Soundproofing garden
As far as I know the more jagged and porous the material is the better. In our library, there is porous material put on walls which are faced toward the source of the sound so that it eliminates reflection. Maybe I would try eliminating those reflections between your house and the fence. But given it is in an open space I doubt it will be significant enough. 



If you are very serious about it, try active noise reduction: ANC - wiki