Mechanical contribution of roots to the shear strength of saturated soils is not the only function of vegetation in favour of slope stability. Plants also cause an increase in the strength due to plant-induced suction, resulting from partial saturation. Although understanding, quantifying and modelling the shearing behaviour of soils is directly related to the volume change characteristics of soil, dilative behaviour of rooted soils has not been of much interest. Large-scale suction-monitored direct shear tests were performed on specimens planted with a mixture of species from different plant functional groups. Increase in matric suction, which was linked to root biomass and root : shoot ratio, resulted in higher maximum dilatancy angles. It was also found that maximum dilatancy is controlled by net normal stress, but not effective stress.