Large amounts of rainfall in humid areas and immature technologies for conservation cause severe weathering and decay of outdoor earthen sites. On the basis of the phenomenon that plants grow naturally on the surface of earthen sites, plants can be used to conserve earthen sites and reduce the impact of open-air environment to some degree. This suggests the possibility of utilizing plant capping to conserve earthen sites. Through investigations of the influence of plants on earthen sites, this paper demonstrates the feasibility of the plant capping technology and discusses several points:types and rates of weathering of sites determines suitable plant capping technology; local dominant plant species are preferred for plant capping; adverse impact of plants should also be analyzed and evaluated before covering.