This is the URL of the site I was talking about:
http://wwwdogpack.com/health/poisonplants.htm
Yes, what margy said is right. Make sure he gets plenty of exercise. This behaviour sounds like frustration and physical energy all bottled-up, to me.
Also, be careful. Some garden plants are toxic. In fact A LOT of garden plants are toxic. I found a site where toxic-to-animals shrubs and plants were listed. I'll try to find it, and post back.
Take him for a jolly good walk and game, until he's burned off some good energy. Then leash him and get him near a plant that he normally would massacre. Every time he shows ANY INTEREST in it, tug a little at his leash and firmly but gently get him to submit to you. Even if he turns his head towards it, do the same. This will mean being very observant, and having time to devote to doing this. Do it until he doesn't show any interest. You might have to repeat this a few times, and keep a watch to stop him. But basically make sure he has a healthy outlet for his (probably tremendous at his age) -energy.
Take him out for walks and exercise, dogs get frustrated if they dont get out and have some stimulation they get bored and look for things to stimulate them, imagine what it would be like in a yard all day, nothing to do... I would chew up every plant I could ....Walks , ball throwing ,playing with him..