Mandi, I can't answer your Q, but my brother gets lot's of lizards in his garden so grasshoppers aren't a problem even when its unbelievably hot. His plants are fairly common though for South of France. ie- you see them in other people's gardens.
I've found that you can vist a garden in France and hear grasshoppers before you enter the garden, then in the garden it's serenely quiet of them, only visited 2 though
so not experienced.
Is it worth either visiting a local garden and asking for their advise or seeking their advise via e-mail... You've probably already tried this...