This paper reports a series of geotechnical centrifuge model tests conducted to investigate the mechanical reinforcement of slopes by vegetation. Some of the model slopes contained young willow trees, which were grown in controlled conditions to provide different root distributions and mechanical properties. Slopes were brought to failure in the centrifuge by increasing water pressures. The failure mechanisms were investigated photographically and using post-test excavation. By measuring the soil properties and pore pressures in each test when failure occurred, slope stability calculations could be performed for each slope failure. These back-calculations of stability suggest that only a small amount of reinforcement was provided by the root system even when it was grown for 290 days before testing. In contrast, the use of the measured root properties and a commonly used root reinforcement model suggests that significant reinforcement should have been provided by the roots. This disparity is probably due to either inappropriate assumptions made in the root reinforcement model or soil alteration produced by root growth. Such disparities may exist in the application of root reinforcement models to full-scale slopes and therefore require additional study. The modelling technique outlined in this paper is suitable for further investigation of root mechanical interactions with slopes.