How to Prevent Fungus Gnats from Taking Over Your Houseplants

published Jul 26, 2019
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Credit: Viv Yapp

Fungus gnats are irritating little insects that are about the size of fruit flies, but closer in shape to mosquitoes, with their slender wings and longer legs. Beyond the ick factor, they’re harmless to people and generally harmless to plants, so long as they don’t start breeding out of control because the larvae will feed on and damage plant roots, according to the University of California Integrated Pest Management Program (UC IPM)

Where do fungus gnats come from?

According to UC IPM, fungus gnats lay their eggs in soil, and their larvae feed on decomposing plant material and other organic matter found in soil. You may inadvertently bring them home with a new plant pal (they can become a problem in greenhouses), but they may also sneak into your house through other cracks and crevices. The little guys aren’t strong fliers, so you’ll mostly see them hovering in the vicinity of infected plants and gravitating toward the windows. Like many insects, they’re attracted to light.

Excess moisture and mold growth in the kitchen, such as from a leaky pipe under the sink, may attract fungus gnats as well, according to pest control company Orkin. However, kitchens are more likely to be a hot spot for fruit flies, which are attracted to ripening fruit and food scraps caught in the garbage disposal. Fungus gnats need soil to lay their eggs, which is why they’re so frequently associated with houseplants.

What should I do about them?

If you develop a fungus gnat problem out of the blue, chances are you’re watering too much or have some other issue with dampness in your home, per Colorado State University Extension. Fungus gnats only lay their eggs in damp soil, so if you can correct the dampness issue, they’ll happily move out (or, you know, die).

UC IPM recommends focusing your energy on drying out the soil and killing off the eggs and larvae instead of the adults you see flying around. The adults will die at the ripe old age of two weeks anyway, so the best course of action for nipping an infestation in the bud is to make the soil an inhospitable place for laying and hatching eggs. And don’t worry about killing your plants—they like the potting soil to mostly dry out in between waterings. In fact, you’re more likely to kill them if you keep up the excess watering and drown the roots.

Step 1: As with all houseplant insect infestations, start by quarantining your infected plants away from other houseplants to avoid spreading the problem. 

Step 2: Stop watering until the soil is almost completely dried out. The surface of the soil needs to be dry to the touch.

Step 3: For extra insurance, get yourself some diatomaceous earth and sprinkle it on the surface of the soil only when the soil is dry, as the package will tell you, because it becomes useless when it soaks up water. Diatomaceous earth is a powdery dust that’s actually made of fossilized algae. To humans and pets, it resembles the consistency of baking soda, but to soft-bodied insects, the crystals are like broken glass. Once they crawl over it, they’re goners. The Old Farmer’s Almanac likes this method.

Step 4: Set up a trap to capture those pesky adult fungus gnats. A classic apple cider vinegar funnel trap works well. You can also get sticky traps. Fungus gnats are attracted to the color yellow, according to UC IPM, so try to get a yellow one if you can, and position it on or next to the surface of the potting soil. 

Step 5: Once the soil has completely dried at least an inch down and you stop seeing signs of fungus gnats, you can resume watering, but don’t overdo it this time! If your pot doesn’t have drainage holes, considering swapping it for one that does so that water doesn’t pool in the bottom of the pot and rot the roots before they can soak it up. (Here’s more on why drainage holes are so important.) Water only when the soil is dry to the touch an inch or two down, rather than on a set schedule, and stop when the water begins to run out of the drainage hole and into the saucer. (And if this is all news to you, take a peek at our comprehensive guide to becoming a plant parent for more tips.)