Fungus Gnats - How To Prevent And Remove From Your Grow Room

Fungus gnats are a common indoor grow room pest, and can be incredibly frustrating.

These small, black, mosquito-like flies reproduce rapidly and endlessly. If you do not take swift action at the first sign of an infestation, you are in for a world of trouble.

Regardless of whether you are growing indoors or outdoors, you can run into these pesky bugs. Luckily, treating your infested grow room or garden can be pretty simple. We will give you a number of ways to eradicate them, ranging from standard pesticides to beneficial bugs.

First, though, we need to go into more detail into what these bugs actually look like, and how to identify them. There are also a few things you need to know to ensure you aren't inviting them into your garden.

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Fungus Gnats - How To Prevent And Remove From Your Grow Room


Fungus gnats are a common indoor grow room pest, and can be incredibly frustrating.

These small, black, mosquito-like flies reproduce rapidly and endlessly. If you do not take swift action at the first sign of an infestation, you are in for a world of trouble.

Regardless of whether you are growing indoors or outdoors, you can run into these pesky bugs. Luckily, treating your infested grow room or garden can be pretty simple. We will give you a number of ways to eradicate them, ranging from standard pesticides to beneficial bugs.

First, though, we need to go into more detail into what these bugs actually look like, and how to identify them. There are also a few things you need to know to ensure you aren't inviting them into your garden.

What are fungus gnats?Fungus Gnats Infestation On Cannabis

As briefly mentioned above, fungus gnats are tiny black flies, and appear mosquito-like sometimes. They are also commonly confused as fruit flies, but using a microscope, they can be easily differentiated.

The fungus gnat life cycle

This common indoor and outdoor garden pest has an endless life cycle. Adults will lay their eggs in your soil, and these eggs hatch into larvae.

The larvae will then feed on your root hairs. This causes serious damage to your plant.

The larvae will grow into a maggot after a few days, and cocoon itself. It then hatches into an adult fungus gnat, and the life cycle repeats.

What do fungus gnats do to plants?

Yellowing of cannabis leaf from fungus gnat infestation This is the damage of a fungus gnat infestation.

We just touched on it a bit, but fungus gnats do their damage in the soil and beneath. The larvae feeds on delicate roots, and causes irreversible damage in some cases.

This damage results in stunted plant growth and yellowing of leaves due to an inability to uptake nutrients and water. As this process runs it course, the plant will slowly die unless corrective measures are taken.

Let's talk about why you may be battling these gnats in the first place.

What causes fungus gnats in the garden or grow room?

There are a number of reasons you may be seeing these guys buzzing around your plants or crawling in your soil.

A lot of it comes down to human error, but there are certain cases where they can be brought in on new plants, or on humans/animals.

Over watering your plants

One of the most common reasons you are battling fungus gnats is probably from giving your plants more water than they need. New growers are notorious for feeding plants more than they need.

This will lead to water-logging, and create a perfect environment for fungus gnats. Warm, wet soil is a sanctuary for fungus gnats to start laying eggs. Once the eggs are present, you are in for trouble.

High concentration of CO2 in the grow room

Serious growers will introduce CO2 into their grow room or grow tent to supercharge plant growth. This is a great way to maximize your yield and decrease your time till harvest, while increasing the potency of your buds.

But the very same tactics used to get the most out of your grow can actually have detrimental affects.

High levels of carbon dioxide will attract fungus gnats, and if they already exist in your grow, it will only make them happier. If you are dealing with fungus gnats, its a good idea to turn off your CO2 producers until its under control.

Learn more about how to properly use CO2 in your grow room here.

What kind of soil are you using?

There are certain soils that are more conducive to fungus gnat infestations than others. While these soils have plenty of benefits, they can also be the reason you have an infestation.

If you are using soilless mixes including peat, coco, or compost, you should be extra wary of fungus gnats.

You can learn more about the different types of soils you can use to grow with here.

Poor ventilation is a welcome mat for fungus gnats

Your atmosphere and ventilation system are crucial to preventing pests like fungus gnats from entering your grow room. If your environment is not on point, you are asking for problems.

A few things you need to be aware of is temperature, humidity, and air circulation. These can be kept under control with a ventilation system.

Stagnant air can cause lots of problems, one of which is pests and diseases. That is why it is important to circulate air with a grow room fan.

Learn more about what makes a good growing atmosphere here.

A ventilation system is essential for bringing in fresh, clean air and expelling hot, stinky air. If your system is not properly sealed, however, it can actually be counterproductive, bringing bugs and harmful contaminants in through your intake or exhaust ducting.

You can prevent this by sealing your system with a HEPA intake filter. This is a high quality particulate filter, and will stop bugs and contaminants in their tracks.

You can learn more ventilation setup tips here in our full guide.

How do you get rid of a fungus gnat infestation?

Catch Fungus Gnats with sticky traps Use sticky yellow traps to slow your fungus gnat infestation!

If you take these precautionary steps to prevent fungus gnats from finding your plants, you will seriously reduce your chances of encountering them.

However, even the best grow rooms can still fall victim to any common indoor gardening pest.

Here are a few different ways you can treat an infestation.

Sticky yellow fly traps

Your first line of defense should be sticky flip traps, preferably yellow in color. Don't ask why, but fungus gnats are attracted to the color yellow.

If you hang a few of these around your plants, they will get caught.

This will slow down your infestation and stop it from spreading, but it isn't going to do the job all on its own.

You should have these in your grow room even before you spot an infestation, as they will alert you that one is starting.

Shop All Insect Traps

Insecticides for fungus gnats

spray fungus gnats with neem oil or pesticides Make sure you completely cover the gnats to kill them!

The most sure fire way to eradicate these pests is with insecticides or pesticides. There are a variety you can use, and we even carry some of these here at Hydrobuilder.

Here are two of our favorites:

Neem oil

Neem oil is a natural insecticide, and can be used to treat a number of infestations. It can be applied as a foliar spray, or spread into the soil.

Use a fine misting sprayer, since it is a contact kill. You need to make sure you are actually covering the bugs.

Hydro Organics goGNATS

This is a fungus gnat specific insecticide, and like neem oil, can be used both as a foliar spray and a soil drench.

It is recommended to do both, to kill adult fungus gnats along with the larvae and eggs.

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Attacking fungus gnats with soil amendments

Since fungus gnats do most of their damage in our soil and root zone, it makes sense that we should take corrective action to our soil.

There are a number of things you can add into your medium to kill fungus gnats and their offspring.

Diatomaceous Earth

This soil amendment can be put on top of your soil to prevent gnats from entering the soil to lay. Diatomaceous Earth is a natural white rock made up of fossilized remains of marine phytoplankton.

It acts like little shards of glass, and cuts open bugs as they crawl around in it. It then dries them out from the inside, killing them. This makes it perfect for disrupting the life cycle of fungus gnats, and preventing those in the soil from escaping.

Growstone Gnat Nix

This popular product is made from recycled glass, and is a great solution for a gnat infestation. It has been tested and proven to be an effective top dress as a defense from fungus gnats.

Gnat Nix works similar to Diatomaceous Earth, in that it traps gnats in the soil and prevents airborne gnats from laying eggs in the growing medium.

Home remedies for fungus gnats

If you are trying to act fast, and don't have any pesticides on hand, there are a few home remedies growers have seen success with.

Hydrogen Peroxide

First, try hydrogen peroxide as a soil drench to kill gnats and larvae. This will kill any microbial life in your soil, which is not ideal, but better than your plants dying from an infestation.

Once your infestation is under control, reintroduce these beneficial microbes with a compost tea mix.

Apple Cider Vinegar

Apple cider vinegar is another great option at home because the gnats are attracted to it. Mix apple cider vinegar with 2-3 tablespoons of dish soap and place the “traps” around the plants. The gnats will get caught in the soapy vinegar and die.

Fighting fungus gnats with beneficial bugs and bacteriafight fungus gnats with beneficial bugs

Fight fire with fire!

There are certain bugs and bacteria strains you can introduce into your infested garden, and they will attack the fungus gnats. The main one growers see success with are beneficial nematodes.

Beneficial Nematodes

These are a great option if you are using an organic soil, as these microscopic roundworms will seek out and kill 200 different species of soil based insects. They attack and eat the gnat larvae, preventing further reproduction.

If you are in a sterile hydroponics setting, these won't be able to help you. However, even soilless mediums can benefit tremendously from the introduction of beneficial nematodes.

Bacillus Thuringiensis

This bacteria is used by organic farmers to control insects on crops that are meant to be consumed. The World Health Organization also uses and endorses this bacteria to control mosquitoes.

Growers have seen success when using a product with BT. Crush the product up into a powder, mix it with water, and apply to the soil.

Final Thoughts

It is important to keep a regular grow checklist, as this will remind you to check for signs of insect or disease issues regularly. When detected early, most pests and diseases can be eradicated.

If you want to learn more about other common issues, check out our full guide on common grow room insects and diseases!