Growing Green

By Robin Trott, Extension Educator

As cooler weather brings houseplants indoors, many plant owners are dealing with fungus gnats. These tiny, mosquito-like insects hover around potting soil and, while harmless to humans, they can stress plants—especially seedlings and delicate roots. Here’s how to identify and treat fungus gnats and keep them from returning.

What Are Fungus Gnats?

Fungus gnats are small, dark flies that lay their eggs in moist soil. Their larvae feed on decaying organic matter and sometimes plant roots, which can cause yellowing leaves, wilting, and slow growth. Fungus gnats thrive in damp conditions, so overwatered plants are especially vulnerable.

How to Spot Fungus Gnats

If you see tiny black flies around your plants or windows, you may have fungus gnats. To check, tap the pot lightly; gnats will take flight if present. You may also see tiny, white worm-like larvae on the soil surface.

How to Treat Fungus Gnats

1. Dry Out the Soil

Fungus gnats love moisture, so let the

top 1–2 inches of soil dry completely

before watering again. This disrupts 

their life cycle, as larvae can’t survive

in dry soil.

2. Use Sticky Traps

Yellow sticky traps, available at

garden centers or online, easily catch

adult fungus gnats. Place them near

the base of plants and replace as needed.

This reduces the number of adult gnats

laying eggs.

3. Top the Soil with a Barrier

Cover the soil with a thin layer of sand,

diatomaceous earth, or fine gravel.

This barrier makes it harder for adult

gnats to reach the soil and lay eggs.

     A layer at least half an  inch thick  is best.

4. Water from the     

    Bottom

     Bottom-watering

     lets the plant absorb

     only what it needs,

      keeping the soil’s sur 

     face dry and discour-

   aging gnats. Set the pot in a tray of water

     for 15–30 minutes, then remove

any excess water.

5. Use Biological Insecticides

A natural bacterium called Bacillus

thuringiensis var. israelensis (BTI),

found in mosquito dunks, works

against fungus gnat larvae. Add a piece

of a dunk to your watering can. BTI is

safe for plants and pets and targets

only fungus gnat larvae.

Preventing Fungus Gnats

After treatment, use these steps to keep fungus gnats from returning:

• Avoid Overwatering: Stick to a watering schedule and use pots with drainage holes.

• Remove Dead Leaves and Debris: Clean up dead plant material, as decaying matter attracts gnats.

• Use Well-Draining Soil: A well-draining mix helps prevent overly moist conditions that fungus gnats love.

Fungus gnats can be annoying, but a few simple changes to your plant care routine can get rid of them. Drying out soil, using sticky traps, and treating with BTI are all effective ways to protect your plants and keep your indoor garden pest-free.

With these tips, you can enjoy your houseplants without the constant buzz of unwelcome pests. Happy gardening!

* * * * * * * * * * * * *

For more news from U of M Extension, visit www.extension.umn.edu/news or contact Extension Communications at extdigest@umn.edu. University of Minnesota Extension is an equal opportunity educator and employer.