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Nature-based mitigation is increasingly proposed as a strategy to cope with global change and related risks for coastal flooding and erosion. Tidal marshes are known to provide shoreline protection as their aboveground biomass attenuates waves and their belowground biomass contributes to reducing erosion rates. The aim of this study was to quantify how effectively wave attenuation rates and erosion reduction rates are sustained throughout seasons in pioneer tidal marshes in the Elbe estuary (Germany). Changes in hydrodynamics and sediment dynamics were measured during 17 months along three sea-to-land transects of 50 m length. Simultaneously, changes in biomass of the monospecific pioneer vegetation (Bolboschoenus maritimus) were measured monthly. This study shows that wave and flow attenuation rates positively correlate with seasonal variations in aboveground biomass, that is: in summer, aboveground biomass and associated wave and flow attenuation rates are highest; while aboveground biomass is washed away during the first storms in autumn or winter, resulting in low wave and flow attenuation rates. Contrastingly, maximum incoming wave heights and flow velocities occur during winter, indicating that wave and flow attenuation is most needed then. However, hibernating root biomass assures low erosion rates in winter, especially at sandy sites. Although wave attenuation by pioneer marshes is highly variable throughout seasons and pioneer marshes alone are not so effective, they might facilitate the survival of higher marshes. Therefore, it is important to conserve or restore a gradual sea-to-land gradient from tidal flats, over pioneer marsh to high marsh to provide nature-based shoreline protection.