My overgrown baby

Titan's the name of latest dog. Already 9 months old and weighing about 50kg, he still thinks that he's a little puppy. After acquiring him for the past 7 months, I've stll yet to train him not to bite at my plants, not that I'm a lousy master or trainer. For the past couple of months, he has been destroying my bonzais like nobody business. The unfortunate thing is that when he's up to his mischief, I'm awalys not around. For 2 weeks now he is isolated at the back of my garden. Oh, don't worry, the whole of my back garden is about 800sq ft; and he has got a companion too. My equally robust 14 mths old Am Staff.