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Fungus Gnats On Marijuana Plants

Fungus Gnats On Marijuana Plants

Fungus gnats are a common problem for soil marijuana growers, but if you know what to do they’re actually easy to get rid of. Fungus gnats are extremely small (two millimeters in length) and are gray, black, or brown in color with long legs.

Female fungus gnats lay two hundred eggs per week, and they are often laid towards the bottom of your marijuana plants. The maggots of fungus gnats have black heads and transparent bodies, but they are microscopic. Both maggots and adults attack marijuana plants.

This article will teach you how to fix your current fungus gnat infestation and make sure you never get fungus gnats again.

What are fungus gnats

What are fungus gnats cannabis

Fungus gnats got their name because they feed on fungus at the soil level. When they have no more soil, they start working on your plants’ root systems instead. They consume the small root hairs and cause damage to the bigger roots. This affects the speed of your plants’ growth, turns leaves into different colors, and causes stems and branches to grow abnormally. The major problem with fungus gnats is they allow your plants to be particularly vulnerable to lots of different fungal diseases.

Fungal gnat droppings are one of the things that cause major problems for your marijuana plants. The droppings, left in the soil, will affect your soil’s drainage and will, therefore, keep the soil too moist. This, in turn, attracts more pests that like the moist environment. Make sure you don’t get a total infestation by being careful not to water your marijuana plants too much.

Download my free marijuana grow guide at this link for more growing tips

Fungus gnats are attracted to your valuable plants in the first place by wet soil and decaying organic matter. This means that overwatering your plants or allowing the dead plant matter to rot will cause fungus to grow. This is often impossible to see with the naked eye – which in turn will invite fungus gnats to come feast and lay eggs.

Not sure if fungus gnats are causing the damage to your marijuana plants? Check the article Marijuana pest and bug control for a list with pictures of all pests and bugs

Signs of fungus gnats

Signs of fungus gnats on cannabis plants

You will notice these tiny black bugs flying around the plants as well as crawling on the soil next to your plants. You may also see the little white or transparent larvae wriggling around in the soil. They are really small and will only look like minuscule white spots in the upper layer of the soil moving around. They may seem small, but they can do some big damage to your plants.

If you think there might be a gnat problem, there are a number of ways to check and be sure. There should be yellow sticky cards available at your local garden center or online at this link here. They should always be yellow because gnats are attracted to that specific color. Buy some of these and put them on the ground, in the soil near your plants. Check these cards a few days later. If there are a high number of gnats stuck to them, you have a problem.

You can also check for the gnat larvae by using a potato. Cut a potato into half an inch deep, and stick it into the ground. Check back several hours (between four and eight hours) later to see if maggots have infested it. If there are a number of maggots there, you need to deal with the problem.

Download my free marijuana grow guide at this link for more growing tips

As far as plant symptoms go, the hungry gnat larvae that chew on your roots will cause a number of plant issues. If your plant is very young or the gnat infestation is bad enough, it could result in the death of one or more of your marijuana plants. Before that you will see signs of slowed growth and leaf problems. Sometimes symptoms of nutrient deficiencies will occur but without explanation, since the pH level, nutrients, and other nutrient-related issues are clearly not the problem.

Fungus gnat larvae generally affect younger, more vulnerable plants, but sometimes they can have big effects on mature plants, as well. If the infestation hits your plants right when they are in the flowering stage, the end yield could be decreased significantly.

It’s important to note that the fungus gnat larvae are not the only source of trouble for your plants. Adult fungus gnats often spread plant diseases with their feet, such as pythium. Pythium, as you may already know, is one of the main causes of root rot.

How to get rid of fungus gnats

Get rid of fungus gnats on weed

Since prevention is the most effective way to deal with any problem that could harm your plant, you will need to make sure you don’t ever overwater your plants. Whether it’s before your very first growing season or your seventeenth, you need to do your best to prevent a fungus gnat infection.

If your last growing season had a gnat problem – even a small one – you need to ensure that the growing area is cleaned out completely. If you’re growing indoors, make sure you don’t have any houseplants with fungus gnats or larvae of any kind near them.

If you’re receiving marijuana clones from elsewhere, you have to be absolutely certain that they don’t have any fungus gnats, spider mites, or other pests. If you are at all concerned about bugs being a problem, you should probably just grow your marijuana plants from seed. Clones are the easiest way of spreading pests to new crops, especially when you are growing indoors.

Even if you have seen gnats flying around your plants, but the infestation hasn’t had any visible effects on your plant yet, it’s important to deal with the problem as soon as you notice fungus gnats.

You can try trapping the adult gnats by placing yellow sticky tape (available here)or sticky cards all around your garden. Gnat larvae, on the other hand, pose the real problem. First of all, make sure the top several layers of soil are allowed to fully dry. You will have to pause your normal watering schedule for a couple days – but it’s worth it. When the soil has dried, make a mixture of 3% hydrogen peroxide and water at a 1:4 ratio. This won’t injure your plants, but it will turn into oxygen and water (that, as you know, are beneficial to the soil). This will keep the gnats away while simultaneously helping your plants out.

To help speed up the process of ridding your plants of this pest, bring in a fan and have it blowing over your soil to help dry it up more rapidly. It also helps keep the tiny fungus gnats from flying around properly, therefore reducing the amount of eggs they are able to lay. A commercial product like Bug Blaster will always to the job. If you want to make sure to kill them all buy something like Bug Blaster.

If you want to be particularly thorough, you can apply some food-grade diatomaceous earth over the areas of the soil that are exposed. This will kill the fungus gnat larvae, keep future infestations from occurring again, and will speed up the eradication process. Diatomaceous earth is organic – it’s simply made of fossilized shells! It kills insects effectively by cutting into their exoskeletons and draining out their bodily fluids. Luckily for you, it is not dangerous for people or pets – you could even consume it and be perfectly fine.

Download my free marijuana grow guide at this link for more growing tips

Another way of destroying larvae that are living in the soil is using an actual pest treatment that contains the Bacillus thuringiensis bacteria. Bacillus thuringiensis sometimes referred to as “BT” or “BTi,” is a type of bacteria that creates a larvae-harming toxin. It essentially disallows them to eat.

The great thing about this bacteria is that, while it wreaks havoc on the tiny larvae, it won’t cause any problems for your plants or their roots. It also poses no threat to your own health. It is highly selective with where it focuses its harm – and it focuses almost exclusively on larvae. Make sure to crush the little pellets that you can buy and then sprinkle the resulting powder onto the soil from there. Dry formulations generally function better than liquids. When you do use this treatment, water your plants plenty afterward. This is a substitute for the hydrogen peroxide/water mixture that was described above.

Once you have eradicated the fungus gnats from your garden, make sure to change your watering habits. Clearly you were overwatering, since that is what must have attracted the fungus gnats in the first place, so make sure to avoid this in the future so you won’t need to deal with another fungus gnat infestation – or worse.

You can keep using those yellow sticky cards to find out what the current status of the infestation is. If it has lowered, there will be fewer fungus gnats caught in the trap – and vise-versa. Don’t relax until you have hard evidence that the infestation is really gone. This could take a couple days, or it could take a few weeks! Don’t lose hope, and keep a careful eye on all the goings-on of the yellow sticky card as well as your garden itself.

Compost Pile
It is somewhat common to discover fungus gnats and their larvae in your compost pile. It is something that you need to pay special attention to since compost is supposed to be there to make your plants strong and resilient – not destroy them from the roots up. So how do you deal with a fungus gnat problem in your compost?

First, you need to ensure that your compost pile isn’t too moist. Excessive moisture is the primary reason for fungus gnats appearing in your compost pile, so take this piece of advice very seriously. A good rule of thumb is that compost piles shouldn’t seem wet, but they should be similar in moisture to a sponge that has been squeezed out.

If you do discover those pesky larvae in your compost pile, you should try sprinkling food-grade Diatomaceous Earth over the exposed parts. This will cause no harm to people, but will kill off larvae and adult fungus gnats. If the problem is particularly extreme, try a pest treatment that has bacillus thuringiensis in it. Mosquito Bits or Mosquito Dunks are some examples of brands you could try. Use a dry formulation of this product rather than a liquid; break the pieces into a finer powder and sprinkle that over your compost. Then you add water (contrary to what we just told you about compost moisture levels), which will activate the treatment and will kill off the larvae.

Download my free marijuana grow guide at this link for more growing tips

Once you have allowed your compost pile to dry out enough, make sure to cover it up with some sort of plastic covering or tarp. If nothing else, this will mess with the fungus gnat life cycle, further weakening them and slowing down the infestation. Another way of accomplishing the same thing is covering it with a material containing plenty of carbon. Some examples of this are cardboard, shredded paper, or brown leaves. If possible, you should have a lid over your compost bin from the beginning, therefore preventing the infestation from happening at all.

To really prevent this, do not add your kitchen scraps to the compost pile if you’re going to be using it for marijuana plants. The best marijuana growth comes from compost that has been maintained properly. This means avoiding kitchen scraps or burying them in the middle of the pile so fungus gnats or other pests can’t reach it.

Ensure that your compost pile is turned over often and consistently, therefore allowing it to dry out enough and keeping everything mixed up. This also will keep the fungus gnat larvae from getting too problematic, in case they have already found their way to your delicious compost pile. If the compost is maintained correctly, it will increase significantly in temperature. This, finally, will roast the remaining larvae, causing them to certainly meet their demise.

Marijuana plant symptoms

Leaf Color:
– Edges brown or burnt
– Pale color
– Yellowing of new growth
– Black or gray patches
– Brown, dark spots

Leaf Symptoms:
– Upper leaves affected
– Newer growth affected
– Small inner leaves affected
– All leaves might look affected
– Leaf edges burnt
– Yellow between veins
– Spots
– Slowed growth
– Twisted, abnormal growth
– Leaves curling under
– Wilting & drooping

Plant Symptoms:
– Slowed growth
– Wilting & drooping
– Twisted, abnormal growth
– Slowed growth of roots
– Bugs
– Buds not getting fatter

Fungus gnats are commonly mistaken for fruit flies – make sure you are aware of which one is buzzing around your marijuana plants. Fruit flies and fungus gnats have very different behaviors (and, therefore, different deterrents), so it’s important to know which one you are dealing with.

Fruit flies have bodies that are brown, yellow, or orange in color; fungus gnats are gray, dark brown, or black. Fungus gnats will appear particularly tiny to you while fruit flies are generally larger and easier to spot.
If your problem is due to fruit flies instead of fungus gnats, you will need to take a different approach to ridding your growing area of them. They are attracted by ripe or rotting fruit (thus their name), as well as improperly composted soil that has your kitchen scraps in it. If you take this food away from them, they will also go away.

Remember that plants with strong genetics have less change of getting sick and are less vulnerable for pests and diseases. So make sure to buy cannabis seeds from a trusted seed bank.

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible


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Comment Section

34 thoughts on “Fungus Gnats On Marijuana Plants

By Anneke Bain on 13 January 2014

Do you have any solutions for root aphids. I am growing in a deep water reseviour and have been infested!

By Elizabeth on 17 January 2014

I got little Root Aphids? How do I get rid of them?

By Dave on 18 January 2014

Keep temperature of the nutes at 21c any higher and you run the risk of aphids.

By Anneke Bain on 19 January 2014

I have been trying TaNLin which appears to be controlling the larvae. Have not observed any flying insects – I did catch them early hopefully I will get a harvest!

By Skiman on 22 January 2015

I have a few fungus gnats. Caught some with yellow stickys. Got good results with hydrogen peroxide but I’m wondering if a garlic solution would be effective on the larvae….

By KODIAK on 23 January 2015


By pou on 12 August 2016

im happy , flys not 🙂 thx

By Trichrometimms on 5 May 2017

This is a tried & true method.
Buy washed/clean sand used in children’s sandpits.
Rinse thoroughly to remove any salts etc.
Apply min 10mm or 3/8 inch.
Can make watering a little tricky, but well worth it.
Instant results.

By [email protected] on 12 February 2015

I use about a 3/4 inch of perlite on the top of all my container plants and it seems to work real well along with hydro peroxide treatments every couple of weeks.

By Mat on 6 June 2015

What do u think of the gnat nix, is a physical barrier I’m using it in my garden I just started using though have you had any expirience also won’t hydrogen peroxide kill good bacteria and micorrizae?

By latewood on 11 June 2015

Best way to proacvtively rid your plants of soft shelled insects is: Food Grade Codex Diatomaceous Earth.

By Kyle Sacker on 24 June 2015

Grow Clean works wonders on any gnats flying around!!

By Marilyn on 27 August 2015

This is some good stuff. It took me some time to locate this blog but it was worth the time. I nitecod this website was buried in google and not the first spot. This internet site has a lot of good stuff and it doesnt deserve to be burried in the searches like that. By the way Im going to save this site to my favorites.VA:F [1.9.17_1161]please wait…

By latewood on 23 September 2015

We are so glad that you appreciate the efforts of ILGM Staff and Workers.

By Scott Dentz on 18 September 2015

I’m wondering if fungus gnats can ruin the taste, causing your crop to have zero flavor and zero smell? Diotamacious earth works great I know I’ve used it many times in the past.

By latewood on 23 September 2015

UNfortunately; I have to say;
If you harvest bugs, harvest will taste like bugs; IN at least part of the grow.”
However; If no nest has been built in your foliage, or, buds; You may not realize any significant damage to the finished herb.

I do not believe that quality would be effected as long as their is not infestation in the foliage. 🙂

Join our support forums for a wider range of opinions and support

By Sherry Weikum on 18 September 2015

On some grows I have had infestation of fungus gnats. One time I tried using the hydrogen peroxide and water solution but I had some limited success, that is the gnats disappeared only to reappear later. The last time I had a fungus gnat infection I read that cinnamon sprinkled on top of the soil works. I had 3 in DWC and 1 in soil. I sprinkled the cinnamon on top of the soil and the expanded clay pellets and it worked like a charm. It took about 2 weeks but all the fungus gnats disappeared.

By latewood on 23 September 2015

We all love home remedies. As long as they work. 🙂

By Macca on 4 October 2015

I’ve not seen any flies but I have got orange fungus very small clusters and they go pale when they dry.. I’m getting worries I case it’s on the roots.. I scooped 2 inches of the top soil away and put in a layer of dry new soil.. The plants genetics must be amazing… I found a seed in a bag of smoke.. Put it in damp tissue it split within 6 hours.. After around 12 hours it had a root half inch long poking out.. I potted it an inch deep in soil.. Watered it a little and the next day it sprouted two little round leaves.. It’s gone from that to having its third set of true leaves in a week.. Yesterday I gave it it’s first dose of nutes.. Even just 24 hours later it’s growth is rapid.. Very unexpected and didn’t even expect it to grow hahaha paranoid now incase it goes south from fungus.. Help

By latewood on 5 October 2015

It is impossible to guess and diagnose the issue you described without pictures. Join our support forum and post a topic with a picture or 2. This will allow us to give you an informed answer.

By Redd on 5 November 2015

I love this site. When I have a problem, I come here. There’s always an answer to my question. Sound advice that I trust every time. The solutions work. I love that most,are just home remedies. Thanks much for all the help. And thanks much for the FREE grow bible. Awesome!!

By Joseph on 10 November 2015

If I’m dealing with a fungus gnat problem where my 3 week old veg plants have withstood the invasion but is showing moderate but not severe damage, still slowed growth yellowing drooping. Twisting but has most of its roots already in place but seemed definitely impacted by these fuckers. Will the plant recover to its full potential? And after the problem is fixed and dries out. How long should I waitIt to start flowering? If I have all my branches in place and topped. Will they recover to normal growth again? ??

By latewood.ILGM on 11 November 2015

Sounds to me like you have a PH or nutrient toxidity issue as well.

The damaged plant will only recover so far as to say, in it’s new growth. The damaged area will always look like it does. If you can get the pests cleaned up; You need to wait and most likely treat the plant again, once the eggs already laid in the plant hatch. Once you clean out those hatchlings you should be able to induce flower..

By Olin on 22 March 2016

Definitely have a fungal gnat issue , gonna try some of the rememdies that ive read here .. the new growth in the tips looks brownish and it starts from the bottom of the plant and goes upward .. some of my flowering plants are looking like they’re dying and its gotta be these fucking gnats .. ph is perfect and definitely not a nutrition issue .. I see a decent amount of adults flying and crawling on top of the soil .. so im hoping once I get control of them I won’t have any other problems..

By jt13 on 2 June 2016

I found that I had these little white mite thingys crawling around and discovered that as my fungus gnat population fell that there was more of these mites. well, what I discovered was that I had a predatory mite called hypoaspis miles. they seek out fungus gnat larvae and eat them. along with hydrogen peroxide treatments, they are eradicated!!

By Rupert on 9 October 2016

Very informative.. You obviously know your stuff..

By allansanderson on 8 March 2017

Can you get in touch with me ? Regarding an order I sent . Regards Allan .

By Lisa on 24 March 2017

I have read all of the above suggestions for gnats – I’ve been trying and trying, with no luck. The peroxide didn’t seem to do anything, so I went and bought a bag of the diatomaceous earth and sprinkled that over the top of the soil. I have also been using the bug blaster as a spray on the plant and soil. There are still gnats all over the place. This is my one and only plant that I have going…. if I lose it I’ll die!

By Paul Davenport on 16 August 2017

G man here I’ve a gnat problem bought 35% food grade peroxide deluded it. To 4-5% didn’t work i put sand on top didn’t work bought fly paper traps trapped quite a lot also just tried yellow sticky traps they work now going to try some bacillus thuringiesis bacteria. Plants need to get transplanted for final time from 1.5 gal to 5gal.1st grow 4 northern lights extreme….gotta get rid of pesky gnats…lotta great here

By ralph on 13 October 2017

is apple cider vinegar and dish soap mixed with water a good gnat killer?

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