How to Prevent and Control Fungus Gnats In Your Cannabis Growroom

By Monica Mansfield
Published: February 26, 2021 | Last updated: May 25, 2021 11:24:52
Key Takeaways

Fungus gnats are a common pest, especially in indoor gardens. While mostly harmless, large infestations can damage plants. Fortunately, as Monica Mansfield explains, fungus gnats are fairly easy to eradicate from your growroom.

There is nothing worse than working in an indoor garden full of fungus gnats. These tiny, mosquito-like creatures can be seen running across the soil surface and flying erratically around your plants. Once they infest your garden, they seem to show up everywhere from your kitchen counter to your windowsills. And, of course, they’ll fly all around you and give you the creepy crawlies.


Outdoors, there is enough room to roam that they don’t tend to be an issue. However, they love to feast on the plants in your growrooms and greenhouses. They tend to be more annoying than harmful, but large infestations can damage your plants. Luckily, once you understand them, they are relatively easy to prevent and control.

What are Fungus Gnats?

Fungus gnats are small flies that tend to develop in the growing mediums of indoor plants. Adults are about ⅛- to ¼-inch long with long legs and antennae. Larvae look like small translucent worms with black heads. Eggs are tiny, smooth, oval, and a transparent white in color. They are weak fliers and tend to stay in one location, however, if a large group of fungus gnats decide to move spots, they will synchronize their flight pattern and resemble a snake as they travel.


Fungus gnats have about a three- to four-week lifecycle, depending on the temperature and other environmental factors. They develop through four stages: egg, larvae, pupa, and adult. Throughout their lives, female adults lay about 100-300 eggs in damp, organic media where the larvae will be able to feed on fungi, algae, organic matter, and plant roots. The larvae are usually located in the top two to three inches of soil and are fully grown in two to three weeks. Then they spend about a week in their pupal stage before emerging as adults that live for another seven to 10 days.

Fungus gnats are generally harmless in small numbers, but a larger infestation can damage your plants, especially seedlings and younger plants. When fungus gnat larvae feed on plant roots, you’ll notice yellowing or wilted leaves and poor, stunted growth. They also spread plant pathogens which can lead to other diseases. Although it is possible, you will rarely see serious damage from fungus gnats outdoors. It is most common in greenhouses, nurseries, sod farms, and indoor environments.

a fungus gnat


How to Prevent Fungus Gnats

In order to control these pests, it is necessary to understand what causes infestations. The most common culprit is overwatering. Fungus gnats thrive in moist environments. Their larvae feed on fungus to survive, which requires moist conditions. It’s important to let soil, especially in indoor plants, dry out between waterings. Along the same lines, don’t allow water to sit in the saucers underneath the pots. If your plants are in large pots, you can use a shop vac to remove the water.

By allowing the medium to dry out, you significantly decrease the survival rate of eggs and larvae. This also deters adults from laying eggs in your medium in the first place. If you already have an infestation, this won’t be a quick fix, but you’ll see the fungus gnat population decrease over time. If you want to speed up the process, you can repot your plant into new soil and be sure not to overwater it. However, this may also stress your plants and isn’t always recommended. The solution shouldn’t cause more damage than the problem.


Read also: Companion Plants for Cannabis

Also be aware that as growing mediums age, they tend to retain more water which can affect the length of time they need to dry out. If your root systems are also growing substantially, this may not be an issue since your plants will be taking up more water.

Another common cause of infestations is bringing outdoor plants indoors. If you want to test a plant for fungus gnats before bringing it inside, you can use a potato slice to detect them. Insert ¼-inch slices into your growing medium and then turn them over to check for gnats after a few days. Fungus gnat larvae will be attracted to the potato to feed on it if they are present in the medium.

If you adjust your watering practices and your infestation still doesn’t clear up after several weeks, it may be time to use an insecticide. If your fungus gnat population is large, you may need multiple applications to clear up the problem.

fungus gnat on cannabis

How to Control Fungus Gnats

Pyrethroid-based insecticides that contain bifenthrin, cyfluthrin, permethrin, and lambda cyhalothrin are effective against fungus gnats. They work by disrupting their nervous systems and subsequently cause paralysis and death. Although generally safe if used properly, pyrethroids have been known to cause some damage to humans and the environment.

If you decide to use these chemicals, make sure you follow the directions closely and take all necessary precautions to minimize these risks. This may include wearing protective gear like gloves and goggles, choosing pesticides with lower toxicity levels, using the correct dosage, staying out of the treated area for the required time listed on the label, and ventilating indoor areas.

Bacillus thuringiensis var. Israelensis (Bti) is a bacterium found in soils that is commonly used as a pesticide by growers. Pesticides that contain this active ingredient can be used as a soil drench to kill fungus gnat larvae. It works by excreting toxic chemicals once it has been ingested by a pest, and is generally safe for humans. Apply Bti every three to seven days to control the population.

Neem oil is a natural extract from the seeds of the neem tree that has a wide variety of uses, from beauty products to pesticides. It is one of the safer products to use for pest control in your garden and will not harm pollinators, although you will probably need multiple applications to eradicate your problem. It can be used as a soil drench for larvae, and as a spray to repel adult gnats.

Read also: Identifying All-Natural Pesticides and their Benefits for Cannabis Plants

When choosing your neem oil product, be sure to choose a cold-pressed neem oil, which is less processed than clarified hydrophobic neem oil. Cold-pressed neem oil is the most natural and complete form of neem oil and still contains azadirachtin, which is a key ingredient to fighting pests. Azadirachtin acts as a poison to insects when they ingest it. It is also a growth regulator and prevents insects from transitioning into their next stage of development and laying eggs.

Yellow sticky cards are helpful when controlling adult fungus gnat populations. They are attracted to the yellow color and will land on the card. Place the sticky cards under the canopy at the edge of your container. By trapping adult females, you’ll prevent them from laying their eggs and reduce their population.

You can also add beneficial nematodes, such as Steinernema feltiae, or predatory mites to your garden to feed on the fungus gnats. These biological controls are most effective in the beginning of the growth cycle at planting, before fungus gnat populations get out of control. You’ll want to apply them two or three times, about a week apart. Predatory mites will last about six to eight weeks.

Beneficial nematodes reproduce and can potentially last through harvest if their population is established early on.

Fungus gnats are annoying, but relatively easy to manage and control if you understand them. If you make sure not to overwater and use a few simple controls, you can keep them from calling your garden home.


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Written by Monica Mansfield | Homesteader, Owner & Writer of The Nature Life Project

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Monica Mansfield is passionate about gardening, sustainable living, and holistic health. After owning an indoor garden store for 5 1/2 years, Monica sold the business and started a 6.5-acre homestead with her husband, Owen. She writes about gardening and health, as well as her homestead adventures on her blog at

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