Salt marshes attenuate waves and thus have an important function for coastal protection. Biophysical properties of salt-marsh plants play a key role in the process of wave attenuation and can be differentiated by morphological properties such as stem density, vegetation height and aboveground biomass as well as by biomechanical properties related to stem flexibility. Numerical or physical scale models predicting wave attenuation over vegetated surfaces need to include biophysical properties. However, few studies have quantified morphological and biomechanical properties of salt-marsh plants and fewer have considered seasonal and within-marsh spatial variability of biomechanical properties. The aim of this study was to quantify biophysical properties of the common salt-marsh grasses Spartina anglica and Elymus athericus, including stem flexibility and density as well as aboveground biomass, temporally and spatially. Samples were collected in spring and in summer 2014 at a study site located in the Northern German Wadden Sea. Aboveground biomass was harvested in plots of 50 × 50 cm, stem density was determined by counting and flexibility of plant stems was determined with three-point bending tests. Biophysical properties of both species varied significantly between seasons with plant stem stiffness being 5.0 (S. anglica) and 2.9 times (E. athericus) higher and aboveground biomass being 2.1 (S. anglica) and 1.3 times (E. athericus) higher in summer than in spring. Small-scale spatial differences for those biophysical plant properties were found for S. anglica with plant stem stiffness being 4.0 (spring) and 2.8 times (summer) higher and aboveground biomass being 1.6 (spring) and 1.5 times (summer) higher in a landward than in a seaward-located zone. Small-scale spatial differences of biophysical properties were not found in E. athericus. We conclude that variability in biophysical properties should be considered in models and experiments especially for S. anglica when predicting and quantifying marsh wave attenuation capacity.