Updated: Aug 20, 2020
Houseplant pests may be the biggest bummer of all, but none is more obnoxious than the fungus gnat. While these are not the most worrisome of pests you might have to deal with for your plants' health, they are no doubt a total bummer. Fungus gnats are small flies (similar to, but not to be confused with fruit flies) that infest sources of organic decomposition- such as soil or compost. While adult fungus gnats will not harm you or your plants, they are a huge nuisance, (especially if you are working from home- and who isn't these days) and their larvae can do quite a number on your plants' roots!
Adult fungus gnats, like many flying insects, are attracted to light, but because of their weak flying capabilities usually don’t get very far inside your home. This means the little buggers are likely to loom near your houseplants and scurry around on your growing media & foliage. These pests like to lay eggs in moist soil, which is why you will usually notice fungus gnats after a plant gets just a bit too much water. It just takes one pot of soggy soil and one sneaky gnat to get in and take over your plants, a female lays eggs in one pot and the breeding cycle begins!
These pests have a lifespan of over 2 weeks, but the longest stage in a fungus gnat's life is spent in the soil as an egg, larvae, and pupa, so that is the best place to target them. Although the flying adults are a pain in the alocasia(😂), they are relatively harmless and will only be around for a short period of time (unless the cycle continues)! Yellow sticky traps are my preferred way to deal with adult fungus gnats, that, and just let them die out. The real trick is killing off the eggs, larvae, and pupa in the soil so that the cycle ends and no more flies are born.
The only way that I have been able to completely eradicate a fungus gnat infestation has been to completely repot all of my plants with fresh medium. I have a lot of plant babies, so this was no easy decision (or feat) but I had tried all of the other recommendations, and although some have helped, none got rid of the problem as my method did. It only takes one plant with fungus gnats to infect your plant collection, so take action as soon as you see one flying around. Find the source plant (or plants if it has spread by the time you notice) and isolate them. If you can't tell which plants are the problem, I highly recommend repotting all the plants in the vicinity to ensure you completely take care of the problem in one go, (let's face it, none of us have time or money to repot our plants every 4 months because we left one plant untreated) so as much as we might not want to do a complete repot, you will really be happy you did down the road. I recommend throwing out the existing potting medium and repotting with fresh soil (whatever you prefer) and amending the soil with Mosquito Bits. These boys are the greatest addition to my plant collection, they will help keep these flying pests from bothering you. If you're unsure you want to introduce a foreign substance to your soil, here is the skinny on Mosquito bits:
Mosquito Bits include a substance called Bacillus thuringiensis sp israelensis, or BTI. This is a good bacteria that kills mosquito and fungus gnat larvae before they have a chance to turn into the flying nuisances we all can't stand. The substance does not seem to have an effect on other organisms. Per the product website; The larvae of these pest insects die within 24 hours of nibbling on the baited bits. Bti is harmless to all other forms of wildlife making it safe for use in ponds, aquaponics, etc. where fish and beneficial insects are present.
I have had great success using this product, I am not sponsored by or affiliated with the company in any way, it is just what has worked for me.
To control fungus gnats in my home, I simply mix the bits into my potting soil prior to using it to repot my plants. When you water your plants, Bti will release into the soil.
This is just one part (but one very key part) to being sure you eliminate all your fungus gnats. If you have an abundance of fungus gnats, it is quite likely you also have some soggy soil or a tendency to overwater your plants. Part of what will help is understanding there is no such thing as a "plant watering schedule", watering plants is more of an art than a science.
Your plants are living things! They don't always want the same thing all the time, and before you have had your plant for a few months and understand its general needs, I recommend feeling the soil (or you can get a moisture meter if that is more to your liking) each week to see how the plant is doing- simple rule of thumb, if the soil is wet or even moist, don't water your plant! Pretty much all houseplants don't like wet feet, but you will notice all plant varieties also have different watering habits, if you are not sure, it is always better to err on the drier side in my opinion.
If you are still worried about overly moist soil, there are other things that can help you out.
- Remember that most plants want to at least begin to dry out between waterings, and the less sun a plant gets, the less water it can utilize. So even if you have two of the same plant in different areas of the house, they might have two completely different watering needs.
- Make sure your plant is in a pot with drainage so excess water has somewhere to go. If you have a tendency to overwater, go with something porous like terracotta that will help absorb some of the excess water.
- Use the right potting medium. If you are potting a basic houseplant like a pothos or philodendron, your medium will likely be a soil mix. If you want something easy, I would go with a moisture control blend that is made for houseplants like Miracle-Gro Indoor Potting mix. If you are just starting out, this is an excellent go-to soil, it is easily found online and at big box stores like Lowes, is inexpensive, and does a decent job right out of the bag. I always recommend adding in mosquito bits, even if you don't have a gnat problem, but that is up to you. I have also heard great things about Black Gold potting mix, they offer a moisture control option that I have not personally tried yet but comes highly recommended.
I personally make my own soil blend. This is what has worked for me when ridding my home of gnats. A basic soil blend to make at home if you are struggling with a fungus gnat infestation
What you'll need:
Soil mix or soil base like peat moss or coco peat (if using a mix I recommend moisture control to start)
Mosquito Bits (can be found online and even on Amazon)
Perlite and/or vermiculite to aid with drainage and moisture control (I use both, and have tried many varieties with no particular preference)
Horticulture charcoal as a soil conditioner, can also help with drainage and absorbed unwanted impurities (I use Hoffman horticulture charcoal, it comes in very small pieces so it can be used with all of my houseplants and not obstruct roots)
Coarse sand is optional but helps with drainage as it adds airspace in the soil. Avoid fine sands- as they won't do the job. (Sand or pea gravel can also be used as a "topsoil" so that fungus gnats cannot burrow into the soil to lay more eggs)
Fir Bark or another type of plant-friendly bark. This is optional, I generally use this with my orchids, but if you are potting a plant that likes to really dry out between watering like a cactus or succulent, bark can really help with drainage.
Coco Coir this is completely optional. I like to use both coco coir and a bit of peat moss for my mix. Coco coir is great, offers water retention, drainage and aeration, it is sustainable and has a near-neutral pH balance (while peat moss can lower pH)
Slow-release fertilizer. I like to include a slow-release fertilizer but this is optional, I am currently testing out Osmocote Plus with no complaints, but I also add ground oyster shell and like worm castings when I can get them. If your base is a soil mix like Miracle-gro, don't worry about adding fertilizer- these mixes almost always have some sort of fertilizer already mixed in.
I mix all of these (fresh/new not reused) ingredients together and repot my plants in a pot with drainage. I always mix my soil together first to ensure all the ingredients are well blended and evenly distributed. The amount of each ingredient used (if desired) depends on what kind of plant you will be potting in it. for things like snake & ZZ plants, I often use a bit more sand, for things like ferns I add a lot of coco coir, no bark, and minimal sand. The number one thing is to make sure that you include mosquito bits in all of your pots (I also always add perlite/vermiculite and horticultural charcoal in basically all of my soil mixes, regardless of the type of plant just to aid with drainage. A good example of basic ratios for my base soil is as follows:
For each quart of soil mix or peat, I usually add between 1/4 to 1/2 cup of mosquito bits, 1/2 cup of charcoal, and about 3/4 cup of perlite/vermiculite to start, and I build from there.
Once I have repotted all of my plants, I top off the soil with pea gravel for the first two weeks to discourage adult gnats from burrowing in to lay more eggs (coarse sand can also be used) and keep yellow sticky trap paper around all of my plants. After two weeks, any of my plants that like the soil to "breathe" more get relieved of the pea gravel- by that time you should notice just about all (if not all) of the gnats have vanished.
The mosquito bits have exterminated all of the eggs & larvae, and adults are either stuck to a sticky trap or have completed their life cycle after two weeks. No new eggs should be in the soil with the pea gravel deterrent and if they are, the mosquito bits should keep them from maturing, thus eradicating all of these pests from your home! The mosquito bits should continue to work for some time. If you see another sign of fungus gnats after about a month or so, you may need to replace the mosquito bits or add some to the top of your soil, but in my experience, this won't happen. I brought a plant home about 9 months ago that had gnats, thinking I could take care of it with, cinnamon, the problem quickly spread. After I repot my plants with mosquito bits and followed the directions listed, I got rid of them all after about a week and a half. It has been about 7 months and no new sightings of fungus gnats. It is the only thing that has worked for me, and I highly recommend trying if you are sick of living with them also!
If you're not quite ready to spend a day repotting all of your plants in new soil, here are some alternatives to try! I get it, what I just explained above is a lot of work and can be wasteful if the soil isn't reused (I mixed my old soil with mosquito bits and transported it to my outdoor garden, as I don't mind if a few gnats linger out there). No matter what, if you have more than 10 plants, repotting them all will take a lot of time and we all know disrupting a plant's roots can be risky if it's not necessary. So here are some alternatives, I have had some luck with a few of these, and not much with some others. A few really popular pest repellents such as cinnamon and neem oil are good things to keep in your toolbox, but probably won't solve your fungus gnat problem completely.
Neem oil is an awesome product to keep and I recommend purchasing it concentrated and diluting it yourself when possible. I spray my plants preventatively with the stuff so that if certain pests do find their way into my house, my plants may be left unharmed! Spider mites, in particular, are often remedied with neem oil, my one houseplant experience with them was cured pretty easily, however, my fungus gnat issue did not seem to change one bit with the introduction of regular sprays of neem oil. Neem oil works because there is a compound that causes insects to stop feeding, so they starve. It can also prevent larvae from maturing or interrupt mating behavior- it works on a wide variety of common pests, it just isn't my go-to for fungus gnats (at least not alone). Be aware that neem oil also has a pretty foul odor that takes a few minutes to settle, especially if you spray a lot of plants. If you are interested in learning more about the basics of neem oil, check out the National Pesticide Information Center's page on it!
Bti or other similar products to Mosquito Bits: this is what I would recommend. You can even make a "tea" out of mosquito Bits and use the water for your plants rather than mixing the material into your potting medium. Similar products to Mosquito bits include Microb-lift BMC, and Fungus Gnat Control from "Pretty in Green" is a powder that can be mixed into water to make a larvacide. They also sell a Gnat Fighting Kit which includes this powder plus gnat sand & sticky traps to target adults. (Arbico Organics, makers of Mosquito Bits, also offers a bundle like this, with NemAttack and sticky traps, if you are interested in buying all your gnat fighting tools in one swoop!) If you have concerns or want more information about using a product with Bti, check out this fact sheet from the National Pesticide Information Center.
Sand or other soil covering: Adults lay their eggs in the top portion of soil, generally, moist soil. If you dress the top of your soil with a layer of course sand or pea gravel, it won't stay wet enough for the gnats to lay their eggs. They won't be able to burrow into the soil through the sand or gravel to lay their eggs either. I don't prefer this for a long term solution, I like to be able to feel my soil and monitor it to know when the optimal time to water is. I also like the humidity that top watering can provide (keeping sand on top of the soil means you may have to begin bottom watering for best results), and am not a big fan of how dry sand is. I used sand and pea gravel for about 2 weeks after repotting to optimize my fungus gnat control, but I removed it once I noticed the pests were no longer present.
Cinnamon: I recommend keeping cinnamon powder around for a variety of houseplant purposes- it is a natural fungicide that causes a bit of drying also. This makes it good for damping off, and keeping fungal issues (like mushrooms) from growing in your containers. Cinnamon also happens to also have properties to encourage root growth while fighting fungus. It helps reduce gnats by destroying the fungus that the larvae feed on. I would say using Cinnamon can be effective if you are willing to be patient, and if you only have a few houseplants that might be impacted.
Peroxide: Keep hydrogen peroxide around for your houseplants, even if you don't have fungus gnats. It is almost a necessity if you cultivate orchids, it is an anti-fungal that does not act as a long-term drying agent (once the bubbles are done, what remains is essentially water) it gives plants an oxygen boost as well. I always use Peroxide 3%, I don't think any other percentage is widely recommended. For pest control, add 1 teaspoon per cup of water in a spray bottle and mist the plant and soil. Alternatively, you can make a peroxide wash (1 part peroxide 3% to 4 parts water) to soak your soil and flush pests. Fungus gnat larvae in the soil should be killed on contact. I use peroxide for a variety of reasons, and preventatively for seedlings and orchids, but it does also work as a pesticide.
Diatomaceous Earth: (food grade) diatomaceous earth (DE) is mineralized fossil dust (from diatoms) that is natural and non-toxic to the environment (as long as you get food grade!) DE also contains sharp, microscopic bits of silica that will slice pests when it goes through them. You can mix some into the top layer of soil to aid in fungus gnat control, or use it as "topsoil" (instead of something like sand) to kill any pests that come in contact with it.
DE works the same way to kill other insects so don’t use DE in any environment that includes beneficial living organisms like springtails or earthworms... for obvious reasons. I use DE on my outdoor plants when I see any sign of pests, I use a powder duster to distribute DE over the plants and soil.
A similar product, GnatNix is made entirely from recycled baked glass, I haven't used it but I assume it is meant to get rid of gnats in a similar way to Diatomaceous earth. More information on DE from the National Pesticide Information Center
Insecticidal soap: I don't know about anyone else here, but I haven't had a lot of luck using soap or insecticidal soaps that are marketed to get rid of pests like fruit flies. I have tried many of the options from big box stores like Lowes and Home Depot and I have not been impressed. However, I did come across Bonide's Insecticidal SUPER Soap while looking for some links to their other products, (I wasn't aware they made an insecticidal soap!) I would probably give that a try before I write this method of pest relief off completely.
Vinegar or other liquids sitting around in bowls: This method has never worked for me. I have had better luck accidentally catching fruit flies or fungus gnats in my red wine at the end of the day. I would skip these altogether if you know you have a fungus gnat problem, and stick with
Other products to consider (but that I haven't much experience with personally):