9 Best Ways to Kill Fungus Gnats on Cannabis

So there I was checking my plants after a hard days work.

Everything was going smoothly or so I thought.

Until I noticed something in the corner of my eye.

It couldn’t be.

It was.

There were FUNGUS GNATS on my plants!

These little buggers have decided to invade my plants right under my nose how could I have missed these bastards.

And so started the war.

But I have good news.

I won!

I was able to save my plants and have an OK harvest.

So not to worry if you have fungus gnats on cannabis plants there are simple ways to eradicate these F*&^ers and save your plants!

You will learn everything you need to know about fungus gnats on how to identify and determine if you have fungus gnats or another type of fly.

The causes of fungus gnats and how to prevent fungus gnats on marijuana and most importantly.

How to kill fungus gnats for good!

Now let's get right into it,

What are Fungus Gnats

Fungus gnats are tiny black flies that if seen with a magnifying glass resemble the appearance of mosquitos.

When I mean tiny, they are anywhere from 1/16 of an inch to ⅛ an inch tall. 

Where do fungus gnats come from ?

Gnats are your most common pest in the house when you bring home plants from the store or flowers they usually appear after a couple of days.

Then they start to go to work. In not only your houseplants but can make their way to the grow room.

How to Identify Fungus Gnats on Cannabis

A lot of people including myself have been confused if they have fungus gnats or fruit flies.

At first, that's what I saw or thought when I first saw them years back. Not Fungus gnats.

Fungus Gnats vs. Fruit Flies

Fungus Gnat


Fruit Fly


You can see the difference between the two different insects; one is black in color the other is more of a yellow, orange color.

Not only visually are they different they also feed and live differently.

The fruit fly lives around rotting fruit and vegetables.

Fruit Flies will lay eggs on the skin of the fruit such as bananas and not in soil.

Fungus gnats live around damp soil or coco coir, rock wool and can also be gray and brown in color.

Lifespan and Damage Fungus Gnats can do to Marijuana

The life cycle of the fungus gnat is an endless cycle unless it is stopped or they run out of food.

It all starts with the adult flies that are about a ⅛ inch in size and lay their eggs. Which then hatch in 4- 6 days.



The fungus gnat larvae are born, and they will immediately start to feed on the root hairs on your marijuana roots.

The larvae grow into a maggot like a form and are visible. Become fully grown within two weeks.

After that the cocoon and turn into the notorious fungus gnat in 7 days. And well you get it, the cycle continues.

Referenced from the University of Vermont

Fungus Gnat Damage

A simple question anyone can ask are fungus gnats harmful to cannabis? The answer is YES!

Fungus gnats infest the soil and decomposing organic matter.

The larvae feed off this and also eat roots hairs of your new marijuana seedlings or your growing plants.

Fungus gnat damage stunts growth and can start to turn leaves yellow. 

The plant slowly dies as the roots cannot uptake the nutrients and water it needs due to root damage.

Causes of Fungus Gnats on Cannabis


Damp growing mediums caused by over watering.

It's one of the primary reasons.

So it's critical to control your water and how saturated the plants get.

Fungus gnats are also attracted to excessive carbon dioxide or CO2.

Have you ever been is your house or outside and little flies come up to your face. Annoying yet this is a great warning sign!

If you are growing indoors, turn off CO2 tanks so you can root out the problem before your roots become the problem!

Make sure your grow tent or grow room is properly sealed.

Believe it or not, they are attracted to soil mixes with compost or peat.

Saturated coco coir medium.

Houseplants and flowers bought at stores which were not properly looked after.

Open windows without screens.


Now onto my favorite part, and what you have been waiting for,

How to Kill Fungus Gnats


When I realized these flies were not fruit flies I immediately took action.

I knew they were already in my crop. So that was my priority.

Obviously, I have to protect my medicine at all costs.

I sealed the room and started to see if there were any air holes from the outside coming into the grow room and tents I had setup.

Also made sure all windows were sealed and closed properly.

Then I tried to see if all my plants are infested or just a one.

From what I could tell a couple had been infected.

There are many ways to kill fungus gnats you can go the natural way of remedies.

Or if you do not want to mess around go with organic food safe foliar sprays.

Each method is safe for your marijuana plants.

So not to worry if you are growing medical marijuana.

I will go over some of the best ways to get rid of fungus gnats on your cannabis.

Then I will give you my exact steps I used to win the war.

Home Remedies for Fungus Gnats

Yellow Sticky Trap paper is a great first solution as supposedly theses flies are attracted to the color yellow.

These fungus gnat traps are perfect cheap first defense.

Just place theses traps around your infected plants and watch them stick and die!

Apple cider vinegar smell also attracts the fungus gnats.

Mix in a jar or cup or bowl, the apple cider vinegar about two tablespoons or more with some dish soap and water. Then place it around your plants.

Hydrogen Peroxide can be used if the fungus gnats have infected the soil and you suspect larvae are growing.

One of the ways to get rid of the fungus gnats in larvae stage is hydrogen peroxide.

  • But beware it can harm the roots if used to much.

Make sure you dry out the soil, then mix 1 part hydrogen peroxide to 4 parts water and then water your infected cannabis plants.

If it starts to foam, it is killing the larvae.

Sand this another significant step that can be done when transplanting.

Place and inch on the bottom and top of your pots. It acts like shredded glass, and they cannot get inside it impenetrable.

Preventive Measures

The growing trend in the growing community is the use of “living organisms and bacteria” to help fight pests and insects in the garden.

One of them is nematodes,

Beneficial Nematodes are microscopic roundworms that seek out and kill 200 different insects in soil or growing medium.

That's 200!. No wondering it is starting to be a trend.

Not only are they great a getting rid of fungus gnats but are also beneficial to kill many other pests.

The great thing about them is they do not bother ladybugs or earthworms. Simply mix with water and water your plants.

Here is short video about using nematodes 

Natural Organic Remedies

Neem oil can be used on fungus gnats it is an effective solution.

Mix with water and soak the plants after the soil has been dried out for at least two days. The same as hydrogen peroxide.

​Bacillus Thuringiensis

A bacteria called Bacillus thuringiensis, it's used by organic farmers to control insects that eat crops.

Also by the World Health Organization to help control mosquitoes.

The bacteria is is an another safe alternative can be used and when growing cannabis.

The bacteria comes in pellet form like:

Mosquito Bits 


Just crush up some pellets and place them in your growing medium when transplanting or even on top when an outbreak hits.

It wipes out the fungus gnat larvae and does not harm the roots of the plant.

Diatomaceous Earth is a food supplement that contains silica and is also good at destroying fungus gnats.

It has a whole lot of beneficial properties to humans and pets

  • Used to detox the body and kill parasites/worms
  • Removes heavy metals scrub the intestinal walls

Nicknamed the “grass of the sea”

Mix Diatomaceous Earth in with your soil for maximum benefits.

For proper handling here is a resource from: The International Diatomite Producers Association (IDPA).

Growstone Gnat Nix is made up of recycled glass this a a excellent solutions for your gnat outbreak.

The Gnat Nix has been tested in Universities and is proven to be an effective top dress on your plants  against fungus gnats. 

This acts like sand or diatomaceous earth at keeping the gnats inside and the soil to die as the turn to adults.

Also you keep them from laying eggs on the grow medium.

If you have on hand some SM-90 spray this can also work in a pinch and is also perfect for another issue in the garden such as powdery mildew.

How I got Rid of Fungus Gnats

The battle plan that I found to be the best for me was to isolate the major problem area, my medical marijuana.

Contained the area.

Find and isolate the plants.

I immediately used the yellow sticky trap paper by the two plants that I noticed them walking and flying around.

Then I sprayed a light diluted mixture of SM-90.

Waited a day.

The fungus gnat tape seemed to be working well and got covered in gnats.

I replaced them once more to let the soil dry out as much as possible not to stress out my plants.

Once dry enough I then made a mixture of Beneficial Nematodes with water.

I then watered my plants and sprayed another dose of SM-90 just to be safe and replaced the tape one last time.

Over the course of about a good ten days, the flies all but vanished!

I was able to stop and kill fungus gnats on cannabis.

But the most frustrating came when having to try and find the cause in my house.

I am a green thumb and like to have my houseplants as well.

That took about another week to find and get rid of the rest of the fungus gnat invasion.

Do you have fungus gnats, try some of the solutions I know you can win the war just like I have.

What solution do you use and what works for you let me know in the comments and share.

Grow on my friends.​

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16 thoughts on “9 Best Ways to Kill Fungus Gnats on Cannabis”

  1. I been fight with gnats for a long time and failed always eventually had to scrap my grow.. this time i’m gonna try Microbe lift BMC heard a lot about it..
    But my Basic question to you is All though my plants already infested by gnats larvae coz i see leaf browning and drying, will my plants again bounce back its vigour after winning war with gnats?
    Plz kindly reply waiting….

    • Hi Tushar,
      Fungus gnats can be a real pain. I have not used Microbe Lift BMC personally, I have used Nemotodes with great success. With your leaf dying,and drying going on. How are the upper leaves are they healthy? If so you plants just may bounce back. It honestly really depends on the plant. If the are strong good genetics they might . I really cannot say. I hope they do. You can also use sand on the top and bottom of your plants for extra defense to prevents these bastards from infesting your plants again.

    • I have been using a ‘living soil’ that includes red wriggle worms.
      I did notice the gnats in my
      House plants preciously although my cannabis is in a separate far away room . I use the yellow strips, tried a neem oil(which I won’t use again due to door) , a miticide and I cannot eradicate them. I have lost two ‘crops’. Do I have to
      Get rid of the soil before starting over.

  2. I’ve been extremely frustrated with a gnat infestation problem. After having put a lot of energy, time, and money to purchase a lot of indoor plants, not only to decorate the house but to enjoy more greenery at home (lifting my spirit), I felt very discouraged to have this gnats problem. I’m not 100% sure but I believe they invested my plant through a bad quality potting mix since all the plant I potted were infested very rapidly. So I repotting every plants, about 10 of them, taking drastic measures at times by ringing out the soil to the roots when possible. I placed yellow stickies which have captured many of them. And while I have seen great improvement on some plants, I’ve also still see percistent gnats on some of them, and i’m concerned they are going to expend again.

    I’ve been looking for various ways to kill the larvae and found your article/post.

    I have a question though: Are Nematodes bad for the plants. I’ve read that they are parasitic to plants and while it may kill the Gnats, it may also damage the plant? Or are there different types of Nematodes, some non invasive?

    Also, what’s SM-90? Looking online, I’m not understanding fully what it does to this fungus Gnats problem.
    Thank you and great post!

    • HI greg,

      Gnats can be a real problem and one of the main causes is potting soil. To answer your question on nematodes. Yes there are good and bad nemotodes out there. The beneficial ones are just that beneficial they eat larvae. They do not harm your plants roots. On to Sm-90 it’s wetting agent and a foliare spray. The ingredients has 1% coriander oil , it kills some insects and 94% sulphonated canola oil . I paired it with neem oil to stop the fungus gnats from running all over the plant. I had it on hand at the time. Keep growing.

  3. Thanks so much for this! I have a rooftop garden in NYC and have had a big problem w/ fungus gnats. I think i got it after I got a big batch of compost from NYC composting non-profit…timing was too big a co-incidence. My friend who is an organic farmer told me big composting companies are often not careful to heat their batches enough to kill all eggs of different pests. the gnats are now invading my apt. even WITH SCREENS AND I DUCT taped around where that don’t fit perfectly! They’re attracted to lamps in my place..Since they’re torchier lamps, they get trapped in there. But it’s disgusting! Have you had any luck with diluted dish soap? That’s my latest ‘eco-friendly effort’.But I better try nematodes for plants in my garden…There are areas of algae on my roof, around leaking water pipes…Could they be attacted to that…or more likely my garden? Thanks!

    • Hi Susanna,
      Your welcome! I have not tried dish soap out but have heard of maybe apple cider vinegar ,water and dish soap may work in trapping them. For the nemotodes They will eat the larvae of the gnats and keep any other root eaters away. Not sure on your algae on the rooftop is what is bringing them into your garden. Good Luck.

  4. Growing Organic? The “brute force” methods above (SM-90 or Hydrogen Peroxide) will kill the fungus gnat larvae, but will also kill many beneficial microbes in the soil. You can add beneficial microbes back, but it will take a while for them to repopulate your soil. This problem only really affects organic soil growers. If you’re providing nutrients to your plants with bottled nutrients, you’re already providing nutrients in a way that your plants will be able to use even if the microbe populations are down. In any case, if growing organically, an organic version of bacillus thuringiensis (subspecies israelensis) is the best option for getting rid of fungus gnats.

    • Hi MIke,
      I do agree with you some of the above methods are pretty harsh on the root zone. Growing organic your best bet is an organic version of bacillus thuringiensis like mosquito dunks, microbe lift.

    • HI Marc,

      Great question , When I first had an outbreak, taking pics were the least of my worries. Although after I wish I did take some pictures of these gnats. Another grower I knew had some. Went to take pics of them and man are they fast buggers , I didn’t have a high powered camera at the time and my pictures did not turn out . In the future if I do come in contact with the fungus gnats I will definitely add some pics to the post.

  5. i had a pretty bad infestation on some of my plants from over watering, your plant will look like it has a nutrient problem with yellowing of leaves and brown spots starting to appear everywhere eventually i discarded the affected plant and went back to watering less often

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