The shear strength originating from the root system of vegetation that mechanically stabilizes a soil slope against shallow landslides is called “root cohesion”. The magnitude of the root cohesion depends on the root architecture and root tensile strength, which are well known to have high variability even from the same species. In this paper, a probabilistic framework is proposed to describe the intra-specific variability of root system and estimate the root cohesion. The two components of intra-specific variability of root architecture, root number and root diameter distribution, are modeled by an extended enumerated variable and the log-normal distribution, respectively. The intra-specific variability of root tensile strength is also modeled by incorporating the variabilities of fitting parameters in the inverse proportional relationship between root tensile strength and root diameter. A Monte Carlo-based procedure that estimates the root cohesion in a probabilistic manner is adopted in the proposed framework. A hypothetical vegetated slope is modeled. Both deterministic and probabilistic analyses are then undertaken to estimate the root cohesion resulting from four Hong Kong native species and examine the effects of spatial and material variability on the safety margin against shallow failure. The study offers insight into the potential merits of vegetating a slope against shallow failure.