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Fungus gnats are common pests of indoor plants, specifically when moisture and humidity are high. The interesting thing about fungus gnats is that it is not the actual flying gnat which concerns growers; it is the larva of the gnat in the soil which feeds on your cannabis plant’s roots.

Fungus gnats are members of the Mycetophilidae and Sciaridae family. Fungus gnats are less than 3.5mm in size, and they are greyish/ black in color. They have three pairs of legs, antennae and long transparent wings. They lay small eggs in the top layer of soil, which later hatch into larva. The larva are 6-7mm in size, and have long milky white bodies with shiny black heads.

Adult gnats live for about ten days, and lay up to 300 eggs in their lifetime. Within five to six days, tiny larvae emerge and bury down into the soil where they begin feeding on plant roots. The larva feed for 10 – 14 days before changing into pupae in the soil. After five to six days young adult gnats leave the soil and begin the next generation. The entire life cycle from egg to adult may be completed in as little as four weeks, depending on temperature.

An area infested with fungus gnats will be hosting the gnat in all stages of its life cycle. For this reason, it’s important to attack each stage to get rid of fungus gnats quickly and permanently.

They are most abundant in rich, damp soil; and they feed on root hair, fungi and other organic materials. In other words, Just Cannabis soil is the perfect environment for these little buggers, and they will be most attracted to it. Luckily, they’re not too difficult to deal with, and preventing them is even easier.



Spotting adult fungus gnats is relatively easy. Owing to their size, you will notice them flying around your grow area or running around on the surface of your soil.

The eggs and larva may also be seen in the top layer (first two inches) of the soil.

With more serious gnat infestations, your cannabis plants can develop the appearance of over-watering, nutrient deficiencies and PH problems. The leaves will show signs of ill-health, while younger plants or seedlings are at risk of total devastation.



Generally, fungus gnats thrive in environments with high humidity. Overwatering is the most typical cause of fungus gnat infestation.

Because they are flying insects in their adult stage, they are able to fly around in search of a favorable home if need be. They are attracted to the organic matter in rich soil, so keeping them at bay requires some attention.

The following are organic methods which can (and should) be put in place as a preventative measure. Prevention is ALWAYS better than cure, especially when growing with living soil and organic techniques.


Companion plants

A great environmentally friendly way of helping prevent a fungus gnat outbreak is by making use of companion plants.

Companion planting is a process whereby the growing environment is improved by planting beneficial plants close to one another. Companion plants protect other plants against pests by emitting a strong odor that is said by many to help deter fungus gnats from reaching your cannabis plants.

Some of the best companion plants for keeping fungus gnats away from your cannabis plants include marigolds, chrysanthemums, and pitcher plants.

Mulch cover

The use of a mulch cover is truly something that cannot be stressed enough. Mulch, mulch, mulch! Covering the surface of your soil with a mulch cover will prevent adult fungus gnats from successfully laying their eggs when landing on it. Quite simply put, the lifecycle of the fungus gnat cannot begin. For this reason, mulch covers remain the ultimate prophylactic for a host of pests, including the fungus gnat. For more information and ideas on mulch covers, please see the section on mulch covers.

Use a fan

Using a fan to circulate the air in your indoor grow area is of utmost importance, even without considering the matter of fungus gnats. A fan creating wind will assist in keeping the top layer of the soil dry, and will prevent adult gnats from being able to fly around and lay more eggs.



Once you’ve identified fungus gnats in your grow area, it’s time to attack the little bastards. It is essential to ‘attack’ all stages of the fungus gnat’s life cycle in order to combat the infestation most effectively.

The following are organic measures, which should collectively assist in getting rid of your fungus gnat infestation.


 Kill the adult fungus gnats

  • The logical first step would be to physically remove as many of the adult gnats as possible, and yellow sticky traps are incredibly effective in doing so. The adult fungus gnats are attracted to the yellow color, and when they land on the sticky trap they, well… stick to it. These yellow sticky traps are available at most garden centers or hardware stores in a variety of shapes and sizes.
  • Fill a small glass jar with apple cider vinegar. Pierce small holes in the lid and cover it. Place these glass jars in areas where there are high populations of fungus gnats. The gnats will crawl down into the vinegar and drown.


Kill the larvae

  • We recommend that you start by drenching the soil with a mixture of Margaret Roberts’ Biological Mosquito Insecticide and water (as per manufacturers instructions). This product works wonders as a microbial insecticide for fungus gnat larvae in the soil. It will kill off any larvae it comes into contact with, which is why we recommend that you drench your soil with it. Make sure you get all of those little suckers. After this exercise, you will want to make sure that you dry out the top layer of your soil as soon as possible.
  • A mixture of food grade H2O2 (hydrogen peroxide) and water can be used as a soil drench, and will achieve the same results as the above mentioned method. (Some of you might be thinking of your mother’s hair bleach right now, and wondering how that’s organic. Actually, hydrogen peroxide is just water with an additional oxygen molecule. Natural AF.) This mixture kills fungus gnat larvae on impact, and is neither dangerous nor harmful for your plants at all – if implemented correctly and safely. The concentration of hydrogen peroxide and its correct dilution in water is of utmost importance: if it’s too strong you will burn your plants! For more information on H2O2 please see the section on hydrogen peroxide.
  • Neem oil is another product which can be diluted with water and used as a soil drench to kill fungus gnat larvae.
  • Place slices of potato (about 3cm thick) on the surface of your potting soil, and check on them every four hours. Dispose of the larvae, rinse the potato and do it again. (According to us, this method is better suited for assessing the extent of the infestation than it is for combatting it. But hey, whatever works for you.)
  • Food grade diatomaceous earth in the dry, top layer of your soil will kill any larvae that come into contact with it. The sharp edges of the diatoms pierce the soft tissue of the larvae, as if they were crawling through shards of broken glass. Be careful not to inhale diatomaceous earth as it can damage the soft tissue in your lungs. For more information on diatomaceous earth, please see the section on diatomaceous earth.
  • Using a proper mulch cover will prevent any more eggs from being laid on your topsoil. Using a mulch cover is quite possibly the most effective way to control fungus gnats. For more information on mulch covers, please see the section on mulch covers.
  • The Hypoaspis aculeifer, more commonly known as fungus gnat predators, are useful predator bugs which can provide a slow and steady decline in the number of fungus gnat larvae. Fungus gnat predators creep around on the soil and feed on the larvae.



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