So in order to get rid of fungus gnats, let the top 2 inches of soil dry before watering, you can also use bottom watering technique, hydrogen peroxide, BT solution, mosquito bits, and spinosad to get rid of fungus gnats as well.
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.
One surefire way to get rid of them is using Vinegar. It is an effective method for controlling gnats in indoor plants. In addition to sticky traps, this is another effective strategy. A simple solution is to fill a shallow basin with vinegar and add a few drops of dish soap. And place it beside your plant.
Because fungus gnats thrive in moist conditions, especially where there is an abundance of decaying vegetation and fungi, avoid overwatering and provide good drainage. Allow the surface of container soil to dry between waterings. Clean up standing water, and eliminate any plumbing or irrigation system leaks.
Homemade gnat killer spray: A mixture of half a cup of water, half a cup of isopropyl alcohol, and a teaspoon of dish liquid can be filled in a spray bottle. This mixture can be sprayed on the gnats directly to kill them.
Use Lavender Spray Or Plants
Gnats are not fans of the aroma of lavender, so whether you want to plant your lavender because you enjoy the look as well as the scent or choose to use a liquid substitute to this scent, gnats will avoid it.
You just need to use a few drops of dish soap and a table spoon of vinegar for each cup of water. With this these destructive gnats will parish with just a few spritz of your new chemical weapon. The best part is that you don't have to worry about this solution harming your indoor plants.
2 Steps to Gnat-Free
Discard the soil immediately, and replace this top layer with sand, which is free of organic matter and dries out quickly—an unappealing medium for fungus-gnat eggs. Let the soil below dry out completely before the next watering.
With all that in mind, the EPA says that hydrogen peroxide in low doses is indeed safe for plants. The caveat is of course, in the concentration. Too much can damage plants.
In cooler weather it can take 4 to 5 weeks or more to complete the life cycle, so winter treatment can take longer. That's why I always recommend treating for 2 months (8 weeks), but at least 4 weeks minimum if you've caught it early and only have a tiny infestation.
However, it is also a great solution for a gnat infestation. Combine 20 drops of tea tree oil with a litre of water. Water the infested plants with this mix, and you should help remove any larvae in the soil. For the adults, use some adhesive traps around the plant.
The only way to get rid of gnats without killing them is to repel them with scents they don't like: vinegar, vanilla, pine oil, peppermint, lemon, eucalyptus, and lavender. Even dryer sheets could help. Of course, the first line of duty would be to get rid of the infestation's root cause.
A fungus gnat infestation is a common problem that's not likely to go away on its own.
Use Hydrogen Peroxide
“You can use 20% hydrogen peroxide (preferably 3%) mixed with 80% water to kill gnat larvae immediately,” Schutter says. “Slowly drench the soil until the mixture comes out of the drainage holes.”
Nematodes such as Steinernema feltiae and predatory mite Hypoaspis miles are both predators that attack the fungus gnat larvae in the soil. Both predators are very popular with experienced growers as a preventative measure.
Combine equal parts vinegar and soap and then add a few tablespoons of sugar – once you mix it all together you can place the bowl wherever the gnats have been gathering. They will be attracted to the sugar and vinegar and drown in the mixture. Traps can also be made out of red wine and dish soap, or rotting fruit.
If you're spraying or watering the soil for fungus, eggs or larvae, be certain you really soak the soil. If you have any of these problems, you can use hydrogen peroxide on your plants once a week.
Prevent Fungal Disease
MAKE IT: Mix 1 teaspoon of baking soda and 2-3 drops of liquid soap in 1 liter of water. Spray the solution on the infected plants. Baking soda helps the plants become less acidic and prevents fungal growth.
To get rid of fungus gnats, mix a solution of one part hydrogen peroxide with four parts water. Pour the mixture onto the soil around the infested plant.
For hydrogen peroxide, mix one part 3% hydrogen peroxide with four parts water. Carefully pour this solution onto the top of the soil, ensuring it reaches the root zone. The hydrogen peroxide will help kill the gnat larvae and any eggs in the soil without harming your plant.
Simply cover your potting mix with a small layer of sand and you are good to go. This creates a barrier that the gnats cannot get through. If they can't lay the eggs, they can create an infestation.
Steaming: Steaming is considered one of the best ways to sterilize potting soil and should be done for at least 30 minutes or until the temperature reaches 180 degrees F (82 C). Steaming can be done with or without a pressure cooker.
Gnats can't stand the smell of lavender, so consider planting lavender in your garden. You can also use lavender oil in a spray bottle with water. Spray it in areas where gnats are a problem. Lemon is another scent that drives gnats away.
Vinegar is a contact herbicide, so you can unintentionally kill plants in your garden if you accidentally spray them with vinegar.
Mix one tablespoon of lemon-scented dish soap with two liters of water. Pour into a spray bottle. Spray the infested houseplant liberally with the soap mixture. Be sure to coat the soil, stem and leaves thoroughly.