First, I need to figure out when he is digging. Since I know the dog and his lifestyle, I can rule out several factors such as boredom or puppyhood or gophers, etc...
I noticed that every time he would start digging holes he was in the yard playing with a friend's dog, unsupervised.
So, I first need to MAKE SURE that it was ACTUALLY MY DOG that was the perpetrator. A quick look at his feet would suggest that it was.
Next, I needed to figure out if he would dig ANY TIME he was left alone in the yard or if it was only when another dog was present.
To figure this out, I simply left the dog in the yard alone with access to the rose garden several times... and came back to find that he had not dug.
So... it stands to reason that the only time my dog is digging in the yard is when there is another dog in the yard. (Who knows why? There could be a million unexplained reasons that only the dog knows. All I need in order to fix the behavior is knowledge of the dog and the circumstances).
Now, I know that to fix any behavior problem I need to make the dog experience a NEGATIVE ASSOCIATION with the actual ACT of doing that behavior. In this case, digging in the garden.
And he needs to experience that same negative association EVERY TIME HE DIGS!
In this case, I must be 100% diligent to never leave Forbes unsupervised in the yard when there is another dog in the yard.
Of course, if he was digging by himself, then I'd need to confine him to a kennel run where he cannot dig when I'm not supervising him. Or if there is another dog visiting then I will need to bring Forbes inside, put him in the kennel run, or use the presence of the other dog as a "set up."
The next step is to make sure that he associates that negative (correction) just as he starts to dig.
There are two ways I can do this: The Lazy Man's Way and the Old Fashioned Way. Both methods are based on the same principle.
The Old Fashioned Way to make sure that the dog gets a motivational negative association when he digs is to:
Step 1.) Leave a pinch collar and tab (one foot leash) on the dog when he's outside in the yard with another dog.
Step 2.) Bury hardware mesh or chicken wire in the spot where he's been digging. The chicken wire should be buried two to three inches below the surface. Dogs don't like scraping their paws against this stuff. So, right off the bat you've got an immediate negative association.
Step 3.) Spy on him and just wait until he start to dig.
Step 4.) As soon as he begins to dig, yell "No No No!" as you run outside and give the dog a correction. As long as you continue to say "No no no" as you run to the dog, the dog WILL still associate the correction with the behavior.
Step 5.) Be 100% consistent until you are 100% sure that the dog isn't digging any more.
The Lazy Man's Way to fix this problem behavior is to use a remote electronic collar (e-collar). Everything else remains the same. (Click on the link above to read about my recommendations for buying a remote electronic training collar).
When using the e-collar for this behavior, I'd turn the setting up to the high level. Your goal is to create absolute avoidance to this behavior (digging in the garden). And you want him to think that the dirt just jumped up and bit him! Usually if you correct the dog with the electronic collar for this type of behavior, you've only got to do it twice before the dog decides that it's in his best interest to leave your garden alone.

To read more of my dog training ramblings, read about my book (click below):
Secrets of a Professional Dog Trainer!