How To Control Fungus Gnats On House Plants
Gardening and plant experts Geoff Hanbury & Andy McIndoe discuss fungus gnats.
Andy McIndoe teaches courses including Gardening for Wildlife, Shrubs & the Garden Design Diploma
If you're interested in doing a course that teaches you how to Maintain Plant Health including getting rid of pesky fungus gnats, have a look at our RHS Level 2 Certification - Principles of Plant Growth and Development.
Andy: Fungus gnats are those annoying, tiny little black flies that you find wafting around your pot plants.
They are notoriously difficult to get rid of and I never know how to advise gardeners on this one. I was particularly interested to hear from Geoff Hanbury who has researched all the possible solutions:
Geoff: Despite being one of the most common indoor plant pests, fungus gnats are notoriously hardy and difficult to get rid of.
They are drawn to moist conditions and, as their name suggests, feed on fungus and other organic matter. In their larval form they eat plant roots, which can make them a potentially lethal infestation for young or vulnerable plants.
In their adult form they can be identified by their long legs, squat body and their semblance to tiny mosquitos. In their larval form they are white or translucent and around 5mm long, with a distinctive black head.
There are no-end of “effective” remedies online in gardening forums, but it is hard to know exactly which method is the most effective.
I decided to put my recent fungus gnat infestation to good use by putting the most popular remedies to the test.
Quick Fixes
1. Drying Out Soil
Many infestations happen as a result of overwatering plants. By waiting longer between watering sessions, theoretically you can make your plant soil dry and inhospitable to gnats.
Method
Turn up around 2cms of soil in the plant. This exposes the larvae and eggs and helps dry out the soil more quickly.
Wait until the soil is dry to a depth of about an inch and a half before watering again.
Did It Work?
It did reduce the number of larvae wriggling about underneath the soil, but it didn’t come close to eliminating the problem completely.
Because many plants can’t be chronically under-watered for a long time, it means that once you resume normal watering, the gnats will just multiply again.
2. Cider Traps
Gnats are drawn to apple cider vinegar, which makes it the perfect bait in this handy trap.
Method
Fill a wide brimmed jar or bowl with apple cider vinegar, cider or beer
Add a drop or two of washing up liquid to get rid of surface tension
Cover the rim with cling film and pierce with a fork a number of times
The gnats will now crawl through the small holes in the cling film, but will be unable to escape and will drown in the vinegar
Did It Work?
The main limitation of this method is that it only kills adult gnats, and therefore doesn’t do anything to kill the hundreds of larvae and eggs which still remain in the soil.
That said, it did kill a fair deal of adult gnats, meaning that the breeding population of gnats was heavily curtailed and it therefore reduced overall numbers.
3. Potato Slices
Gnat larvae love raw potato, which makes potato slices a cheap way to measure how heavy the infestation is and get rid of some larvae in the process.
Method
Slice or dice a potato into inch wide blocks
Bury potato pieces just beneath the soil surface
Wait 4-8 hours and then pull out the chunks. If the infestation is heavy, the potato will be literally crawling with larvae.
Throw away the potato to get rid of all the larvae cling-ons
Did it Work?
Each potato slice can only get rid of dozens of larvae at a time. Since adult gnats can lay 200-300 eggs at a time, getting rid of a heavy infestation with potato slices alone would be impossible.
However, as a way to measure gnat population size and see if your other methods are working, it is fantastic.
4. Chamomile Tea & Cinnamon
Chamomile and cinnamon are powerful natural fungicides, which kill off the gnats’ primary food source, therefore making the soil inhospitable.
Method
Brew a litre of strong chamomile tea with boiling water, allow it to cool and mix the tea with 4 parts water. Use tea mix to water plants as usual.
Sprinkle cinnamon all over the surface of the soil for a fungicidal double whammy.
Repeat for 3 weeks, or until gnats are gone
Did It Work?
Within days the chamomile/cinnamon dousing had dramatically reduced the gnat populations, however, over the next few weeks it couldn’t quite kill off the remaining population.
Weapons of Gnat Destruction
If these cheaper, natural methods still haven’t managed to rid you of your gnat infestation, then you can escalate things with these more powerful gnat killers.
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All Gardening courses5. Hydrogen Peroxide
A peroxide solution apparently kills larvae and eggs on contact, effectively killing off all infant gnats and disrupting the lifecycle.
You should be able to buy hydrogen peroxide from any high street pharmacy or online.
Method
Mix 1 part 9% hydrogen peroxide with 6 parts water.
Allow top layer of soil to completely dry out before drenching the soil in the peroxide solution.
After fizzing for about 30 seconds, hydrogen peroxide will break down into oxygen and water, both of which are harmless to your plants
Did It Work?
The peroxide drench was very effective in killing larvae, although it never completely killed off the gnat population within the 3 weeks I used it.
6. Food Grade Diatomaceous Earth
Diatomaceous earth (DE) is made from microscopic fossil shards and shreds the larvae and the gnats upon contact.
You can buy DE from garden centres or on Amazon.
If you buy DE, make sure you buy the food grade version, not the product made for pools.
Method
Apply a layer of DE to the top of the soil, or mix it into soil mix if you are repotting a plant.
It is a good idea to wear a mask while handling DE
Did It Work?
DE was very effective in killing off both larvae and gnats, with good results within a week. After around three weeks, the population was all but gone.
Although it is effective, it is quite expensive and, when used outside, will also kill helpful insects such as worms.
7. Bacteria Thuringiensis
Bacteria Thuringiensis (BT) is a natural bacteria which is hugely effective in killing gnats. However, these products are very expensive to buy.
Fortunately, a much cheaper option is to buy something called mosquito dunks, or mosquito bits, which contain the same bacteria, although in a lower concentration.
Method
Mosquito Bits. Sprinkle bits on soil, generously covering every bit of the surface
Pour a handful of bits into a gallon of water, and leave it to infuse for a day or two
Use this infusion to water the plants as normal
Does It Work?
BT is very effective in killing off the larvae and will disrupt the lifecycle of the population, meaning that you can be totally gnat free within a fortnight or so.
Within days of applying the bits, you’ll notice a huge dip in gnat numbers.
However, if you have a lot of plants which are infected, buying BT products can get expensive.
8. The Ultimate Gnat Killing Solution
While by themselves the different methods I used had varying degrees of success, you can use a number of them in conjunction to completely eradicate your gnat problem fairly quickly.
I’ve put together this gnat killing regime which is effective and cheap.
- Till the first inch or so of soil
- Sprinkle ground cinnamon all over the surface of the soil
- Set up vinegar traps around the plant and change them daily
- Insert slices of potato daily
- Do not water the plant until the top layer of soil is totally dry
- When top soil is dry, thoroughly drench the soil with the hydrogen peroxide solution
- Once the hydrogen peroxide solution has stopped fizzing, repeat step 1 onwards
Repeat steps 1 to 7 for a fortnight or a month, depending on the severity of the infestation.
If the gnats are still not wiped out, you’re going to have to invest in the more expensive options and follow these steps:
- Sprinkle diatomaceous earth all over the surface of the soil
- Till the first inch or so of soil
- Keep vinegar traps around the plant
- Insert slices of potato daily
- Soak mosquito bits in a gallon of water. Use this “bits-mix” to thoroughly drench the plant when the top soil is dry
- Repeat step 1 onwards
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