Houseplant pests series part 1: Soil or fungus gnats

This is my first in a series of posts about common household pests. I am starting with soil gnats (also referred to as fungus gnats) because in my opinion, these are the most common houseplant pests. Unlike most houseplant pests, which usually only effect certain types of plants, soil gnats can effect any plant that is potted in dirt. 

Soil gnats

You will notice them crawling or flying out of the soil around your plant when you water or otherwise disturb the soil. 

Soil gnats look similar to fruit flies, and I have seen many people mistaken a soil gnat problem with fruit flies. 

Soil gnats lay their eggs in moist soil where the larvae will hatch and feed on small roots, fungus and other organic matter in the soil. 

They are mainly just a nuisance and are rarely destructive to the plant. Sometimes they can cause root damage if the infestation is heavy, but normally they only eat rotting roots.  

A soil gnat infestation can come from anywhere. They can be in the soil of a newly purchased plant or a bag of potting soil, they can come in with a plant that was outside during the summer, they can even come through the screen of an open window during the summer. Even if you are successful in eliminating an infestation from your house, recurring problems with soil gnats are difficult to prevent. The good news is that soil gnats are one of the easiest houseplant pests to control. 

Soil gnats are difficult to eliminate if you have a large number of plants. The adults can easily fly or jump from one plant to the next, laying eggs wherever they find moist soil. Like fruit flies, the adult gnats only live for a few days. Once all the larvae are dead, your problem will go away. 

As I stated earlier, soil gnat larvae thrive in moist soil. Actually, they can’t even survive in dry soil.

Sticky paper fly trap to capture soil gnats

Therefore, the easiest and most effective way to control and ultimately eliminate soil gnats is to make sure you do not regularly overwater your plants. 

 Always check the soil before you water a plant by sticking your finger into the soil. 

Do not water the plant if the top inch of dirt is moist. 

Be careful though, you don’t want to allow the entire root ball to dry out on most houseplants; and you definitely don’t want to wait until the plant has wilted before watering it. 

Here is a list of other things you can do to control a soil gnat infestation:

• Water the plant from the bottom. Soil gnat larvae live in the top inch of the soil. Watering from the bottom will make it easier to maintain dryer top soil without risking the overall health of the plant.
• Put a fly trap near the plant to capture the adult gnats. Note that this will only be effective to control the adult population, it will not take care of the problem at the source (the larvae).
• Pour or spray insecticidal soap into the top of the soil to kill the larvae. This should be effective after a few treatments. You can also use a mixture of dish soap and water rather than an insecticidal soap.
• Remove the top inch of soil and replace it with new, dry soil. This will remove the eggs and larvae. The larvae could still hatch and mature in the soil after you remove it, so make sure you remove the soil from your home immediately.
• Replace the top inch of soil with a layer of sand, gravel or decorative moss. This will help detour the gnats from laying eggs in the soil, and also adds a decorative touch.
• Store potting soil in a sealed container. I store mine in buckets that have a tight lid rather than in the bag it comes in. Also, don’t store potting soil outside in the summer.

Please feel free to leave a comment and share your tips on how you deal with soil gnats. I would love to get more ideas from others out there that have found a better way!

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