Physical processes limiting colonization of bare tidal flats by pioneer mangrove species have commonly been described but not yet quantified. Understanding thresholds to early seedling establishment is critical for successful restoration and management of mangrove forests. We determined how seedling establishment of Avicennia alba Bl. is affected by the combination of increasing hydrodynamic forces and sediment dynamics. As the buoyant propagules of A. alba are dispersed by water, early establishment on bare tidal flats requires propagules to quickly anchor to stay in place. Flume studies and field observations showed that 3 thresholds have to be passed until the seedling is successfully established: (1) stranded propagules need an inundation-free period to rapidly develop roots that are long enough to withstand displacement by flooding, (2) roots need to become long enough to withstand seedling dislodgement by hydrodynamic forces from waves and currents, with the required root length being proportional to the force that needs to be resisted, (3) even longer roots are needed to survive high energy events that cause sheet erosion and can thereby induce seedling dislodgement. This sequence of thresholds implies that establishment of the pioneer mangrove species A. alba requires a suitable window of opportunity to pass all thresholds and underlines the importance of rapid root expansion as a crucial pioneer trait for the species.