The gnats in plants kept indoors are fungus gnats, sometimes referred to as soil gnats. They aren’t the same as fruit flies, although they do resemble them a lot.
Fungus gnats are attracted to moist soil, while fruit flies are attracted to anything fermenting. You’ll see fruit flies flying around fruits and your garbage bin. You’ll see fungus gnats flying around your plant, but mainly they’ll be hovering closer to the soil because that’s the ideal ground for them to lay their young for a healthy place to hatch into larvae and feast on your plant’s roots and the soft parts of stems.
The gnats themselves are an annoyance. They may fly around your home, but you’ll notice they’ll only fly around near your plants, particularly, the top of the soil. Larvae is the main problem, and if left to feast on plant roots and stems, they can eventually cause your plant to wilt (see my article for other causes and fixes) and damage the root system.
If you aren’t sure if you have gnats or fruit flies, a handy way to tell is to consider the food sources available to them. Fruit flies are almost always going to be a kitchen problem because they want your food scraps – or the bowl of fresh fruit on the countertop. You’ll find gnats in any room of your home with plants that have excess moisture in the soil as the larvae feed on plant roots and fungi in the soil.
Soggy soil is what’s going to attract gnats to your plants. Think of them as doing you a favor when they turn up because when you have gnats feasting on your potting soil, it’s a sure-fire sign you’ve over watered your plants.
Getting rid of gnats is only part of the equation. Understanding where they come from, what attracts them and what happens when they invade your plants… well, that’s the knowledge that’s going help you not only get rid of gnats in plants, but to nail your watering routine. Keeping your potting soil at just the right moisture level for the plant to get enough hydration will help with not encouraging gnats in the first place.
How Gnats Get Into Your Plants
Gnats can get into your home through an open window, open door, through vents or cracks and crevices in your homes walls. Because they’re so tiny, it’s difficult to keep them out. Bringing plants from outdoors inside is another way you can bring in both gnats and/or their eggs. The same is true with taking cuttings from flowers to grow inside. If the eggs are laid, larvae will feed on the potting soil then grow up into flying pests.
It’s possible for gnat problems to start in the soil before you’ve even used it. As gnats are attracted to moist soil, if your potting mix has been moist prior to you using it for your house plants, it is possible that gnats could have laid their eggs in the potting soil.
The Life Cycle of Gnats
This is the scary part of a gnat problem. The adult gnats – the ones flying around being a nuisance, driving you up the wall… they can lay hundreds of eggs in your potting soil. And they will if it’s too moist because moist soil is a perfect feeding ground for larvae.
It only takes three days for eggs to hatch into larvae. The larval stage lasts about ten days. During these days, the larvae in your potting mix is feeding on your soil and decaying plant matter, starving your plant of nutrients.
After around ten days, they go into the pupal phase for roughly four days, after which, you have potentially hundreds more gnats starting the cycle all over again.
How to Control Gnats in Plants
In most cases where gnats present as more than a few annoying pests and more like an invasion of your home, there’s usually more than one plant close together.
Gnats only want the moisture from the soil of plants. The more gnats there are, the less they’ll be able to feast in the soil. Instead of fighting for the same source, they’ll fly over to nearby plants and delve into the soil of those.
As each gnat to fly around your plant is capable of laying hundreds of eggs, the more plants you have, the more gnats you can find yourself fighting.
For that reason, gnat control starts with isolation. The first plant you see gnats floating around your soil, separate from all your other plants. Also, inspect your plants too just in case there are tiny flies crawling on top of the soil.
Any plant you see with gnats, move to another room, or put it on your bathroom either in the bathtub or shower tray, just to prevent the gnats from flying onto other plants. Also, if plants nearby the one you’re isolating are showing signs of wilting, move them too just in case the wilting is being caused by the larvae from gnats in neighboring plants eating away at the roots and stems.
If gnats aren’t your only problem, see my article about stopping other common insects from attacking your plants.
Treat the Culprits
The culprits are the gnats laying eggs that’ll be harming your plant. An effective way to catch these is to trap them with fly paper (affordable and readily available on Amazon) and you may need a lot as gnats are often in high numbers and continue to multiply until the larvae problem is fixed. An alternative is to use a bug killer spray, however, most are not organic.
A slower process that is organic is to make a cocktail of apple cider vinegar, sugar and some dish soap added to a cup or bowl, put some plastic wrap over it and pierce some small holes in the top. The gnats are attracted to the apple cider vinegar, get in through the holes then drown in the liquid. It’s a slower process than using fly traps but is still effective.
The apple cider vinegar trap works like this:
Dehydrate Your Plant
Overwatering your plants is what attracts gnats to lay their eggs in your potting mix, but the larvae can’t survive in dry soil. So, the simplest solution to try first is to stop watering the plant so the soil can dry out. Larvae needs moisture to survive. Starve them of moisture, they’ll die.
Now, if you’ve really overwatered your plant, you’ll want to take additional steps to get rid of the excess moisture. This could be as simple as poking some holes in the soil to aerate it, increasing the temperature in the room the plant’s in, or removing the plant from its container and wrapping the roots in newspaper to dry it out.
Treat the Larval Stage of Gnats with BTI (Bacillus thuringiensis israelensis)
If letting your plant dry out doesn’t work, or you’re frightened you’ll harm your plant by not watering it, the next thing to do is treat the soil with BTI (Bacillus thuringiensis israelensis). This a naturally occurring bacterium that is not harmful to your plants, pets, kids, or any wildlife. In fact, it is only harmful to small flying insects, mostly mosquitoes, gnats, black flies and certain types of midges.
A popular product used for treating the larval stage of an infestation is Mosquito Dunks (view on Amazon), which contains BTI. You only need a little bit of one dunk, crushed up into a 2L bottle of water. Leave it to stand for 24-48 hours and then water your plants with it (it’s what’s sprayed into the soil in the video above). The larvae will die off, hopefully within a month or sooner.
Once you’re sure they’re gone, put your plant back where you had it, but pay close attention to your watering routine. Remember the reason gnats are attracted to house plants – moisture. Moisture and humidity control are your best method for long-term gnat control.
Watering Guidelines for Gnat Prevention
There’s a number of ways to test your soil to see if it’s needing watered, helping to prevent overwatering. You can use your finger to feel if the soil’s dry or moist, or use a moisture meter to tell you if the soil’s wet, dry or in between.
Only water your houseplants when the soil is dry, paying particular attention to the top two inches of your potting mix as that’s the part adult gnats will find attractive to lay their eggs.
Top Watering vs. Bottom Watering House Plants
Top watering is what most people do with all their plants. Add water directly to the top soil. Top watering is best done with containers with drainage holes in the bottom. When watering, you can keep adding water until you see water coming through the drainage holes. It’s a good way to flush the soil, getting rid of any salt build-up that’s accumulated.
Bottom watering on the other hand is when you use a larger container than the one your plant’s in, or a saucer, and let it soak in the water. Instead of you pouring the water into the soil until it pours through the drainage hole, you leave the plant in standing water long enough for the roots to absorb moisture. This can take anywhere between ten minutes to a half hour.
Don’t leave the plant in standing water continuously or it will keep feeding on the water. A moisture meter can be used to indicate the moisture level in the soil, or you can just leave the plant soaking in the water until you can poke your finger through the top two inches of the soil and feel that it’s wetter.
As the top two inches of your soil is where gnats are interested in laying their eggs, causing a larvae problem and eventually more gnats, switching to bottom watering will help make sure the plants are watered while the top soil isn’t being drenched.
For house plants, bottom watering is a better method as it prevents excess moisture on the top soil. But you still need to top water the plant at least monthly to flush excess salts.
In terms of the frequency for watering, there are none. In fact, if you try to keep to a watering schedule, you’re probably going to do more harm than good. Plants are best watered when they’re thirsty and not when it fits into your schedule.
A Quick Checklist to Get Rid of Gnats in Plants
- When you find you have gnats flying around your plants, isolate it by putting them in a separate room.
- Check all plants near the one the tiny flies are flying around for signs of wilting as that can indicate there’s larvae feeding on the soil nutrients.
- Use fly paper to trap adult flying gnats.
- Let the soil dry out.
- Treat the soil with BTI (Bacillus thuringiensis israelensis) to kill larvae before it goes into the pupal stage to prevent more gnats from starting the cycle over.
- Keep the plant isolated until the gnats are gone.
- Return the plant(s) and start watering only when the plant’s soil is dried out and not on a set frequency.
Switching to bottom watering can help prevent overwatering your plant and making sure the plant roots get enough water while not drenching the top soil, making it attractive to gnats.
For other common houseplant pests, check out my guide for getting rid of bugs on your plants.