our 6 month old is still on long line in the house and outside. I wouldn't give that much freedom yet to a pup so young to ever allow him in the garden in the first place. He is curious and young and still learning boundaries. I know it's a pain in the butt to always have to be there, but when he goes outside, he needs a lead on him and you should be there. That way, if he heads to the garden, you can say "Ah ah or leave it" and lead him away from the garden by the lead. If you do this now, overtime you can sit on the patio enoying your cup of coffee while he explores the back yard and if he heads to the garden, you can "Ah ah or leave it" from your chair and he will remember what you taught him while he was young and on the lead. Think about it this way: If he has never been taught to leave the garden alone, then is it really fair to punish him for exploring and digging in it? It sounds more time consuming to do it this way, but really if you put in the effort now, it will payoff big ways for the full life of the dog.
I did a parenting class a LONG tiime ago at my church and the one thing that always stuck with me is this: The word discipline is derived from the word disciple which means "to teach" (not to punish)...so put teaching in the front of your mind and correct only after you are 100% sure that the behavior has been fully taught.
The other thing is, if you are laying on the ground "taking bites", you are allowing your dog to treat you like a littermate. At 5 months, you can start working on controlled playing. Make the dog sit for a toy or before taking the tug toy (we use the command "take it"). The dog is not allowed to take the toy no matter what until you say the command. Also start practicing "out" or "drop it" using a treat. And give lots of praise when they are behaving appropriately. If they get too rough, game over.
Good luck! I think it was less work raising my human children! The good thing aboutt dogs though is that you can crate them when you can't supervise!