Destructive Black Lab

Question
Hi, I have a 18 month old male Black Labwho is neutered, who is absolutely wonderful apart from the fact that he is trying to destroy my yard. He lives outside during the day and in the past to control destructive behaviour I have had him tied-up while we are at work. I saw that tying him up was not a "normal" life for him, so on and off for the past year i've been letting him run free around the yard all day when we're both at work. We provide him with numerous toys, food treats that he has to work for (ie. knog ball filled with treats or food) and big chewing bones every couple of days. As well as this we walk him at least once a day for 40-60mins, and most days i take him for a strenuous run for 40-60 also. Now my problem is that whenever i let him roam free during the day he eats the shrubs in the garden. At present he is systematically destroying our plants one by one. When i come home i tell him off for destroying the plants, but i'm never there to see him doing it and catch him in the act. This has been going on for a while now, and i've started getting really angry at him, which i think is making him scared. I really love this dog, and want to find a way for us to live harmoniously together. Any help would be greatly appreciated.

Thanks.

Answer
I doubt there is any training him to leave things alone.  Could you divide the yard leaving him a nice area with at least one large tree for shade but not access to the shrubs he is destroying?  I usually suggest leaving the dog inside in a crate, but that requires finding a way to give him a mid day break and he might not adjust well to it at his age,  Another alternative  if available is doggy day care.