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.
- Make the pesticide
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.
- Apply the pesticide
Spray the pesticide on both sides of the leaves of the gnat-infested plants. Save the leftover pesticide for future use.
- Apply a nematode soil treatment
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.
- Put sliced potatoes around the plants
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.