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Communities are increasingly harnessing the coastal protection functions of marshes and other coastal ecosystems within built infrastructure, developing nature-based designs to stabilize coastlines. These “living shorelines” often include planting ecosystem-engineering plants, which have traits that attenuate waves and facilitate sediment accretion while limiting erosion. However, failure is common during plant establishment, requiring interdisciplinary approaches to inform planting designs that enhance short-term sediment stability. Here we combine hydrodynamic modelling with mesocosm experiments to assess different planting approaches for the marsh grass Spartina alterniflora. The model, parameterized with traits measured in the experiments, showed that random arrangement of plants outperformed regular arrangements, reducing areas of high flow velocities and increasing tortuosity, facilitating sediment stability. Furthermore, wide-diameter Spartina clumps with increased biomass reduced flow better than small-diameter clumps, even when the area occupied by the vegetation site-wide is identical. Our experiments revealed multiple factors that influence the diameter and biomass of Spartina clumps, including plant source, sediment characteristics, and spatial arrangement of propagules. While some sources performed better than others, their relative performance varied with time and environment, suggesting that practitioners plant multiple sources to ensure incorporating high-performers in variable and often unexamined planting environments. Furthermore, clumping propagules during planting best generated the large, dense clumps that facilitate sediment stability.