Fungus Gnats and How to Deal With Them
Fungus gnats are an insect every plant owner is familiar with. There are various remedies going around the internet on how to rid your plants of them. Spoiler Alert: Most of those "treatments" don't actually work, or do work, but harm the plant in the process. Here I will bust the myths of three of the most commonly recommended remedies, talk about how to best control them, and why they don't cause the harm you think they do.
There was a study done that treats fungus gnats specifically with cinnamon. The part you might not know is the concentration level and the type of cinnamon. Using grocery store cinnamon does nothing to control fungus gnats. The level of concentration rivals that of essential oils and the type needed is not available in the grocery store.
2. Hydrogen Peroxide
Hydrogen peroxide (H2O2) is a stress hormone in plants. When this chemical comes in contact with cells, it is working on destroying the cell membrane and the DNA housed inside the nucleus. While this process isn't as fast on plants since they have the cell walls, it still happens. The plants also actively try to rid themselves of this chemical, just as your body actively tries to rid itself of cortisol (the human stress hormone). Nothing grows well when under stress.
3. Bottom Watering
Bottom watering, when done properly will soak the soil all the way to the surface. This immediately negates the theory that because the water isn't reaching the top of the soil, there will not be fungus gnats. There is some truth in this that will be addressed later in this article, but just know for now that if you are not leaving your plants to "butt chug" long enough for the top of the soil to look wet, you are not giving your plant significant water. Some soils are too chunky or have too little adhesive abilities for the water to wick itself up. Those soils should not be bottom watered.
What do we do to actually control these nuisances? First, it has to be recognized that, that is all they are: a nuisance. Fungus gnats bother humans significantly more than they bother plants. The way they survive is by eating fungi that grows in potting soil during their juvenile state. While it is true that, if food runs low and populations are high, they will start to nibble on roots, the chance of this happening is less than likely. It would have to be such a severe infection that the whole plant room, house, greenhouse, etc would have clouds of adults flying through the air. Second, the most common way fungus gnats continue to thrive is due to over watering. Some plants, like ferns, carnivorous plants, and others, require wetter soil. This cannot be helped and fungus gnats are a fact of life as a plant parent with those plants. Allowing plants to dry properly between watering is the best way to treat these little pests. If plants are allowed to dry completely at least two inches down, it will kill both juveniles and the fungus the gnats feed on.
This does not provide a fast result and if you water one pot while letting another dry, the gnats will hop from pot to pot. It is best to let them all dry at once. Their populations will lower and potentially disappear over time. If you have a plant that requires wetter soil and cannot allow it to dry without risking the health or life of the plant (i.e. sundews, maiden hair ferns) mosquito dunks are an option. I don't recommend them often, but they do actually work and will properly provide the relief you may need from the annoying flying black dots.
P.S. Yellow sticky cards were never meant for control. They are meant to monitor populations. They will not attract adults to them, they just exist near your pots and if one lands on it, they will become trapped.