cory Hughes
Published: May 15, 2017 | Last updated: April 23, 2021 02:58:06
Key Takeaways

Mold is a sneaky thing. It can find ways to infiltrate your perfectly clean growroom. Thankfully, you have the ultimate power to control this Sneaky Weasel of Indoor Gardening.

Even if your indoor cultivation area is clean—I mean really clean—mold can still rear its ugly head.


Mold is one of the biggest preventable problems experienced in most gardens and growrooms. It can ruin a perfectly good crop before you even realize you have a problem.

There are a handful of different molds common in cultivation, and they are all introduced and spread through circumstances completely controllable by you.


So, what can you do about it?

Before we talk about how to control mold in your garden, let’s look at what exactly mold is and what types you are most likely to find in your garden. Mold is a fungus. It is a living, breathing life form, and a common plant pathogen.

While there are many different fungi and molds encountered in nature, there are only two types you should come across in your growroom that will negatively affect your plants. The most common type is powdery mildew.


Powdery mildew is considered a fungal disease, and it is caused by several different types of spores (the primary culprit is called Podosphaera xanthii). Powdery mildew appears as a white powder, primarily on your leaves. The white on green is easy to spot, so catching powdery mildew early shouldn’t be a problem.

The next type of mold that can greatly affect your harvest is white mold. It forms on the fruit of your plants and looks like small puffs of cotton in the beginning. As it becomes more advanced, it can lead to cankers and a number of other problems.


We have all seen what happens to fruits and vegetables that we leave in the bottom drawer of the fridge for too long. The same thing can happen on the vine if the problem isn’t caught in time. Allowing white mold to spread can result in diseases ranging from crown rot to stem infections.

So, if you start with a clean room, how does mold find its way into your garden? Contamination. Contamination is the biggest threat faced by your garden. Every time you enter or exit a room, you take something with you and you leave something behind.

Unfortunately, when you are in there trying to flex your green thumb, the things you drop off can have adverse consequences. When you walk across a lawn or stand near a tall tree, you pick up mold, pollen, and a slew of other contaminants. The impact to your plants can be devastating, hence the need for ongoing observation and maintenance.

Once mold spores are introduced, they require fairly warm and moist conditions to thrive. The surefire way to stop the spread of mold in its tracks is to remove the environment favorable to fostering its growth.

Sounds simple enough, right? Maintain the proper temperatures with a nice neutral humidity level, and the spores won’t be able to take over.

If you have your environment on lock down, you shouldn’t have a problem. Unfortunately, balancing temperature and humidity is the ongoing battle faced by most amateur horticulturists.

To prevent mold, you need to make sure your air is flowing and excess moisture is kept to a minimum. One of the worst things you can do is leave unnecessary pools of water on your trays and floor.

Excess water leads to mold; there is no way around it. If you don’t have the benefit of a pump and drainage system, make sure excess water is vacuumed up every time you feed. If water soaks up into your drywall, it could cause black mold to form along your baseboards.

This, combined with your good airflow, which, paradoxically, helps spread spores when you get a mold infestation, will act as a source of contamination until it is repaired. You really don’t want to let a problem get to that point. An ounce of prevention is worth a pound of cure.

Once you find you have powdery mildew or the beginnings of a mold infestation, don’t panic. If you can manually remove infected leaves, that’s a great place to start. If the problem spreads, use an organic fungicide. Many like to avoid pesticides and fungicides, but there are a lot of perfectly safe organic products that work wonders in the garden available at your local grow shop.

One of the absolute best fungicides available is a concentrated hydrogen dioxide (H2O2) solution, sold commercially under a variety of different names. When applied by foliar application, it instantly dries out the spores, killing their ability to reproduce. Once the spores lose that ability, it is only a matter of time before they themselves die.

Hydrogen dioxide (also called hydrogen peroxide) also functions as a general disinfectant and if it’s applied to your clean, dry workspace before your first plant goes down, you increase your chances of a clean harvest. Another incredible method of mold control on your plants is the use of beneficial fungus.

Bacillus amyloliquefaciens, often referred to as Bacillus A, is a beneficial microbe available over the counter. Coat your garden with a layer of these beneficial fungi, which will then form colonies. These colonies, which are invisible to the eye, kill competing spores and prevent them from taking hold and forming colonies of their own.

If you are just setting up, take preventative steps to ensure you don’t run into problems with mold. Start with a clean, dry room and make sure you have plenty of airflow. Make sure you have no leaky windows and do the absolute best you can to avoid inadvertent contamination from outside sources.

If you are so inclined, or have concerns about mold, give your garden a pre-transplanting wash with a solution of H2O2. Keep your trays and floor dry and if you feel the need, throw Bacillus A into a bi-weekly treatment schedule. Keeping mold at bay isn’t difficult, it just takes diligence and a watchful eye.


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Written by Cory Hughes | Commercial Grower

Profile Picture of Cory Hughes

Cory Hughes is a former police officer turned full-time commercial grower in Denver, Colorado.

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