Root biomass and plant age have been recognized to influence the hydro-mechanical properties of root-permeated soils, but its direct field observation was scarce. In this study, influences of Chrysopogon zizanioides (vetiver grass) of various plant ages (up to 2 years) on soil-water retention curves (SWRCs), saturated permeability, specific water-retention capacity and soil aggregate stability have been investigated. Undisturbed soil samples were obtained at different locations, representing the root zone, upslope, and downslope from the vetiver hedgerow at three bio-engineered slopes in Western Thailand. As root biomass increased, air-entry suction of soil slightly decreased, and porosity significantly increased due to the formation of macropores and aggregated soil structure in the root zone (0-15 cm depth). Nevertheless, below the root zone (15-25 cm depth), the air-entry suction of soil increased with root biomass due to the occupancy of soil pores by roots. Saturated permeability shows a positive correlation with plant age in the root zone, while a slightly negative correlation was found below the root zone. The upslope and downslope soil samples showed a negative correlation due to the effect of sediment trap. The specific water retention capacity and soil aggregate stability became higher in the root zone and positively correlated with plant age.