Houseplants brighten our homes and give us a sense of bringing the outdoors inside.
However, if you recently brought a houseplant, such as a sansevieria, into your home and noticed lots of small black flies flying around it, you may have brought a bit more of the outdoors inside than you really want.
In that case you probably have a case of fungus gnats, which are some of the most frequently occurring houseplant pests.
Fungus gnats are small flies that are attracted to moist potting soil and compost. They lay eggs in the soil and their larvae feed on rotting plant material and the soft roots of young plants. They don’t usually cause too much damage to indoor plants, but they can be a pest.
If you’re unsure if you have fungus gnats, place several slices of raw potato on top of the soil, and check them after a few days. If you have fungus gnat larvae in the soil then you are likely to find them clinging to the bottom of the potato slice.
There is a whole section below which gives more detail about fungus gnats and their lifecycle. But let’s get straight into the various ways of preventing and getting rid of fungus gnats naturally as part of your house plant care routine.
- 16 different ways to get rid of fungus gnats in houseplants naturally
- 1. Isolate new houseplants
- 2. Let the soil dry out before watering again
- 3. Remove decayed plant materials
- 4. Change to better potting soil
- 5. Use diatomaceous earth
- 6. Use horticultural sand or grit
- 7. Use Cinnamon
- 8. Attract and Trap the Fungus Gnats
- 9. Use Sticky Traps
- 10. Kill adult fungus gnats with soap solution spray
- 11. Use hydrogen peroxide to kill the fungus gnat larvae?
- 12. Bacillus thuringiensis var. israelensis
- 13. Beneficial Nematodes
- 14. Neem Oil
- 15. Repot Your Plant
- 16. The regime
- What are fungus gnats?
- The lifecycle of fungus gnats
- Protect your houseplants today
16 different ways to get rid of fungus gnats in houseplants naturally
You can spray fungus gnats with fly killer but, as well as being unhealthy, that will not be a long term solution. It may kill fungus gnats flying around, but won’t necessarily prevent fungus gnats coming back, because it won’t impact any larvae under the soil surface.
It is worth understanding that the adult fungus gnat is short lived and does not feed, although they are pretty unpleasant to have around. Any damage to plants is caused by the gnat in its larval stage, which lasts longer than the adult lifespan.
Therefore most of the techniques for dealing with fungus gnats target the fungus gnat larva and the management of the potting mix in your containers.
Fortunately, there are natural ways to prevent and resolve gnat infestation in your houseplants. So, read on for a variety of suggestions on how to get rid of fungus gnats in houseplants naturally.
1. Isolate new houseplants
Usually, fungus gnats enter your home and infest houseplants when a plant has been outdoors or has comefrom a gnat-infested environment (perhaps from a shop pr nursery). So, if you’re bringing a new plant into your home, prevent gnat infestation by keeping it away from other plants first.
Quarantine the new plant in a separate room for at least 18 days, during which time the gnats will have rcompleted one entire lifecycle. Using potato slices, you can check if the plant already has larvae during this period. If there’s no sign of larvae or adult gnats in your new plant, you can move it in with your existing houseplants.
2. Let the soil dry out before watering again
Since fungus gnats thrive in moist environments, try to completely dry your plant’s soil between waterings. Ideally, don’t water your plant for several days until the soil is dry at around one to two inches deep.
This is especially important in autumn when the plant growth slows down and the plants will take up less water. So, unless you reduce your watering regime at that time, the the soil will become too wet.
If you don’t keep your soil dry, fungus gnats will lay their eggs in the moist soil. Their eggs are almost impossible to see, and you’ll only notice them once they’ve grown.
You can also create a DIY sub-irrigation or self-watering system to keep the top of your plant’s soil dry. For example, place the pot in a deep saucer or bowl and water into that bowl/saucer so the plant sucks up its required moisture from below.
3. Remove decayed plant materials
Fungus gnat larvae feed on decayed, organic matter like dead leaves and roots. Removing decayed plant materials will eliminate some of the gnat’s food source and prevent them from thriving. So, check your pots regularly, and clean out damp surface debris.
4. Change to better potting soil
Potting mixes retain moisture and this is a great attractant of fungus gnats. So, it is a good idea to ensure that when you store your potting composts, you prevent them from becoming too wet.
When topping up or changing the soil in your pots, use fresh, dryish mix. This is a balancing act though, because potting mixes that dry out completely can become difficult to re-wet. This is know as soil hydrophobicity Completely dry potting mix is to be avoided because it will not retain the water that the plants’ roots need.
Some potting soils are sold as sterile. However, I am pretty dubious about these. This is partly because some are treated with chemicals, which does not accord with the way that I want to garden or raise plants. But it is also, because so-called sterile soil can never remain sterile for long once it is outside the processing plant and stored in the home environment.
5. Use diatomaceous earth
Diatomaceous earth is a naturally occurring mineral product. It is a chalk-like substance made from siliceous sedimentary rock and helps control pests by drying them out and killing them.
Place a layer of diatomaceous earth on the top of the potting soil, or mix into the top layer. Either use a wettable form or wear a mask when handling the product, as the fine white dust can get into your lungs.
Also make sure you use the food grade version of the product, not the type that is used for swimming pool filters.
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6. Use horticultural sand or grit
Horticultural sand is a gritty sand made from crushed sandstone, granite, or quartz, and it helps with soil drainage. Horticultural grit is, as the name suggests, more gritty, with larger size particles than the sand.
A layer of sand or grit on top of the soil in your pot will make it difficult for the adult gnat to penetrate the soil to lay eggs. Sand, in particular, will help dry out the top layer of potting mix and therefore create an inhospitable environment for the gnat eggs and larvae.
7. Use Cinnamon
Dried cinnamon powder is also a useful weapon against fungus gnats. Cinnamon is known to have anti-fungal properties, so it works to eliminate the material in your potting compost that the gnat larvae feed on. I haven’t seed any convincing evidence that it kills the larvae themselves, nor does it kills adult flies. But when used as part of a multi-pronged approach to fungus gnat control, it can be an effective weapon.
8. Attract and Trap the Fungus Gnats
If fungus gnats are already flying around your plant and home, you can reduce the impact by trapping and killing them.
One natural way to do this is by putting red wine or apple cider vinegar in a deep bowl or glass and adding a few drops of liquid dish soap. Then, put the bowl near your gnat-infested plant, and change the solution every couple of days. The solution will attract the gnats and trap them, leaving them to drown.
A variation on this approach is to cover the bowl with cling film and pierce it with a few holes. The flies will crawl in to get to the fluid, but will not be able to get out.
Clearly, this method deals with the nuisance impact from the adult gnats, but will not affect the larvae or eggs that are already in the soil.
9. Use Sticky Traps
Another way to deal with adult fungus gnats is with insect sticky traps. These are often used in greenhouses and are not the most attractive thing to have on display in your home. But if you hang the sticky trap up near your infested plant for a few weeks, you have a good chance of killing enough adult gnats to diminish the infestation significantly.
You can also create your own insect traps by using a piece of yellow cardboard and applying vaseline over it. Ensure that you’re using a yellow colour since it attracts fungus gnats. Then, place your trap above your plants horizontally, and wait until it attracts and traps fungus gnats.
10. Kill adult fungus gnats with soap solution spray
A spray made from liquid dish soap or Castile soap can be an effective home made insecticide. It works by penetrating through the fungus gnat’s protective body surface and kill them via suffocation and dehydration.
To create the solution add one table spoon of soap to approximately two pints or 1 litre of water and shake well.
11. Use hydrogen peroxide to kill the fungus gnat larvae?
Hydrogen peroxide is said to be one of the best options for direct control of the fungus gnat larvae.
Dilute one part of hydrogen peroxide with four parts of water and pour them into your plant soil. This solution is safe for your plants and best used weekly until the gnats are gone.
I have seen lots of websites repeating the idea that hydrogen peroxide directly kills the fungus gnat larvae. Admittedly, I have not tried this method myself (as I said above, I am pretty wary of any chemicals), but neither have I seen any direct evidence that hydrogen peroxide works in this way.
What I have seen, is evidence from the US Environmental Protection Agency that hydrogen peroxide can kill “microbial pests on crops growing indoors and outdoors” and that the “active ingredient prevents and controls bacteria and fungi that cause serious plant diseases”.
It therefore sounds to me as if hydogen peroxide kills the fungi within the damp potting soil that fungus gnat larvae feed on, rather the larvae themselves.
12. Bacillus thuringiensis var. israelensis
This naturally occuring microbe, commonly know as BTi which is available in solution form can be watered into the soil. The microbes will attack and kill the fungus gnat larvae.
However, BTi is not necessarily effective on the larvae at all stages of the larvae lifecycle. Therefore repeated use is needed in order to ensure an infestation can ultimately be controlled.
BTis are also found in mosquito bits, which operate to kills mosquoto larvae, as well as fungus gnat larvae.
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13. Beneficial Nematodes
Nematodes are microscopic thread like worms that attack and ultimately kill fungus gnat larvae. Various commercial preparations are available. But they usually come in packs that need to be mixed with water and then watered into the potting soil.
The Nematodes are live creatures, so the packs need to be refrigerated and have a short ‘shelf life’.
Don’t use nematodes at the same time as using other drenching techniques, such a hydrogen peroxide or Neem oil (see below), as these will kill the nematodes.
14. Neem Oil
Neem oil is a naturally occurring pesticide derived from the seeds of the neem tree. It works by affecting the lifecyle of the fungus gnat larvae (and other pests) by interfering with feeding, egg-laying and mating.
Neem oil needs to be mixed with soap and water in order to create a solution that will allow the oil to be applied effectively to the potting soil around the plants.
Add 1 tablespoon or liquid soap to half a gallon of warm (not hot) water and then add a tablespoon of neem oil. Mix thoroughly and drench the top layer of potting soil. Repeat every seven days until the infesation seems to have gone.
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15. Repot Your Plant
If all else fails, or if you want a quick solution to fungus gnat infestation, take the plant out from the pot, remove most of the soil without harming its roots, and transfer it to another pot with healthy, gnat-free soil.
Then, don’t forget to place the gnat-infested soil into a bag, seal it, and throw it away. Unfortunately, that soil will be unusable since it’s already crawling with fungus gnats or larvae, and using it again can still spread the gnats.
16. The regime
If you’ve read this far, you’ve probably realised that the best way to deal with a fungus gnat infestation is through a combination of the methods outlined above. Here is a suggested regime:
- Use a sticky trap or vinegar trap to reduce the nuisance from the flying adult population;
- Use a soap spray to kill any adult gnats on the leaves or surface of the potting soil;
- Use potato slices to draw the larvae to the surface. Check and renew every few days and dispose of the infested slices;
- Modify your watering regime so the top inch or two of potting compost dries out between waterings;
- Pour a solution of neem oil over the potting soil every 7 days until no more gnats are appearing.
- As an alternative to neem oil, use a BTi or nematode preparation.
What are fungus gnats?
It is worth knowing a bit about fungus gnats and their lifecycle, because this will help you understand what works best when dealing with these nuisance gnats.
Fungus gnats are small dark coloured flies of various species and families, such as Mycetophilidae, mainly from the superfamily of Sciaroidea. They are often known as Sciarid flies and sometimes called plant gnats or soil gnats.
Fungus gnats can be confused with fruit flies, which are attracted to over-ripe fruit and vegetables in your kitchen. Fruit flies are lighter in colour than fungus gnats (more tan that brown) and have red eyes.
Fungus gnats typically infest moist or unsterilised soil and potting mix in house plants, or in plants in containers in a greenhouse or conservatory. They don’t bite or cause any harm to humans or pets. However, they can be a problem.
They feed on fungi and organic matter, and the larvae are known to feed on tender young plant roots. So, although a fungus gnat infestation does not do much damage to mature plants, the gnats can stunt the growth of young plants, by damaging the immature roots and affecting their ability to take in nutrients.
Therefore, fungus gnats are mainly just a nuisance when they are in the potting soil of any mature house plants. But if you are raising cuttings or seedlings indoors or in a greenhouse, they can cause real damage.
The lifecycle of fungus gnats
Female fungus gnats lay eggs in tiny ribbons in the gaps and spaces in moist potting soil. The fungus gnat larvae hatch in about 3 to 4 days and then develop through 4 larval stages over about 10 days until they develop into rounded, oblong pupae.
After another 4 days or so the adult gnats emerge from the pupae. In fact it can be only 17 days or so between successive generations of fungus gnats. In warmer conditions development can be speeded up and more generations can occur, more quickly.
Protect your houseplants today
So, although fungus gnats aren’t dangerous for plants, people, or pets, they can limit your plant’s nutrient intake and stunt growth in young plants.
Moreover, having them in your home can be annoying and unhygienic, especially when there’s already a swarm of them.
Try the methods recommended above to prevent or get rid of gnats infestation in your houseplants. Once you’ve tried them, let us know which one is the most effective for you.
Martin Cole has been an avid gardener for more than 20 years and loves to talk and write about gardening. In 2006 he was a finalist in the BBC Gardener of the Year competition.
He previously lived in London and Sydney, Australia, where he took a diploma course in Horticultural studies and is now based in North Berwick in Scotland. He founded GardeningStepbyStep.com in 2012. The website is aimed at everybody who has been bitten by the gardening bug and wants to know more.
Gardening Step by Step has been cited by Thompson and Morgan, the UK’s largest mail order plant retailer, as a website that publishes expert gardening content.
Check out my comprehensive step by step guide, with plain language explanations and ultra-useful images and illustrations. This is for you if you love dahlias and want to the best out of the dahlias you grow.