The first thing to say is - is it really worth getting into a battle over? I wouldn't have thought so.
If you want a technical answer as to whether he is in breach of some civil law, I suspect it depends on who owns the pedestrian access. Sometimes these access routes are part of a public right of way and a foopath is supposed to be a minimum of one metre (it might be 3 feet) wide. I suppose if it was less than one metre wide, he could do this.
Alternatively, the route may be privately owned by each of the gardens adjoining, and each of you has a private right of way across it. In that situation I suspect it is for the owner of the land including the path to decide how long he wants his plants to be.
Perhaps a quiet word - 'the plants look rather good at this time of year - is it really necessary ............. (and so on)'