Fact: Dense foliage can make a slight difference—if it’s tens of metres thick
There is a lot of misplaced confidence in the ability of foliage to attenuate sound. This is partly due to psycho-acoustics (out of sight, out of mind) and the belief that if you can’t see the noise source, it is perceived to be quieter.
Foliage can also mask (hide) other sound when there is some wind, although the masking sound level depends not only on the wind speed but also on the type of foliage and time of the year!
For foliage to attenuate sound to any significant extent, it needs to be dense and tens of metres thick. There is some research into the use of trees as ‘sonic crystals’ to provide some attenuation in specific frequency bands, but this is based on the size and very dense spacing of the trunks instead.
In addition to generating sound in a breeze, foliage can also increase the noise level when broad-leafed trees are on top of an earth bund (acoustic barrier), in which case high frequency sound that is scattered by the leaves can be reflected to the quiet side of the bund, potentially reducing the screening attenuation provided by the bund!