Fungus gnats come from soil that stays damp at the top. Let the top few centimetres dry between waterings, catch adults on yellow sticky traps, treat larvae with a BTI product used exactly as labelled, and repot in fresh mix if the problem keeps coming back.
Those small, dark flies hovering around your plants are annoying, but they are easy to manage once you know their life cycle. This guide uses simple, low-risk steps and no home-remedy chemicals.
What fungus gnats are
Adult fungus gnats are tiny, slim, dark flies, about 2 to 4 mm long. They fly weakly and tend to hang around pots, windows and lamps.
The adults do not hurt your plants. The larvae are the problem. They are small, translucent worms with black heads that live in the top few centimetres of damp mix, feeding on fungi and decaying plant material. In large numbers they can nibble fine roots, which matters most for seedlings, cuttings and young plants.
A female lays hundreds of eggs in damp soil. The cycle from egg to adult takes about three to four weeks at room temperature, so treat the problem as a month-long project.
Why you have them
Gnats thrive where the top of the mix stays moist. Common causes:
- Watering on a schedule instead of checking the mix
- Pots with poor drainage or a saucer that always holds water
- Dense, peaty mix that stays wet for a long time
- New plants brought home with gnats already in the pot
- Winter: lower light means plants use less water, and the mix stays wet longer
If this sounds familiar, our post on overwatering vs underwatering can help you read what your plant needs.
Step 1: Let the top dry out
This is the most effective step. Larvae and eggs need damp mix. When the top layer is dry, eggs fail to hatch and larvae die.
- Check by pushing a finger 3 to 5 cm into the mix. Water only when it is dry to that depth, for plants that tolerate it.
- Empty saucers 15 minutes after watering.
- Move plants to brighter spots if possible so they use water faster.
For plants that like constant moisture, such as African violets, you cannot let them dry fully. Use the other steps more heavily and keep the surface as dry as the plant allows.
Step 2: Water from below
Bottom watering keeps the top layer dry while the roots drink.
- Set the pot in a bowl with 2 to 3 cm of water.
- Wait 15 to 30 minutes until the top feels slightly damp.
- Drain well and return the pot to its saucer.
Adults prefer to lay eggs on a damp surface, and a drier top makes the pot much less appealing. For a similar approach with seedlings, see how to start seeds indoors.
Step 3: Use yellow sticky traps
Yellow sticky traps catch adults before they lay eggs.
- Place a trap in or just above each affected pot, flat against the surface or on a stake.
- Replace traps when covered with flies.
- Keep traps out of reach of children and pets.
Traps are a monitoring tool as much as a treatment. When you stop catching adults for a couple of weeks, the cycle is broken. They will not clear larvae already in the soil, so combine them with the other steps.
Step 4: Treat larvae with a BTI product
BTI stands for Bacillus thuringiensis israelensis, a bacterium that targets the larvae of fungus gnats and mosquitoes. It is sold in several forms, such as granules, dunks or liquid concentrates, labelled for use against fungus gnat larvae.
- Look for a product registered for this use in Canada and read the label first.
- Use it exactly as directed, including the amount, how often to repeat, and any safety precautions.
- Repeat treatments as the label says, since new larvae hatch over several weeks.
Do not make up your own recipes or mix household chemicals into the pot. A properly labelled product has been tested for this use, and home remedies can burn roots or harm your plant.
Step 5: Replace the mix if it keeps coming back
If gnats come back after a month of careful watering, the mix is likely too wet, too old or full of decaying material.
- Take the plant out and gently shake off old mix.
- Trim away any brown, mushy or smelly roots with clean scissors.
- Wash the pot with soap and hot water.
- Repot in fresh, free-draining mix.
- Water lightly and let the top dry before watering again.
Fresh mix removes eggs and larvae in one go. Choose a mix that suits the plant. Chunky mixes with bark, perlite or pumice dry out faster at the surface and are less friendly to gnats. Our Aroid Mix and Orchid Mix are examples. For other plants, the guide to the best soil for indoor plants can help you pick. Ordinary potting soil is fine for many plants, so long as you let it dry between waterings.
When you repot, our post on signs your plant needs repotting is a handy check.
Step 6: Prevent a repeat
- Quarantine new plants for two weeks, and check the surface of the pot.
- Keep bagged mix sealed and dry. Damp, open bags can harbour gnats.
- Do not let leaf litter or dead flowers rot on the surface.
- Add a thin top layer of coarse sand or fine grit if your plant tolerates it. A dry, gritty surface is harder for adults to lay eggs in.
- Clear dead leaves and keep the area around your plants clean.
A realistic timeline
- Week 1: Stop overwatering, put out traps, begin BTI treatment.
- Weeks 2 and 3: Keep the surface dry, repeat treatment as labelled, keep trapping.
- Week 4: You should see few or no new adults.
- After that: Keep checking the mix before you water.
If adults are still appearing after six weeks, look for a hidden source such as another damp pot, a tray of seedlings, or a dish of water under a plant, and treat that too.
A note about fertilizer
Our mixes contain no fertilizer. Overfeeding and decaying organic matter on the surface can make gnats worse, so apply fertilizer only as the label directs.
