Castor oil does not kill moles. Rather, the oil is an ingredient in various home remedies and commercial products that will merely evict them. Castor oil upsets a mole's digestive tract and causes stomach upset. Topically, it makes them itch. It does not cause permanent irritation or damage, but in theory, the moles will reroute their tunnels and feed elsewhere. Choose a commercial product containing castor oil or try mixing ingredients at home. Experiment on a small area of the yard. Moles prefer loose, moist soil in shady areas and sandy loam to clay.
Purchase a bottle of castor oil or castor oil based mole repellant from a lawn and garden center. Follow the directions on the commercial repellant for dilution and use.
If you mix the solution yourself, purchase oil marked "unrefined" to cut costs. Pour one gallon of water into a bucket. Mix 6 ounces of castor oil and 2 tablespoons of an oil based or vegetable based detergent into the water. The detergent acts as a carrier for the sometimes sticky oil.
Pour the mixture into a spray bottle or a hose sprayer.
Turn on the lawn sprinkler or hose. Saturate the target area with approximately one 1/4 inch of water.
Saturate the soil where the moles appear with the castor oil solution. Spray the roots of freshly transplanted flowers and shrubs in frequently attacked areas.
Follow immediately with another inch of water to thoroughly soak the solution into the ground.