I know I am coming into this post almost 2 months after you posted it, but perhaps it can still help you or somebody that has the same problem.

First recommendation is probably pretty expensive and a pain in the neck, but I would say to get an invisible fence. You install wires underground and the dog wears a collar. If they get too close to the fence, they get a very slight shock. It sounds cruel but if done correctly it can be safe and not too bad. Get one of the fences that gives an increasingly potent shock relating to closeness to the fence...that way the dog learns that it is the area to avoid as they get closer. If it's a crappy fence that just shocks them straight out of nowhere with no increasing intensity based on distance, they will freak out and be terrified to go outside.

Other idea is to sprinkle powdered cayenne pepper around the garden bed. It's safe and non-toxic...will just give a burning sensation if the dog gets in it, but it will wear off. Similar to pepper spraying a human criminal.

That's my advice, but you could speak with a professional dog trainer or veterinarian.