How to Keep Fungus Gnats Out of Your Houseplants

by Suzi Tehan

These minuscule gnats (3.2 mm) are less plant pests than people pests if truth be known, but the houseplant is the source. They inhabit damp soil areas and fly feebly around the room getting in your face, basically just being annoying.

The root-chewing larvae are what irritate your indoor plant.  The fungus gnat larvae stress seedlings, cuttings, and young growth. In large numbers they can damage a mature plant.

  • The entire life cycle takes place in about a month.
  • Eggs, larvae, and pupae live together in your plant's pot.
  • They thrive in moisture and feed on decay: fungus gnats are a probably a signal something is wrong with your watering routine.

Your Options:

  • The real solution for these common houseplant pests is to eliminate all dead/decaying material and avoid soggy soil. 
  • Always let the top of your soil dry out to the recommended amount for your plant variety.
  • Consider adding a thin layer of sand on top: it dries out quickly and discourages the hovering adults.
  • Placing yellow sticky traps near the soil to help reduce adult numbers and help you monitor their presence.
  • One easy treatment is to water the plant with one part of 3% Hydrogen Peroxide to four parts water.  Drench the plant and let it drain out of the holes. This kills larvae on contact, though it doesn't affect the flying adults: an organic pesticide spray can clear the air.

Note that Hydrogen peroxide, like Eco-Neem Oil (used as a soil drench), disturbs the beneficial bacteria in your soil. So is something to consider.  Once your on top of your outbreak, cease the treatment and let the soil regenerate.

Alternatively you could also give some gnat-loving nematodes a go. These are (insect killing) nematodes that won’t harm the plants, or any beneficial bugs and nor will it harm the bacteria.

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