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Soil that’s constantly damp attracts pests, including gnats, and promotes mold growth. While you shouldn’t let your plants wilt or turn yellow, try to hold off watering until the soil is completely dry.  To test the soil, stick your finger about 1 to 2 inches (2.5 to 5.1 cm) into the surface. If dirt sticks to your finger, it’s probably best to hold off watering, unless your plants look sad. It’s also best to use containers with drainage holes in the bottom. If your pots sit on saucers that collect excess water, be sure to drain them after you water your plants. Nematodes are tiny worms that eat the larvae gnats, fleas, and other pests. You can purchase them at a garden center; look for a product labeled for gnat control that contains the nematode species Steinernema feltiae.  Typically, you mix beneficial nematodes with water, then add the mixture to your soil. Check your product’s label for specific instructions. Beneficial nematodes are safe for people, plants, and pets, so try not to get grossed out or worry about harmful health effects. Mix a teaspoon or less of gentle hand or dish soap with a 1 US pint (470 mL) of lukewarm water. Add the mixture to a spray bottle, then spritz your plants’ leaves. After 2 or 3 hours, rinse the leaves thoroughly with water.  While they’re more expensive, you can also purchase insecticidal plant soaps at a garden center. Using a homemade soap spray increases the risk of leaf damage. It’s wise to test the spray on a small area before spraying an entire plant. As a rule of thumb, if a soap dries out or irritates your skin, it’s probably too harsh for your plants. your plants if other methods don’t work. If you haven’t had success keeping flies out of your plants, it’s time to transplant them. Water the plant, remove it from the old pot, then gently shake away as much soil as possible from the root system.  Fill a new pot nearly to the top with fresh potting soil. Dig a hole in the center large enough to accommodate the roots, bury them, then cover them with soil. If your gnat problem persists after transplanting, you might need to resort to chemical treatment. Head to your local garden center for a plant-safe liquid insecticide labeled for flying insects.

Summary:
Allow the soil to dry out before you water your houseplants. Add beneficial nematodes to your soil. Make a homemade gnat spray with gentle liquid soap and water. Repot