Root tensile resistance (Rr), either expressed in stress or force term, plays a crucial role in assessing the mechanical reinforcement provided by roots, and evaluating the use of vegetation as a nature-based solution for shallow slope stabilisation. In contrast to man-made structures such as nails and geotextiles which exhibit limited variation in their mechanical properties, biological materials, such as plant roots, present inter-and intra-specific variabilities associated with genetic and environmental determinants. The effects of such variabilities on the estimation of the root reinforcement have been shown to be considerable, necessitating thorough characterisation in modelling efforts. Although our understanding of factors influencing Rr has been advanced by extensive experimental research, the insights that can be learned from these findings regarding the characterisation and modelling of the variability in Rr are inadequate. The aim of this work is to systematically review studies addressing the various effects on the variabilities in Rr and identify major challenges for future study in the prediction of Rr for engineering applications. Given the considerable number of spatially and temporally varying factors influencing Rr and their coupling effect, future study should focus on (i) establishing a standardised testing protocol with minimum measurement-induced Rr variability, (ii) developing statistically robust sampling framework associated with the number of samples and the sampling location, and (iii) advancing next-generation models that can (a) provide predictions of Rr and the elastic modulus (Er) for estimating the mobilisation of Rr during shearing of rooted soils; (b) exploit practically measurable predictors; and (c) characterise the residual Rr and Er variabilities that model predictors cannot explain.