Castor oil, derived from the seeds of the Ricinus communis plant, is a thick, pale yellow oil known for its wide range of applications, from medicinal uses to industrial lubricants. In the context of lawn and garden care, this naturally sourced oil is associated with controlling unwanted activity in turfgrass. The primary query for many homeowners is whether this substance kills the grass itself or if its effect is limited to managing pests. This article clarifies the actual role of castor oil as a lawn treatment.
Does Castor Oil Harm Grass?
Castor oil is generally not considered an effective herbicide and does not typically kill established, healthy turfgrass when applied at recommended rates. Its chemical composition does not contain the systemic agents required to interfere with plant growth processes or photosynthesis. The oil is intended to penetrate the soil, not the grass blades, to reach the underground environment. However, any oil can potentially smother or “burn” small, isolated patches of turf if applied in excessive, undiluted concentrations that coat the leaves and block gas exchange. When properly diluted and emulsified with water and soap, the solution passes quickly through the turf canopy and into the soil without causing damage. Therefore, castor oil acts as a non-damaging soil amendment, not as a lawn killer.
How Castor Oil Functions as a Yard Deterrent
Castor oil is widely discussed in lawn care due to its effectiveness as a repellent against tunneling animals, particularly moles and voles. These small mammals are not repelled because the product harms them, but because it contaminates their living and feeding environment. The oil’s slightly acrid taste and persistent odor are highly unpleasant to these pests.
The repellent works primarily by coating the underground food sources of moles, such as earthworms and grubs, making them unpalatable. Moles are voracious insectivores, and the contamination of their diet forces them to abandon the treated area in search of a better food source elsewhere. This effect is purely behavioral, driving the animals to relocate. The oil also saturates the walls of their tunnels, further encouraging them to move away from the protected lawn.
Safe Application Guidelines
To use castor oil effectively as a pest deterrent without harming your lawn, it must be properly prepared to ensure it penetrates the soil. Since oil and water do not naturally mix, a liquid detergent or dish soap must be added to act as an emulsifier. A common homeowner recipe involves mixing approximately six ounces of unrefined castor oil with two tablespoons of liquid dish soap.
This concentrated mixture should then be diluted in a gallon of water before being applied with a garden sprayer, covering about 300 square feet. It is beneficial to water the lawn lightly before applying the solution, and then water again afterward for about 10 minutes to help the oil penetrate deep into the tunnels and soil. Because the oil can break down or leach out, especially after heavy rain, reapplication every few weeks is necessary to maintain the barrier.
While generally safe for pets and children when used as directed, ingesting the treated soil or oil mixture could cause temporary digestive upset.