While gnats may not actually harm your houseplants, they can be annoying -- especially if they start to fly around your home. Gnats can quickly become a problem with houseplants when excessive moisture leads to fungus within the soil. The gnats consume the fungus for nutrition and continue to reproduce, causing the population to grow out of control.
Glue boards are an effective way to reduce the population of gnats and keep the annoyance to a minimum. Glue boards work much like fly paper, trapping the gnats that fly near them and get caught up on the gluey surface. Glue boards are a remedy for the adult gnats, but will not solve the problem completely. Larvae and any other gnats that do not happen to fly or land on the glue boards will still be a problem.
Dry the Soil
Allowing the soil to dry between watering can reduce the fungus in the soil. Fungus is what the larvae count on to survive and reproduce. Reducing the fungus will help to control the number of larvae that grow to maturity and reproduce as well, resulting in a continuous decrease in population. Placing the plants outdoors can help, but placing them in direct sunlight can harm the plant. Shaded outdoor areas are best for drying the soil without harming the plant.
Pesticides that are specifically formulated for gnats are the most effective way to completely remedy the problem in houseplants. The manufacturer's directions must be followed exactly and applied in the recommended amounts. When used correctly during the first watering of the day, pesticides can kill larvae and adults continuously until the entire population is eliminated. Suitable pesticides are widely available at home improvement and garden supply stores.