Soil gnats (also referred to as fungus gnats) are probably the most common (and annoying) houseplant pests. The worst part about soil gnats is that they can infest any plant that is potted in dirt. You will notice them crawling or flying out of the potting soil around your plant when you water or otherwise disturb the soil.
Soil Gnats or Fruit Flies?
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 have no interest in fruit. If you see gnats flying around your plants – those are soil gnats. The ones flying around the fruit or the garbage disposal are fruit flies.
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Soil gnats 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 soil gnats only eat rotting roots.
Where Do Soil Gnats Come From?
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.
How to Control Soil Gnats
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. So, once all the larvae are dead, your soil gnat problem will go away.
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- Soil gnat larvae thrive in moist soil, and they can’t survive in dry soil. So, the easiest and most effective way to control and ultimately eliminate soil gnats is to make sure you do not regularly overwater your plants. Be careful though, you don’t want to allow the entire root ball to dry out on most houseplants. Use a soil moisture gauge to help maintain the right level of moisture for your houseplants.
- 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 yellow sticky trap near the plant to capture the adult soil gnats. This will only be effective to control the adult population, it will not take care of the problem at the source (the larvae). But it definitely helps to keep the adult soil gnats from flying around to other plants.
- Pour or spray mild soapy water (I use Dr. Bronner’s Baby-mild Liquid Soap) or neem oil mixture into the top of the soil to kill the larvae. This should be effective after a few treatments. Neem oil works great to kill houseplant pests, and has a residual effect. You can buy neem oil here.
- Remove the top inch of potting soil and replace it with new, dry soil. This will remove soil gnat eggs and larvae. The larvae could still hatch and mature in the soil after you remove it, so make sure you take it outside to the trash.
- 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. You could also use a product called Gnatnix, which is a non-toxic soil cover that will eliminate soil gnats.
- Store potting soil in a sealed container. I store mine in buckets that have a tight fitting lid rather than in the bag it comes in. Soil gnats can’t survive without oxygen.
Even if you are successful in eliminating a soil gnat 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.
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Please leave a comment and share your tips on how you deal with soil gnats.