I’ve had houseplants for about 15 years, and found this information useful. There are several ways to kill gnats that are infesting your house plants without killing your plants. The first way is to prevent the gnats before they even start, it’s also the easiest.
When you purchase soil, open up the bag and let that baby dry out! If you don’t live in an environment where your soil can dry out, you can microwave it. Yes, I know it sounds odd, but microwaving the soil will kill off the gnat eggs that are infesting your soil. You can also freeze the soil, stick it outside in the cold for a few nights but be sure it’s below twenty-two degrees outside. If you don’t live in an environment where that’s an option to place your soil outside to get cold, you can place it in your freezer (if there is room of course). If however, your houseplants are already planted in a gnat-infested soil you can move on to the other options below.
First thing you need to do is stop over watering your houseplants! This makes the gnat eggs ripen more. Make sure your houseplants have good drainage holes and place in direct sunlight in a drafty location.
Let your soil dry out, once the soil has dried out enough pour a bit of rubbing alcohol on the soil. Doing this will dry out the gnat eggs and kill them off. Only do this once, and I have to make a side note here, in England the rubbing alcohol contains oil for some odd reason in it, do not use that.
Another option is a tiny amount of Murphy’s Oil Soap per gallon of water, use only about a cap full or less. The oil will suffocate the gnat eggs. Using this option you have to make sure the soil is dry again and you basically drown the houseplant with it. It will take longer than the rubbing alcohol though, and yes, it is safe. Only do this option about once every week until the gnats are dead. This should only take a month maximum.
Going to extremes, if over watering your houseplants isn’t your problem and you have tried all the above and are still infested, you can purchase sand and coat the top layer of the soil with it. This will prevent gnats laying more eggs. Do not suffocate your house plants, merely sprinkle the top of the soil with it
If you have a bowl under your plants that they are draining into, be sure to empty that often, the less moisture the better. Once you notice no gnats in your houseplants, you can go back to watering them as you see fit. However, be sure to wait a month to double check.
If the following doesn’t help with gnats infesting your houseplants they may be infesting something else in your house. Be sure to check around for other moist environments.