Last year a plant mysteriously grew in my flower bed, it smelled strongly of mint and the bee's loved it. I left it growing. I soon found out that it will take over your garden so I removed it. This spring little offshoots of this plant all over my flower bed! I've pulled and pulled, sometimes the roots are over a foot in length. How can I get rid of this plant? I've tried weed killer and all the pulling but it keeps coming back!! Help!