How do you keep your pets from you garden beds?

I have a puppy 4 mos old. I know he is young and hopefully will grow out of it, but for right now the little monster has a huge appetite for my hostas, coneflowers, etc etc. I usually let him have free reign of the backyard, but this just isn't working out. So I have two questions, #1 do you think he will grow out of eating plants or is this just a dog thing in general and not a puppy thing? and #2 do you think it would be ok to set up some sort of kennel in the yard and keep him in there for outside time unless he is supervised by me or my husband? I have no idea where I would put it or even what size I should get. He is a mini schnauzer. He is an inside dog, so I he wouldn't be in the kennel all the time, just really when he needs to go out and DH and I are not able to go out with him.

Any advice would be taken to heart. I'm really enjoying the beautiful plants that I have bought this summer and I'd really like to buy more, but I can't keep spending this kind of money on puppy snacks!

Thanks!

Jen