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.