Get rid of fungus gnats for good

Fungus gnats are a real nuisance and if you leave a fungus gnat infestation untreated you can set yourself up for a problem in the future.

Here is my tried and tested 3 step method to get rid of a fungus gnat infestation for good.

1. Set up sticky traps
2. Cover the surface of the soil with a layer of sand or stones
3. Perform a hydrogen peroxide/water or Neem oil/water drench of the plant’s root system

Now step back and observe for a week or two. Should the fungus gnat infestation persist, simply go through all 3 steps again.

As with all things related to pests and diseases, early prevention and treatment is key. I have found that treating plants for fungus gnats before the problem gets too severe is key to getting rid of the problem in the first attempt.

Should the infestation be a bit more severe, you may need to go through several cycles to get rid of the fungus gnats for good.

Step 1. Set up sticky traps

Fungus gnat caught in sticky paper trap
Fungus gnat caught in trap

Whilst it’s important to rid your plants of fungus gnats, the flies themselves do not harm your plants. This does not, however, mean that they are harmless. 

Fungus gnats are in fact the direct cause of many problems. They lay the eggs that eventually hatch into the larvae, that in turn damage the roots of your plants.

And this is why every fungus gnat caught in a trap is a small victory. 

Step 2. Cover soil with sand or stones

Fungus gnat eggs and larvae thrive in wet and moist soil. Covering the soil with a 1-2 cm (½ inch) layer of stones or coarse sand will effectively deny the fungus gnats access to the moist soil they need to grow and thrive. 

Using small stones like driveway shingle or pebbles work well when covering the soil. Sand tends to “disappear” as it becomes part of the soil over time.

The key here is that stones and sand drain and dry quickly leaving the fungus gnats an unwelcoming and dry environment.

Step 3. Perform root system drench

The last of the 3 steps is directed at attacking and killing any fungus gnat infestation in the actual soil.

And we do this by drenching the plant with a solution that will clean the soil of fungi and kill any fungus gnat larvae in the soil. 

You can use a water and hydrogen peroxide (3%) solution or a Neem oil and water solution.

Use hydrogen peroxide to kill fungus gnats

Treat your plant with hydrogen peroxide by following these simple steps:

  1. Let the infested plant dry out
  2. Mix 1 part hydrogen peroxide (3%) with 4 parts water
  3. Drench the plant and let the excess run off

Please note that we always use the readily available 3% hydrogen peroxide.

Observe the plant over the next week or two.

Does Neem oil kill fungus gnats?

Use Neem oil water solution to kill fungus gnats
Use diluted Neem oil to kill fungus gnats

We find Neem oil mixed with water to be very effective in getting rid of fungus gnat infestations.

But it does help to start your Neem oil treatment before the fungus gnat infestation gets too severe.

There are many different types of Neem oil products in the market. Follow the instructions on your Neem oil packaging.

  1. Let the plant you are treating dry out
  2. Mix Neem oil with water and ecologically friendly dishwasher detergent
  3. Drench the plant using the mixture and let any excess run off

We use 100% cold pressed pure Neem oil and mix our solution per instructions below:

  • 5 ml Neem oil 
  • 1 litres of water 
  • about 1 tablespoon of ecologically friendly dishwasher detergent.

But again, you should dilute the Neem oil with water per your product manufacturer’s directions. 

How you know you have a fungus gnat problem

As with all pests and diseases you want to catch the problem early on. And visual inspection is of course one of the easier ways to detect small flies in the air.

Pay special attention when top watering your plants as it will disturb the flies and cause them to fly off. 

But besides visual inspection you should also set up traps right from the beginning of the season. Yes, set up the traps early on before the growing season has started.

Now observe and see if you catch any fungus gnats. And when you do, consider starting the treatment early to avoid facing a fungus gnat infestation.

I use two different types of fungus gnat traps

1. Yellow sticky fungus gnat traps

Once a year I buy a pack of yellow sticky paper traps from an online retailer. I place these yellow sticky traps strategically in pots, containers and grow bags taking care not to touch the actual plants.

2. Apple cider vinegar fungus gnat traps

The idea of an apple cider vinegar trap is to create a mixture that will attract the fungus gnats. The fungus gnats will attempt to land but will get caught and drown.

I use a plastic PET bottle, apple cider vinegar, a few pieces of sweet fruit and a few drops of washing up liquid detergent to make our apple cider vinegar traps.

7 steps to making a fungus gnat apple cider vinegar trap (video below)

  1. Remove cap from plastic bottle
  2. Draw a line ⅓ from the top of the bottle
  3. Cut the plastic bottle along the marked line
  4. Put 3-6 cm / 1-2 inches of apple cider vinegar in the bottom part of bottle
  5. Add a couple of small pieces of sweet fruit to the apple cider vinegar
  6. Put 1-3 drops of washing up liquid detergent in the apple cider vinegar and stir gently
  7. Place the top half of the bottle inside the bottom half – pointy side down

Your trap is now complete and ready to be placed among your plants. Be careful not to mix in the washing up liquid detergent too aggressively – you do not want foam or bubbles to form. 

Watch this video on YouTube.

The role of the washing up liquid detergent is to break down the surface tension of the apple cider vinegar making it impossible for the fungus gnats to land on the surface. 

How to avoid problems with fungus gnats

When it comes to pests and diseases there is only one thing that beats early detection and that is prevention.

And there is a lot you can do to minimise the risk of attracting pests like fungus gnats. 

Here I will list methods and good habits that will help you protect your plants and growing environment against fungus gnats and many other pests and diseases.

1. Do not overwater 

Fungus gnats thrive in damp soils and you should take care not to overwater your plants. 

This is especially true during times of the year when plants use less water like winter or early spring. Also be mindful of over-using organic materials like peat moss that retains water and moisture extremely well. 

2. Allow soil to dry out between each watering

Fungus gnats lay their eggs in moist soil and these eggs later develop into the larvae that feed on the root system of your plants. 

But the eggs and larvae cannot live in dry soil. When you allow the soil of your plants to dry out between waterings, the eggs and larvae will die.

You should of course not let your herbs and vegetables dry out to the point where they wilt or look distressed. But it is safe to let the top layer of soil in any pot, container or grow bag to dry out between waterings.  

3. Bottom watering beats top watering

When possible, always opt for bottom watering your plants to avoid the top of the soil getting overly damp and moist.

To bottom water your plants simply place the pot in a large container with water. Allow the plant to absorb water through the drain holes in the bottom of the pot. 

Remove the plant from the water bath when the soil starts to change colour on top. 

Now place the plant in a place where it can drain off any excess.

4. Avoid high levels of humidity

Watering plants in combination with high temperatures can lead to a high level of humidity. 

This humidity will in turn keep the soil damp and moist for the fungus gnats to thrive and develop.

We sometimes get problems with high levels of humidity when the weather breaks from cold to warm. Typically, nights are cold but days are warm. 

As nights are cold, windows are closed to protect our plants. But this also leads to poor air circulation during the day when the temperature heats up. 

The result is sometimes high levels of humidity catching us by surprise.

5. Potting soil that drains well

Soil that drains well typically dries more quickly. And as the surface of the soil dries first the fungus gnats find themselves without any damp soil. 

I cannot stress the importance of soil that drains well enough. 

And I still maintain that more seeds, seedlings and plants die from overwatering compared to those than die from drying out. 

6. Do not let plants sit in water

If your pots or containers are sat on a plate or saucer, always empty any excess that collects after watering. 

Plants should never sit in the wet. 

This is also why it is important to let pots drain after watering. You always want excess water to drain off. 

7. Use good quality soil

It is so easy to say “always buy new soil and never reuse old soil”.

And if it is an option for you, great. Always using new and fresh soil is a good thing.

But we reuse soil all the time. We will of course improve the soil before reusing it. But it just gets too expensive for us to buy new soil for all our herbs, vegetables and plants every time we pot-up, plant or transplant. 

And we have found a method that works for us. Soil where we have had problems with fungus gnats is placed in a sealed plastic container. This container is placed in a location with full sun for at least four weeks. 

The process is referred to as solarization [1] and is a non-chemical process for getting rid of soil borne pests and diseases.

We are unable to use the soil straight away, but we are able to reuse it and it does save us a lot of money.

Fungus gnats and what to do when all else fails

It is important to remember that you can do everything right and still suffer from fungus gnat infestations. After all, it only takes an open window for a fungus gnat to enter into your growing environment.

And there can be times when the infestation has got to the stage where it is hard to treat effectively.

We always have sticky paper and apple cider vinegar traps on the go. And if needed we typically drench the plants using a Neem oil and water solution.

If we still experience problems our last step is to repot the plant and remove as much soil as we dare from the plant’s root system. 

We then add new soil and top the soil with a layer of stone shingles.

So far, these tips, tricks and methods have been enough to control the fungus gnat problems we encounter every year.

Understanding the 4 stages of development of the fungus gnat

  1. The most effective way to fight off an infestation is to break the cycle that feeds the process.
  2. Fungus gnats go through 4 stages of development: egg, larvae, pupae and adult flies.
  3. Eggs hatch in about 3 days, the larvae take another 10 days or so to develop into pupae and finally after an additional 4 days the adult flies appear. [2] And the warmer it is the faster the process. 
  4. This is why we have a 3 step process that attacks the mature flies as well as the soil borne eggs and larvae at the same time.

Again, it is critical to understand how it all fits together to be a successful gardener.

DIY fungus gnat traps to get rid of fungus gnats for good
Sticky paper fungus gnat trap protecting overwintering herbs in one of our greenhouses

[1] http://ipm.ucanr.edu/PMG/PESTNOTES/pn74145.html

[2] http://ipm.ucanr.edu/PMG/PESTNOTES/pn7448.html

https://pss.uvm.edu/ppp/pubs/el50.htm

Meet the author: Mattias is an experienced gardener spending most of his free time on his knees among herbs, plants and garden vegetables. For the past two years he has been sharing gardening projects and how-to tutorials on the NordicLavender website and YouTube channel.