Sound absorption by different green wall systems at normal incidence is investigated between 100 and 1000 Hz. Measurements are conducted in an impedance tube in order to estimate the effective acoustic properties of plants and soils. Using these properties, the transfer matrix method is employed to simulate absorption coefficient and surface impedance of multi-layer green wall geometries. Results show that an adequate choice of layer geometries may result in an efficient sound absorption in a broad frequency range due to two main mechanisms: thickness resonances of the wall structure and quarter wave transformer effect of plant layer between air and soil media. To optimize the absorption bandwidth of green facade, continuous living wall system or modular living wall system, effect of plant, soil or air layer thicknesses on simulated absorption coefficient are also provided. With a 16 cm thick optimized system, average absorption coefficients typically reach 0.2 between 300 and 1000 Hz for a green facade, 0.2 between 200 and 1000 Hz for a continuous living wall system and 0.9 between 300 and 1000 Hz for a modular living wall system.