<p>Whether the contribution of grass root systems can be neglected in slope stability assessments is not well understood. This study aimed to characterize the depth-dependent shear strength profile of grass-reinforced lateritic soil and its influence on slope stability. Three typical grasses were used in planting experiments and direct shear tests to examine root distribution and soil shear strength profile under different seeding densities. Empirical models of soil shear strength parameters were established and then incorporated into a finite element framework to evaluate slope stability under rainfall infiltration. The results demonstrate that the proposed empirical models could well characterize the shear strength profiles of vegetated soil under different grass types and seeding densities. The incorporation of plant age through a new form of the Richards growth function captured the temporal evolution of shear strength induced by grass growth. Slope stability was governed by the spatial coupling among the root-reinforced zone, the rainfall-saturated zone, and the potential failure surface. This revealed that the effectiveness of grass roots depended on their overlap with the active failure zone, providing a unified and practical criterion for determining when the effects of grass roots should be considered in slope stability analysis.</p>