A series of experiments on spectral wave transformations over an artificial Posidonia oceanica patch were performed at full scale. Submergence ratio and stem density were found to significantly affect wave height decay along the patch, confirming previous observations on monochromatic wave height decay (Stratigaki et al., 2009a, b). Wave dissipation was increased amongst the peak spectral components. Wave-induced flows are significantly reduced in the lower canopy especially for high submergence ratio and stem density. The flow attenuation in the lower canopy was higher for the longer period components of the spectra.