Dear Garden Guru,
I’m hoping that you are able to help me. I have a gnat problem in one of my indoor houseplants. I have tried everything – repotting the plant 3 times, spraying indoor houseplant bug spray.. I haven’t overwatered the plant at all since it’s last repotting. What can I do? Ellen
Dear Ellen,
Fungus gnats are bothersome. It may help to understand the environment they prefer and their lifecycle. Fungus gnats lay their eggs on very moist, damp, warm soils containing lots of organic matter, compost, and fungi. The fungi are existing in these soils to break down the organic matter. Fungus gnat larvae feed on decaying organic matter and fungi. If the population is large enough, they will also feed on the plant roots.
Once the larvae are full-grown, they pupate and emerge as pesky gnats. The adults mate and the cycle begins anew. The cultural controls for the gnats are to reduce watering, allowing the plants to become fairly dry between watering. Do not let plants sit in water filled saucers or cache pot. Putting plants in containers that are too large for them prevents the soil from drying out sufficiently. You are better to plant in a container that is no more than 4″ wider than its original size.
You mention that you have repotted 3 times. This is one of the methods of control. All the soil should be washed away from the roots before putting in fresh soil. Choose a sterile potting soil that does not have a lot of extra amendments that would feed the gnat larvae. We love our Organic Potting soil, but to be on the safe side in a recurring situation, the Sloat Bay Area Blend potting soil is a better choice. It drains faster, has fewer compost nutrients and less organic foodstuffs to decay.
If none of this seems to work for you, there are beneficial, fungus gnat nematodes that can be applied to the container soil. The nematodes attack and feed on the larvae. These have been very successful. There are also Gnat Trap Stix which are small, sticky cards that can be attached to the pot to trap the adult gnats. Using the sprays that are available to homeowners are not effective in the long run.
Good luck!
