Gnats In Houseplants Hydrogen Peroxide. The hydrogen peroxide solution also controls other insect pests of houseplants. Experts recommend hydrogen peroxide to kill gnats because only one treatment of it will surely end the life cycle of these flies.
Gently spray the potion on the leaves and stems of the plant where the adult gnats are present. Gnats in houseplants hydrogen peroxide. Mix one tablespoon of 3 percent hydrogen peroxide into a gallon of water.