In a study of eight different vertical greenery systems on eight concrete walls in HortPark in Singapore, five systems were found to reduce noise levels by around 5-10 decibels in the low to middle frequency range. For the greatest reduction, there should be no empty gaps within the system.
Wong, Nyuk Hien, Alex Yong Kwang Tan, Puay Yok Tan, Kelly Chiang, and Ngian Chung Wong. (2010). Acoustics evaluation of vertical greenery systems for building walls. Building and Environment, 45(2), 411-420.