To kill gnats outside, make a solution of pepper, soap, water and oil, spray the plants, apply a nematode treatment, and put potatoes around the plants. The supplies you need are a jug, water, cayenne pepper, oil, Castile soap, a spray bottle, potato slices, a hose, and a nematode treatment.Continue Reading
Fill a gallon jug with water, and add 1 teaspoon of ground cayenne pepper. Pour in 2 tablespoons of vegetable oil and 2 tablespoons of liquid Castile soap, close the jug, and turn it upside down to combine the ingredients. Pour some of the liquid into an empty spray bottle.
Spray the pesticide on both sides of the leaves of the gnat-infested plants. Save the leftover pesticide for future use.
At dusk, water the soil with a hose fitted with a pressure nozzle. Attach a bottle of commercial nematode soil treatment to the nozzle and spray it over the soil according to the manufacturer's instructions. Detach the bottle, and water the soil again.
Scatter sliced raw potatoes around the plants, and check them daily; gnat larvae burrow into potatoes to feed on them. Throw out infested potatoes, and replace them with fresh slices.