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Stop Fungus Gnats

Prevention

Gnats showed up after a new plant?

Written by Mustafa Ayan Fact-checked against university extension/IPM sources Last reviewed: September 19, 2026

Direct answer

A new plant is one of the most common ways fungus gnats enter a home — eggs or larvae can already be present in nursery mix or bagged potting soil that was stored somewhere damp, arriving before you ever see an adult fly. If gnats appeared within a week or two of a new arrival, that plant's mix is the most likely source, not the rest of your collection.

If gnats have already appeared

  1. 1

    Move the new plant away from others

    Adults wander freely between nearby pots. Isolating the likely source slows how fast it spreads while you treat it.

  2. 2

    Set a sticky card in that pot specifically

    This tells you whether the new plant is really the source, or whether an existing plant was already harboring a smaller population.

  3. 3

    Check and adjust the mix's moisture

    Nursery mix is often kept wetter than houseplants need long-term — let it dry appropriately for that plant type before resuming a normal watering routine.

  4. 4

    Follow the standard elimination steps if it's confirmed

    Once you're sure this plant is the source, treat it like any other infestation — traps, moisture control, and a labeled Bti product if needed.

Quarantine, for next time

Keep a new plant away from your existing collection for about a week, with its own sticky card at soil level. If the card stays clean and the soil dries at a normal rate for that plant, it's reasonable to let it join the rest of the shelf. This single habit is one of the more effective, lowest-effort ways to avoid reintroducing an infestation you already cleared, or starting a new one from a fresh source.

The same logic applies to bagged potting soil: if a bag was stored somewhere damp (a garage, a shed, a warm stockroom) before you bought it, treat it with the same suspicion as a new plant rather than assuming sealed packaging means sterile.

Full prevention guide

Sources