It won't work. I have a 7/8 foot brick wall around my house, they get up it in a flash. One of them can do it in pretty much 1 jump, the others kinda get 3/4 of the way up and seem to literally climb the wall the rest of the way.
I think your best solution would be a cat enclosure, but that will eat into your garden too. From pictures I've seen they can look pretty cool though.