How to Deal With Fungus Gnats in Cannabis?

Ever left a damp towel in the corner of your room for too long, only to find it has somehow become a science experiment? That’s basically what happens when fungus gnats show up in your grow room. 

These tiny, flying bugs thrive in moist conditions, and once they move in, they bring their whole family. While they won’t munch on your buds, their larvae will go to town on your plant’s roots, causing all sorts of trouble. 

But don’t worry — you don’t have to let these pests take over. And that’s what you’ll learn in this article. 

What Are Fungus Gnats?

Fungus gnats are tiny, dark-colored insects that resemble fruit flies. If you've got an infestation, you’ll probably notice them bouncing around on the soil’s surface, looking like they own the place.

Unlike other pests that target your plant’s leaves or buds, these little bugs are all about the roots. And, since roots are the lifeline of your plant, a heavy infestation can do some real damage in just a few days.

Their life cycle runs about a month. It starts when a female lays up to 300 eggs in the soil during the final week of her life. Once the eggs hatch, the larvae emerge and start feasting on delicate root hairs and soft tissues. 

After two weeks of non-stop munching, they wriggle up to the soil’s surface, pupate for a few days, and then turn into full-grown gnats. The cycle repeats, and before you know it, you're dealing with an army.

The good news? Fungus gnats aren’t the worst pests you could have. A healthy, mature plant with a strong root system can usually handle them without too much stress. But if you’re growing seedlings or young plants, it’s a different story — those tender roots are prime targets, and a bad infestation can take them down fast.

Why are Fungus Gnats Problematic for Cannabis Plants

Fungus gnats are a real problem for cannabis plants, which is especially true for indoor and greenhouses. The larvae eat organic matter and plant roots, which can cause many problems that impact the plant’s health, yield, and growth. Let’s discuss the 3 main reasons why you need to control fungus gnats the moment you find them.

1. Damage to Roots and Seedlings

The main damage to plants is caused by fungus gnat larvae since they live on the top layer of the soil and feed on decaying organic material, fungi, and small roots.

Young cannabis plants are particularly susceptible due to their roots still being in development. If larvae eat these soft roots, the result can be stunted growth or the plant may die before it has a chance to grow.

Damage to the roots, even to older plants, can cause the plant to not be able to absorb water and nutrients well. If the roots are damaged, the plant will probably grow slowly and have droopy leaves and yellowing leaves. If the condition is serious, even watering won’t help.

2. Increased Risk of Root Rot

Fungus gnats enjoy damp environments, so they are a common issue for overwatered cannabis. If you notice fungus gnats around your cannabis plants, you should know that they prefer wet and moist conditions.

When the substrate is too wet, there isn't enough oxygen in the substrate, which makes the roots more susceptible to fungus. Once roots start to rot, they become soft, brown, and mushy.

When a cannabis plant suffers from root rot, it becomes unable to uptake water and nutrients from the soil. Soon, you'll see that it starts yellowing and wilting as it dehydrates. The larvae that feed on damaged roots weaken the plant further, which speeds up the process of root rot.

If you don't address this issue, your plant will eventually die.

3. Reduced Nutrient Uptake

Fungus gnats can affect the uptake of nutrients. This happens due to damage to the roots, which means that when the roots suffer damage, the plants will have difficulty absorbing essential nutrients nitrogen, phosphorus, and potassium. The plant will also suffer from slow or uneven growth.

If a plant can no longer take nutrients from the soil, growers will notice deficiency symptoms like yellow leaves, brown spots, or a generally weak structure. In addition, the plant will suffer in other ways because when it's deprived of nutrients, it will face stress from temperature, pests, and illness factors.

4. Spread of Harmful Pathogens

Fungus gnats don’t just cause damage with their feeding. They also carry harmful plant pathogens. Fungal pathogens like Pythium and Fusarium may create damping off in seedlings and cause root rot in adult plants. They can also transmit spores of these pathogens between plants.

As the larvae damage roots, they create open wounds that allow pathogens to enter and infect the plant. In a greenhouse or grow tent, an outbreak can easily occur and once it does, the entire crop can become infected.

5. Rapid Population Growth

Fungus gnats multiply rapidly, making them one of the biggest challenges in planting. They are capable of finishing their life cycle in about three weeks. Plus, these females could lay several hundreds of eggs inside damp soil. 

Also, due to how fast they reproduce, infestations can get out of hand in no time at all. You need to intervene quickly so they don’t cause mass damage. Remember — just because you’re only seeing a few adult gnats, it doesn’t mean there isn’t a huge infestation in the soil already, where larvae are feasting on roots.

6. Disruption of Indoor Grow Environments

Growing cannabis plants indoors can be quite the challenge if you get fungus gnats, but you should not panic. Here’s how they can annoy you:

• Constant Nuisance: Even if they aren’t causing significant root damage, adult gnats are a persistent annoyance, flying around plants and grow lights.
• Attracted to Grow Lights: Fungus gnats are drawn to light sources, meaning they can easily spread throughout a growing space.
• Hydroponic Risk: While less common in hydroponic setups, fungus gnats can still infest rockwool, coco coir, and other growing media, particularly if moisture levels are not controlled.

What Causes Fungus Gnats?

Fungus gnats aren’t picky — they don’t care if you’re growing cannabis, tomatoes, or houseplants. If they find what they need, they’ll move in.

So, what attracts them? Two main things:

1. Too much water in the soil – Overwatering creates the perfect breeding ground for these pests.
2. High humidity in the grow space – A damp environment encourages fungus growth, and fungus gnats are all about that life (hence the name).

Fungus gnats don’t just randomly show up for no reason. They come because your soil is full of moisture and organic matter. They start by feeding on fungus in the soil, but once that’s gone, they move on to your plant’s roots.

Since the root zone is already a warm, damp place, it becomes the perfect home for these pests to settle in, breed, and wreak havoc.

Let’s take a look at the causes in detail: 

Overwatering and Excess Moisture

Cannabis grows are often invaded by fungus gnats in the case of overwatering. These pests are like wet areas, so when the soil is too wet, the flies can breed and thrive. Female fungus gnats lay eggs in the first few inches of soil, and larvae start hatching and consuming organic matter and plant roots just a few days later. The soil gets wetter, and they reproduce faster, so the infestation spreads quickly. 

Overwatering attracts fungus gnats and weakens cannabis plants. When the soil is wet for a long time, it curtails the oxygen supply to the roots. This can cause the roots to rot and cause fungal infections. Weak roots get damaged by larvae that eat the fine root hairs and stress the plant more. This also hinders their ability to absorb nutrients.

To prevent overwatering:

• Always check soil moisture before watering. You should wait for the top 1-2 inches of soil to dry before adding water.
• Use a moisture meter if unsure about soil hydration.
• Give your plant plenty of water but only infrequently so the roots can grow strong.
• Don't mist the soil too much, as too much humidity is also favorable for gnats.
• By watering plants properly, growers can limit fungus gnat activity while helping their plants grow stronger and healthier.

Poor Drainage and Soil Conditions

Heavy or compacted soils can prevent drainage and cause moisture build-up and infestations. Also, soil mixes heavy in organic matter, peat moss, or compost will hold moisture for too long, which is ideal for breeding fungus gnats. The same goes for containers that are missing drainage holes which can allow water to pool out of sight at the bottom so fungus gnat larvae can thrive there.

To improve drainage and soil conditions:

• Use a very aerated soil mix that would drain perfectly, while holding a good amount of moisture. An excellent mixture consists of perlite, sand, coco coir, and/or vermiculite for good aeration.
• Don’t use chunky wet soils unless properly amended with anything to aerate it.
• Make sure that all plant containers have drainage holes. You can also use fabric pots that allow excess water to evaporate better.
• Elevate the pots with trays to avoid water pooling beneath.
• When you improve drainage, you lessen the chance the soil will be saturated with moisture, making it harder for fungus gnats to live and breed.

Contaminated Growing Medium

Fungus gnats can get into your growing room via soil, compost, and other organic materials. A lot of store-bought potting soil contains either peat moss, decayed plant matter, or even composted material which could already contain fungus gnat eggs or larvae at the grower’s end.

If you use dirt from outside or any untreated organic matter in an indoor grow, it’ll also increase the chance of fungus gnats and other pests like thrips, aphids, and mites. If you have a medium with fungus gnat eggs, then watering it will activate the problem because the water will get them to hatch.

To prevent contamination:

• Always buy the best sterilized soil from reputable sources.
• Keep leftover soil in closed containers so pests don't lay eggs in them.
• Do not bring outside soil or compost into indoor growing areas.
• If you are incorporating organic compost or making your own soil mix, you should consider heating the soil by baking it at 180°F(82°C) for 30 minutes to kill larvae and eggs.
• To avoid any problem from gnats, spread some new soil in a container and watch it before planting. 
• Growers can prevent fungus gnats by ensuring that grow mediums never introduce the pests into cannabis growing space.

How to Spot Fungus Gnats?

These tiny pests like to hang out in the soil, making them easy to miss. The quickest way to confirm an infestation is to check the soil — if you see tiny bugs hopping or flying around, you’ve got fungus gnats.

Another telltale sign? White maggots wriggling in the soil. These are fungus gnat larvae, the real root destroyers.

Unfortunately, fungus gnats don’t just hang around — they also cause visible damage to your plants. Here are a few signs that your soil has fungus gnats:

1. Small flying insects Around the Soil or Plants

If you see little dark flies swarming around your plants or at the top of the soil, you may have a fungus gnat problem. Fungus gnats look like tiny mosquitoes and tend to fly in short, erratic patterns — particularly when plants are disturbed. They seldom fly above their breeding ground due to weak flight coordination, making it easy to spot them inside grow rooms or greenhouses. Although they may not directly damage plants as adults, their presence shows they have already laid eggs in the soil and that the larvae are most likely feeding on the plant’s roots.

2. Larvae in the soil

The larvae of fungus gnats are like transparent tiny worms with black heads. They hang out just below the top of the soil. These larvae mainly eat dead organic matter. However, they will feed cannabis roots when they get too plentiful. To check for their presence, growers can gently disturb the top layer of the soil and see whether tiny, thread-like larvae appear. 

A good trick to check for an infestation is to place a slice of raw potato on the surface of the soil; larvae will show up on the potato in a few hours making them easier to identify. If growers ignore them, they can quickly become severe and cause root damage. 

3. Yellowing, wilting, or drooping leaves

Plants struggle to absorb moisture and nutrients as the fungus gnat larvae feed on the small root hairs and soft root structures. This stress leads to symptoms that are very much visible, including yellowing, wilting, or drooping leaves. The bottom leaves may become light green or yellow, and the plant may begin to wilt, even though the moisture in the soil is sufficient. A lot of growers misdiagnose these signs as underwatering, overwatering, or nutrient deficiencies, delaying treatments. 

Therefore, stay ahead of it by checking the soil for fungus gnats. This is essential if plants show these signs even with an appropriate watering and feeding schedule.

4. Stunted growth in seedlings and young plants

Young cannabis plants and seedlings are especially affected by fungus gnat infestations because of their fragile root systems that are attacked by larvae after they hatch, causing improper growth and sometimes death. When seedlings stop growing completely, become droopy, or die, growers can suffer serious losses. 

Newly transplanted cannabis plants and clones are also at risk since their root systems are still establishing themselves. If young plants exhibit slow growth, weak stems, or sudden wilting, fungus gnats should be suspected as a possible cause.

5. Sudden nutrient deficiencies

When roots are injured, they absorb less nutrients, which causes some deficiencies. Damage caused by fungus gnats can lead to a variety of deficiencies and yellowing in the recently affected leaves. These problems look a lot like problems with nitrogen or phosphorus deficiencies. Many growers may attempt to correct these deficiencies with nutrients, but if these issues persist after proper feeding, then there’s likely root damage due to fungus gnat larvae.

6. Presence of sticky traps covered with gnats.

Yellow sticky traps can be a simple way to confirm the presence of fungus gnats. When adult gnats are attracted to these traps and capture themselves, growers can assess how severe the infestation is. Sticky traps placed near the soil surface will indicate if fungus gnats are present in the grow room. If you see a lot of gnats getting stuck in them fast, there is an infestation that needs to be dealt with. Checking these traps regularly will indicate if they are being controlled or not.

If growers can notice these signs quickly, then they can control fungus gnat infestation issues before they cause serious damage to cannabis. However, don’t wait too long to treat them after you notice some damage, since these pests multiply quickly. There will then be root damage across many plants and growth will be stunted. 

How to Get Rid of Fungus Gnats?

The good news? Fungus gnats are one of the easier pests to deal with. Even better, their presence is a wake-up call that something’s off with your grow setup. Fix that, and you’re not just kicking these bugs out — you’re improving your plant’s overall health.

1. Stop Watering for a Few Days

Overwatering is the number one reason fungus gnats move in. So, the simplest fix? Hold off on watering for a few days. Letting the soil dry out naturally makes it inhospitable for them, forcing them to pack up and leave.

Going forward, you should rethink your watering schedule. Only water when the top layer of soil is completely dry — this improves aeration and prevents a repeat infestation.

Not sure if it’s time to water? Try the finger test — stick your finger into the soil. If it feels damp, wait a bit longer. Or, lift the pot. If it’s still heavy, hold off; if it’s light, it’s time to water.

2. Use an Electric Bug Zapper

If you’ve ever used one of those electric bug zappers for mosquitoes, you already have a weapon against fungus gnats. While it won’t wipe them out completely, it’ll help cut down their numbers. 

3. Set Up Yellow Sticky Traps

These bright yellow traps are like flypaper for fungus gnats. The gnats are drawn to them, get stuck, and can’t escape. Once the trap is full, just toss it out.

You can grab these from any gardening store for cheap. Just keep in mind that while they’re great at reducing the population, they won’t completely eliminate the problem — you’ll need to combine them with other methods.

4. Use Organic Pesticides

Neem oil is one of the best organic solutions for getting rid of fungus gnats. Another solid option is Essentria IC3, a blend of horticultural oils that works in the same way. These pesticides are plant-safe but deadly for gnats on contact.

Make sure to follow the instructions on the packaging. Also, never spray during light hours, as it can cause burns on your plant. And whatever you do, don’t spray the buds—this can ruin their flavor and aroma.

5. Sprinkle Diatomaceous Earth

Diatomaceous earth (DE) is a fine white powder that’s harmless to humans and pets but lethal to tiny pests. On a microscopic level, it’s made of razor-sharp particles that cut into fungus gnats’ exoskeletons, drying them out and killing them.

6. Point a Small Fan at the Soil

Since fungus gnats thrive in moist conditions, you can make their lives harder by directing a small fan toward the soil. The airflow helps dry out the top layer, making it less appealing for them to lay eggs.

Just be careful not to blast your plants with too much wind, or you’ll risk windburn — which is just as bad as it sounds. A gentle breeze is all you need.

7. Use Biological Control

Growers have been using natural predators and disease-causing microorganisms to manage pests and you could do the same. Introducing beneficial organisms into the grow space can help growers reduce fungus gnat numbers naturally while helping to maintain balance.

1. Beneficial Nematodes

Beneficial nematodes, such as Steinernema feltiae kill fungus gnat larvae in the soil. They are microscopic roundworms that actively seek out and kill fungus gnat larvae in the soil. They do so by entering the larvae, releasing bacteria by which they infect the organism, and killing it in 24-48 hours. Nematodes are a safe and sustainable solution that will not harm cannabis plants, beneficial insects, or humans, unlike chemical pesticides.

How to Apply Beneficial Nematodes

• Purchase live nematodes from a reputable supplier. They are usually sold in a sponge or powder form.
• Mix nematodes with water as per product instructions.
• Put the mixture in the soil while watering evenly.
• For the next few days, keep the soil moist, but do not soak it with too much water for nematodes to spread.
• Reapply every two to four weeks if necessary, especially if the severity of the infestation is high.

Advantages of Beneficial Nematodes

• This product is very effective for killing fungus gnat larvae in soil.
• Safe to use on plants, helpful insects, and pets.
• Control gnats long-term by interrupting their life cycle
• Top choice for growers seeking a non-chemical, natural solution that works at the root level to get rid of gnats.

2. Predatory Insects (Hypoaspis miles)

Hypoaspis miles, now known as Stratiolaelaps scimitus, is a species of predatory mite that feeds on fungus gnat larvae and other soil-dwelling pests. These tiny mites live in the top layer of the soil and actively hunt fungus gnat larvae, thrips pupae, and other harmful insects, helping to naturally control pest populations.

How to Use Predatory Mites:

1. Introduce predatory mites directly into the growing medium near the base of the cannabis plants.
2. Don't use pesticides or neem oil because they kill the mites too.

• Keep a medium humidity of 50-70% to keep the mites alive and working.
• Return the bugs every 4–6 weeks for protection. 

Advantages of using Predatory Mites

• They control larvae before they turn into adult gnats fast and effectively. 
• Will continue to provide continuous pest control without further assistance. 
• Works for other pests in the soil too! 
• Most suitable for cultivators with consistent pest problems that desire an extended biological pest-control solution.

3. Bacillus thuringiensis israelensis (BTI)

Bacillus thuringiensis israelensis (BTI) is a bacterium that occurs in nature producing toxins. They are terrible for fungus gnat larvae but harmless to plants, people, and good insects. When the larvae consume BTI in the soil, it damages their digestive system and kills them within a few days.

You can find BTI in biological insecticides such as Mosquito Bits, Gnatrol, or Microbe-Lift BMC, which can be mixed with water and applied to the soil.

How to Apply BTI

• Dilute BTI granules or liquid in water as the product label directs you.
• Soak the soil with the BTI solution to infect the fungus gnat larvae.
• Apply once more every week to 10 days for newly hatched larvae.
• Apply BTI solution to the soil as a preventative measure.

Advantages of using BTI

• Helps in killing Fungus gnats larvae rapidly.
• It's safe for your cannabis, people, and beneficial bugs.
• Can be used as a preventative for long-term gnat control.
• Ideal for growers who want an organic, user-friendly solution that specifically addresses fungus gnat larvae found in soil.

Choosing the Right Biological Control

Using just one biological method has its pros and negative side effects. However, you will never have to worry if you use multiple biological methods. For example,

If there is already an active infestation, use BTI or beneficial nematodes to get rid of the larvae quickly. Here are a few things to keep in mind:

• Introduce predatory mites for long-term control of fungus gnats.
• To avoid larvae from growing, drop in BTI every few weeks.
• Cannabis growers can use biological control methods to manage infestations of fungus gnats without the use of toxic chemicals. 

How to Prevent Fungus Gnats?

Fungus gnats are a headache, but once you’ve kicked them out of your grow room, the last thing you want is for them to return. Here’s how to make sure they stay gone for good.

1. Water Less Frequently

Overwatering is the root of the problem — literally. To prevent fungus gnats, you need to keep the soil dry enough to discourage them from laying eggs. Only water when the top layer of soil is completely dry, and make sure excess water drains properly.

Every strain has different watering needs, but your plant will let you know if you’re overdoing or underdoing it. Drooping leaves? Might be too much water. Dry, brittle leaves? Not enough. Adjust accordingly.

2. Cover the Soil

Fungus gnats lay their eggs in moist soil, so blocking their access is a simple and effective way to keep them out. You can cover the soil with:

• Gravel
• Sand
• Perlite
• Breathable fabric

3. Keep the Grow Room Clean

Decaying organic matter is a buffet for fungus gnats. Any dead leaves, plant debris, or old soil sitting around can attract them, so keep your growing space tidy. Regularly remove any fallen plant material and ensure your grow medium is fresh and clean.

4. Sterilize Your Soil

Fungus gnats can sometimes hitch a ride in your soil when you buy it, especially if it’s from a local vendor. To be safe, sterilize your soil before using it.

Here’s how:

1. Place the soil in an oven-safe container.
2. Bake it at 82 to 92 degrees F for about 30 minutes.

This will kill off any hidden eggs or larvae, preventing a surprise infestation.

5. Use Yellow Sticky Traps

While sticky traps won’t stop fungus gnats from coming in, they’re great for early detection. If you see a few gnats stuck to the trap, it’s time to check your plants for any signs of a larger problem. Catching them early makes dealing with them much easier.

Summary: How to Deal With Fungus Gnats in Cannabis?

Fungus gnats might not be the worst pest out there, but they’re definitely one of the most annoying. The good news? Getting rid of them is pretty straightforward — fix your watering routine, keep things clean, and use a few well-placed traps. Do that, and your grow room will be a no-fly zone for these pests.