Vegetation is one of the most important components of nature-based coastal protection due to its ability to dissipate wave energy. To quantify the wave attenuation by vegetation, traditional analytical models assume rigid vegetation and use bulk drag coefficient (CD) or effective blade length (le) techniques to consider the effects of blade motion, where CD and le are conventionally fitted as a function of KC (the Keulegan–Carpenter number) and CaL (with Ca the Cauchy number and L the ratio of the blade length to wave excursion), respectively. These parameters do not include the full blade dynamics and so the empirical formulas of CD and le are different for varying vegetation with blade dynamics. To obtain analytical solutions for CD and le, an analytical wave attenuation model for flexible vegetation (and kelp) was developed in this study by simplifying and linearizing the blade motion. Compared with a wide range of experiments for both submerged vegetation and suspended kelp canopies, the simplified analytical model underestimated the wave decay coefficient (kD) by 27%, but with a small NRMSE (normalized root mean square error by the range of the measured data) of 0.054. In comparison, the numerical model with full nonlinearity underestimated the wave decay coefficient by 11.7% with NRMSE=0.063. To reduce the underestimation of the analytical model due to the simplification and linearization, a modification factor defined as the ratio of the numerically calculated kD and the analytically calculated kD was fitted. With the modification factor, the underestimation of the analytical model was reduced to 10.1%. Based on the analytical model, analytical solutions for CD and le were derived, which showed a similar precision with the experimentally fitted CD and le based on KC and CaL, respectively. Thus, the analytical solutions for CD and le could be a reliable alternative when the experimentally calibrated CD and le are not available. Using the analytical wave attenuation model, a case study showed the wave attenuation by cultivated Saccharina latissima changes seasonally with the kelp growth. When the kelp blade reaches 2.4 m long after 7 months of growth, the kelp farms with 50 longlines (over a distance of 200 m in the direction of wave propagation) in 8 m-deep water may attenuate wave energy by 29% for 1 m-high waves with the period of 6 s. The wave attenuation can be enhanced to 43% when the farms are located in 5 m-deep water. To provide considerable wave attenuation of kelp with adequate long blades around the year, biennial and multiple partial harvesting techniques are recommended.