How to Prevent Disease, Pests and Fungi in Marijuana Grow Room?

Like human bodies that are constantly thwarting off pathogens, such as the common cold virus, plants are also under constant attack.

When growing cannabis we will observe many pathogens which threaten our plants. Insects like spider mites, thrips, whiteflies, leafminers, etc. and fungi like powdery mildew, botrytis, downy mildew, pythium, are the main enemies for both indoor and outdoor growers.

Securing Your Grow Space


Building a secure grow space is imperative to protecting your crop from pests. A well-built grow room will provide a impenetrable barrier between your plants and the outside world.

When constructing your grow space, the two most important questions to consider are:

  • Is the room is sealed off from the outside environment?
  • How easy the room is to clean?

Mylar is an excellent option for both of these needs. The white or silver reflective material is easy to clean and apply around the walls of your room. Just be sure to take accurate measurements and overlap the edges. When considering a doorway, you can either build a frame for an actual door or buy a zipper that can attach to the mylar walls.

Pest prevention for indoor cannabis plants

Although these may seem basic aspects for the experienced grower, let’s recall some of the things you can do to avoid problems with pests in your indoor grow room during the hottest months of the year. As mentioned above, spider mites, thrips and white flies are usually the most common pests at this time of year.

Ditch the Debris

While some websites will tell you that using mulch is a good way to protect and insulate you soil, it can actually have the opposite effect.

Insects and fungi enjoy hiding in dark corners, under decaying leaves or rotting mulch.

Unless you are 100% sure that the mulch that you are using is organic and will not permit external forces gathering, it is best to keep the top soil completely clean.

Temperature and Humidity

Temperature and humidity must be kept at certain levels to avoid either the reproduction of insects or fungal diseases. Using air conditioning, cool tubes (air cooled reflectors), improving the ventilation system (take caution with RH drops), using LED panels or CFL lamps or using cold-vapour humidifiers are some of the things you can do to keep these levels under control. You can also put some anti-thrips netting in every air intake/outtake so these insects can’t enter the grow space. Increasing the watering frecuency may also help to avoid dehydration by the plants; weak, dry plants are much more prone to be infected.

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Ventilate and Circulate

Most experienced cultivators understand the value of a grow room with a proper air circulation setup. This helps your plants in a number of ways, not least through the prevention of fungi and pests latching themselves onto your plants. Pests will be literally blown off your grow by the wind, while fungal spores struggle to settle in breezy conditions.

Ventilation also keeps the humidity of a grow room down. Mould thrives in humid conditions, so by investing in a vent fan to remove that moist air, you will remove the danger of mould settling on your plants.

Choosing the correct strain to grow indoors

It is crucial if you want to successfully harvest your plants, especially if you’re growing indoors. You must choose strains naturally resistant to pests, like Heaven’s Fruit or Easy Haze. In this way, many problems can be avoided from the very beginning, since any of the insects mentioned in this article can devastate your plants in much less time than that needed to complete the bloom phase. Furthermore, as often happens, you’ll also increase the chances to harvest without problems by choosing early flowering strains.

Natural products for pest control

We’ll take a look now at the most commonly used natural products to prevent plant pests (we’ll also see some chemical compounds used for this purpose). Note that there is a myriad of products on the market and homemade recipes for pest management, we’ll just list some of the most popular, effective and easy to find ones. Also, keep in mind that a wetting agent greatly improves the properties of some of these products, as well as using RO water. Furthermore, some of them may degrade with light, so they must be used during periods of low light intensity.

Neem Oil

Vegetable oil extracted from the seeds and fruits of the Neem tree. It is widely used as insecticide against spider mites, white flies, aphids or thrips, either larvae or adults. It is toxic to insects upon direct contact and it is used via foliar or in the irrigation water. It is also a good preventive product against moulds like botrytis, mildew or rust. It can be used along with other insecticides like potassium soap. Do not use within 15 days prior to harvesting your plants.

Potassium soap

Made from potassium salts, it is used to treat insects like white flies, aphids, thrips or mealybugs. It acts when touching the insect and can be used until one week before harvest.

Cinnamon extract

Extract made from the inner bark of the cinnamon tree. It is effective to treat a wide range of mites while promoting the regenerative and antioxidant activity of the plant, thus improving its defences against stress sources. It is also used to treat infections of powdery mildew. Do not use within 10-15 days prior to harvesting your plants.

Chrysanthemum extract

Also called pyrethrins, this extract has been used since 1000 BC to treat infections of aphids, thrips, caterpillars, mealybugs or white flies. It can be used mixed with potassium soap to obtain a more potent product. It acts when touching the insect and can be used until 10 days prior to harvesting your plants. It is a photosensitive product, so use in hours of dark.


Micronized sulphur can be used by either covering the plant with the powder or by using sublimators, which heat up the sulphur until reaching its sublimation point. It is used against insects and moulds, although only during the growth stage. It is then a wide spectrum preventive product, although you must be sure of what you’re doing before using it.



Also used to correct deficiencies of this nutrient and to promote the growth of the root system. There are a number of copper-based fungicides on the market to treat infections of botrytis, alternaria or rust, also several pathogenic bacteria. Use with low light intensity to maximize its effectiveness, and avoid using amino acids at the same time than copper.


It’s a substance produced by bees to seal their hives. It has antibacterial and antifungal properties, used by many growers to prevent moulds like botrytis or powdery mildew. It also contains vitamins B, C and E and is an effective tissue regenerator (ideal for mother plants).
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Bees produce propolis to seal their hives (Image: Flickr)


Is an extract of equistaceae plants which contains saponin and flavonoids. It reinforces the structure of the plant and has antifungal properties. It is important to use it alone, without mixing with other products.
Bacillus Thuringiensis: It is doubtless one of the most widely used products to prevent and treat caterpillars. This product prevents the insect from feeding from the plant. Thus, by using it you’re also preventing botrytis infections, which often come together with caterpillars. It is important to start the treatment during the growth stage, especially if you’re growing outside.


This beneficial fungi colonizes the root system of plants and protect them from harmful pathogenic fungi. It also promotes root development by improving the production of plant hormones. It is normally used by mixing it with the irrigation water and is effective to prevent fusarium, pythium or rhizoctonia among others. Since fungal infections in the root area are almost impossible to treat, using trichoderma as preventive is highly recommended.

Natural predators

Besides this products, you can also use natural predators which feed on insects like spider mites or aphids. Insects like Phytoseiulus Permisilis (spider mites), Amblyseius Californicus (spider mites), Orius Laevigatus (thrips), Encarsia Formosa (white flies) or Aphidoletes aphidimyza (aphids) will help you to control diverse pests. Carefully check the instructions if you’re plannig on using any of the aforementioned products together with predators.


While we never encourage the use of chemicals, we understand that sometimes the grower has no other option. Please do not use these products unless strictly necessary, especially outdoors. We recommend using penconazole to prevent and treat powdery mildew, spiromesifen for spider mites, triazole fungicides for botrytis or powdery mildew, thialcropid for white flies or imidacloprid for thrips. We do not recommend to use these products during the flowering stage.


A little bit of effort goes a long way when it comes to protecting your indoor garden space. Without the natural predators that are native to the outdoors, it is imperative to keep your garden free of devastating pests. Keep your garden clean and happy, and your plants will follow suit.

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