How to get rid of fungus gnats on plants

A yellow, purple and red hand all holding objects to hold off a swarm of gnats.

By Jessica RoyAssistant Editor, Utility Journalism

July 14, 2021 7 AM PT

I accidentally brought a plague upon my house.

In November, I perched a newly purchased pinstripe calathea on the dresser in my bedroom.

I have plants in every room of the house and plenty outdoors too. My collection ranges from thumb-size succulents to a dracaena taller than I am. I’m on and off my two balconies a lot to water and fuss over them. So the first few times I swatted a little fly getting up in my face, I assumed one had followed me back inside.

Wrong.

The calathea started to struggle right away. I fretted over it, brought in a humidifier, moved the plant closer to and then farther from the window. Then the ‘Marble Queen’ pothos hanging in the window started struggling. The philodendron ‘Brasil’ began to droop. I took them down to get a look at the soil and was met with a face full of fungus gnats.

That was in early December. It was already too late.

Since then I have been playing whack-a-gnat.

A lot of us became plant parents in quarantine. My collection expanded from a handful of cute little cactuses picked up on a whim at Trader Joe’s to a sweeping collection of more than three dozen posed in corners and on countertops in every room of my house. But as parents of all kinds know, even the cutest little additions to the family come with potentially unpleasant corollaries.

With plants, it’s bugs.

Fungus gnats are one of the most common plant pest problems, and one of the most frustrating (and potentially expensive) to resolve. Google searches for “fungus gnats” and related insect elimination queries have ticked up since the start of 2020, as the people projecting pandemic stress onto their plants have found themselves in the same position I’m in.

I have read just about every article on the internet about eradicating fungus gnat infestations. I have ordered 48-packs of yellow sticky traps from Amazon — three times. I have done toxic soaks and let poison pellets bloom in water overnight. I have top-dressed. I have bottom-watered. (For those of you who haven’t gone off the gnat-inspired deep end, top dressing is covering the soil with something that physically block bugs from accessing it. It’s also done for aesthetic reasons. And bottom-watering is when you let your plant soak up water through its drainage hole instead of pouring it over the top, which keeps that soil at the top from getting damp and attracting critters.) I have gone as long as possible without watering my plants before I start to feel dehydrated on their behalf. I have repotted. I have purchased strange devices, like the Katchy, a robotic insect vacuum that lures its victims with UV light and sucks them to their doom with a fan.

I’ve even done the unthinkable: I’ve had to toss some of my beloved plants. Some wilted and died as the gnat larvae chewed up their roots. Others were so profoundly infected that even multiple attempts at solving the problem failed, and I was forced to make a horticultural Sophie’s choice to save the rest.

I am still subject to the scourge.

At The Times, we have an internal Slack channel for plant lovers where I have posted about my plight. Here’s what I have spent trying to combat the problem:

— Three 48-packs of sticky traps: $44
— Landscaping sand: $15
— Beneficial nematodes: $60
— Steel wool: $10
— Neem oil spray: $11
— Katchy: $40
— Bag of cedar shavings: $10
— Three Venus flytraps and specialty potting mix: $66
— Mosquito bits: $19
— Hydrogen peroxide: $5
— Bonide Systemic Houseplant Insect Control granules: $22

Total: $302. More than 10 times what the original calathea cost me. All of these items promised to ward off, murder, smother or otherwise expunge my pest problem. Only a handful succeeded.

Even worse? The gnats came for the dwarf Meyer lemon tree and the burro’s tail on the balcony. They got into my big dracaena, my fiddle-leaf fig, my bird of paradise. They unleashed their foul spawn on my pride and joy: my two big monstera plants. I’m so paranoid that I’ve caught myself aggressively swatting specks of dust out of the air. At this point, I am legitimately concerned that I have somehow bred a mutant strain that is immune to every treatment out there.

It’s time to bring in some experts. Here’s whom I spoke to:

Yvonne Savio, who retired as the head of the University of California master gardener program after more than 20 years and now runs gardeninginla.net.

Leigh Adams, an interpretive horticulturist who works on projects at the L.A. Arboretum, Farmlab at Metabolic Studio and Studio Petrichor.

Brian Brown, curator of the entomology section at the Natural History Museum of L.A. County.

Robin Jones, self-described “lead garden girl” and beekeeper at Honey Girl Grows.

Before we get into warfare, a brief overview of the enemy: They are related to mosquitos but don’t bite people. They look like fruit flies but lay their eggs on damp soil, not past-ripe produce. They don’t carry any diseases dangerous to humans. The adults aren’t really dangerous to plants either. They are slow, clumsy fliers — rather satisfyingly easy to swat. Their larvae are the root eaters. And even those won’t be able to do much damage to most of your plants unless you’re profoundly afflicted. They have a roughly three-week lifespan, which is why it sometimes feels like you’ve finally, truly gotten rid of them, only for the cycle to begin anew.

So mostly, they’re annoying. As they search for a wet patch of dirt, you’ll often find them buzzing around your face, evaluating your moist holes for real estate potential. Or they’re in your water glass or crawling around your bathroom sink. They’re obnoxiously fertile, and the problem can escalate from nuisance to all-encompassing swarm in shockingly little time.

And so, we prepare for battle. Let’s talk about what’s worked and what hasn’t.

Here’s what works

Eradicate your fungus gnat infestation for good with the following method: Put up a bunch of sticky traps, top-dress the soil with sand and do nematode soaks every 10 to 14 days until the problem is resolved. Also, stop overwatering your plants and eliminate any unnecessary sources of moisture. That’s it.

The thing about fungus gnats is that you have to stop them at every stage of the life cycle to eradicate them. If you kill only the adults flying around by trapping them, some will get the chance to lay another round of eggs in the soil, and the cycle begins anew.

The yellow sticky traps “work” in that they capture a disgusting amount of twitching flying insects. But they capture only adult fungus gnats and not necessarily before the bugs have had a chance to lay eggs. If you catch the problem early, a strong line of sticky trap defenses might be enough to stop it in its tracks, but I was too far gone for this to be sufficient.

“Top-dressing” is when you cover the soil with another medium to make it inaccessible to insects. I initially tried steel wool after seeing a post about it on Reddit. That was not successful; they still had plenty of room to fly around. I tried again with a robust bag of landscaping sand from a big-box hardware store. The sand gives enough coverage to prevent the adults from laying more eggs and traps the larvae in the soil, stopping them from wriggling to freedom. You can still water your plants at the top if you want to. It will just go through the sand. You need only about a quarter inch to a half inch of sand on top of the soil to do the trick. Get it around the base of the plant and all the way to the edges of the pot: Fungus gnats are wily and will gladly take advantage of any opening you give them.

Adult fungus gnats are annoying but largely harmless. The larvae are the ones sucking the life out of your precious plants by chomping on the roots. Enter nematodes, or microscopic roundworms. It might sound counterintuitive to fight your insect problem by bringing in more of them, but these are the beneficial kind. Nematodes will seek and destroy the larvae, which prevents the next generation of adults from forming. (Don’t Google how that process works unless you’re prepared to sleep poorly tonight. OK, fine, if you must.) Because you probably have gnats at various stages in the life cycle, you have to do the nematode soaks a few times to get them all.

I bought some nematode “pot poppers” online; I put the little sachets into my pots and they didn’t seem to do much. After strenuous urging from Jones, the beekeeper, I tried again. I got the kind that looks like a bag of finely grated Parmesan, which you put in water and then pour over your plants. It made a noticeable dent in the gnat population after just one application.

Every person I spoke to for this article told me I was overwatering my plants. I swear I am not overwatering my plants. I don’t water on an app’s schedule like a nervous first-time plant mom. I stick my finger in the soil and check further down with a moisture meter.

But when I got the bug-trafficking calathea, the soil was very damp. Brown, the entomologist, said it almost definitely came from an infested greenhouse. Greenhouses are humid and full of plants that get watered frequently, he said, and thus are a somewhat intractable scene of fungus gnat issues. Similarly, anywhere in your house that has moisture is keeping the gnats interested.

Jones asked me a question that made my blood run cold: Did I have water sitting in my plants’ drainage trays? Guilty. Sure, I was letting the topsoil thoroughly dry out, but I was practically offering a buffet at the drainage holes.

So, to recap: Put a few cheerful little literal death traps around your plants in the form of sticky traps. Cover up the soil with sand. Keep a bag of microscopic roundworms in your fridge. Wipe down your drainage trays and do whatever you can to minimize moisture.

Here’s what doesn’t work

I tried a lot of things I read online. Here are some suggestions I tried that yielded unsatisfactory results.

Neem oil spray can be effective against a variety of plant-destroying insects. But unless you really, really soak the top 3 inches of the soil, it won’t kill all the fungus gnat larvae. At best, it seems to weigh down the flying adults enough that they’re easier to swat.

The Venus flytraps were simply not up to the task. At one point there was a fungus gnat corpse in every single one of their sticky little mouths. They’re so overworked and exhausted that they can’t even close their tiny teeth anymore. The dead bugs have to wait their turn to be consumed. At the height of the issue, I estimate that I would have needed a carnivorous plant on par with Audrey II from “Little Shop of Horrors” to adequately address this. I did get to watch the flytraps bloom, though.

Jones, the beekeeper, recommended I try cinnamon or cedar shavings on the soil. She said the gnats don’t like those scents, so they act as a deterrent. It did not seem that many were deterred.

The Katchy is a device with a little light that attracts gnats and mosquitoes and a window vortex that sucks them down onto a big sticker. It has captured a satisfying number of victims but does not fully address the problem.

Mosquito bits are little pellets that contain a bacteria that’s toxic to fungus gnat and mosquito larvae. You scatter them on top of your soil or let them sit in your full watering can for a few hours or overnight to marinate. I tried both with limited results.

“Bottom-watering” is when you put your parched plant in a bucket of water and let it soak up what it needs through the drainage hole. (It’s very fun. “Like a magic trick,” said Adams.) This keeps the top of your soil dry and therefore gnat-egg-proof. But if the water leaks back out into the tray, or if the hole is accessible to the gnats, you could just end up with the problem at the bottom of your plant instead of the top.

You can also repot your plants in fresh, sterile soil. But unless you repot all of your plants at once, and very thoroughly clean the inside of the containers and get every speck of dirt off the fragile roots, your efforts may be futile.

Here’s what works but you probably shouldn’t try it

There are some things that will help but that plant experts caution against. One is a diluted hydrogen peroxide soak. You dilute hydrogen peroxide in water at about a 1-3 or 1-4 ratio and thoroughly soak your plant’s soil until it comes out through the drainage holes. You’re not just giving your plant a regular watering, Savio said. It’s a thorough drenching of the whole container, ideally two to three drenches in one go to make sure you reach everywhere. The hydrogen peroxide will kill the larvae.

But there is a small problem and a big problem with this solution.

Small problem: Your soil is very wet. Once the hydrogen peroxide is done fizzing, your plant is now a very fertile target for more gnats.

Big problem: The hydrogen peroxide kills everything in your soil, including the good stuff.

“You don’t want dead soil,” Jones told me. Healthy soil is a vibrant ecosystem with beneficial bacteria and microorganisms that help your plants grow. Murdering them is bad. Jones said this is really acceptable only as an “all-out, last-case, last-ditch effort” when you’ve tried everything else in this article. If you do this and you go about five weeks without seeing any new gnats, you have to repot your plant or aggressively feed the soil with things like compost tea or worm castings.

A friend recommended something called “systemic granules.” It’s an extremely powerful pesticide. You can’t let the granules touch your bare skin. You can’t measure the amount you need with any item you use with food. And you can’t use them on any plant that grows something you plan to eat. However, I was desperate. I lined a measuring cup with a plastic bag and used it on my big, profoundly infested monstera plants. And it worked: The evil was defeated. But my beautiful, glossy girls stopped putting out new leaves for almost a month after. Once I feel confident that I’ve gotten rid of every last gnat, I’ll repot them with fresh soil.

“That insecticide stuff — it does work. But I just don’t like to have those chemicals in my life,” said Adams, the horticulturist.

I asked Brown, the entomologist, whether I had bred a new and unkillable mutant species of fungus gnat. He said no. He’s a bug guy, and I’m a plant girl, which put us somewhat at odds in our conversation. I asked why fungus gnats exist at all. What purpose do they serve?

“We definitely need them for something,” he told me. “They’re all part of the ecosystem. They do various things that are necessary to provide us with clean air, clean water, foods, medicine; all these things work together to do that. All the fungus gnats may be obscure and at times inexplicable to humans, but they’re all part of what keeps the world going.”

He did concede that they are “annoying” and “ugly” and, most important, that it’s OK that I’ve killed a bunch of them.