Abstract Perennial crops, such as Silphium perfoliatum L. (cup plant), offer significant benefits for soil and groundwater conservation, owing to their nearly continuous soil cover and extensive root systems. To effectively plan and manage the establishment of cup plant in areas with high erosion risk, it is crucial to quantify its preventive impact on rainfall‐induced soil erosion, expressed as the C factor. This study determined the C factor using a subfactor approach, which incorporated measurements of root mass development, soil cover by mulch and vegetation, and plant height across multiple experimental and practitioner sites with cup plant stands of varying ages. The findings indicate that, over a cultivation period of at least 10 years, soil erosion because of intense rainfall is reduced under cup plant to less than a quarter of the erosion typically observed in conventional arable farming and one eighth of the erosion in maize. While erosion under cup plants is significantly lower than in pure cereal cultivation, it does not achieve the minimal levels observed in grasslands, primarily because of higher erosion risks during the initial years following establishment. This applies equally to stands established through direct sowing and those established with maize as a nurse crop in the first year. A substantial portion of the erosion reduction can be attributed to soil consolidation and intensive rooting in the topsoil. Additionally, the absence of soil displacement because of tillage post‐establishment further contributes to reduced erosion by additionally eliminating tillage erosion.