Sediment stability is optimized by manipulating planting design during coastal marsh establishment.

Seaside communities increasingly harness the shoreline protection functions of coastal ecosystems by constructing nature-based infrastructure. Practitioners often install vegetation into these "living shorelines" because coastal plants have traits that limit erosion by attenuating waves and increasing soil shear strength. However, failure is common during plant establishment, highlighting a need for planting designs that enhance short-term sediment stability. Here we combined hydrodynamic modelling with mesocosm experiments to assess planting approaches for the marsh grass Spartina alterniflora. The model compared random and regular planting arrangements containing plant clumps of different sizes and densities. The experiments evaluated the influence of plant collection source, arrangement, and sediment environment on plant traits. Model results showed random arrangements outperform regular arrangements, reducing areas of high-velocity flow. Large-diameter, high-stem-density Spartina clumps attenuated flow better than small-diameter clumps, even when site-wide vegetation coverage was identical. Experiments revealed multiple factors that influence the diameter and density of clumps, including plant source, sediment organic matter, and plant spatial arrangement. Some plant sources had larger diameters and more biomass than others, yet relative performance of sources varied with time and environment; thus, planting multiple sources would increase the likelihood that high-performers are included in variable and often-unexamined planting sites. Furthermore, a clumped planting arrangement was most effective for generating large, dense clumps that facilitate sediment stability.