We have a very large (15ft tall) cedar hedge. It great privacy, but the roots are invading flower beds and stealing moisture. The roots are shallow running, mostly in the first 7 inches of soil. I dug a trench a foot deep, until I hit some gravel. If I pour concrete into it, will that stop the shallow running suckers, or will they find their way into the bed? Is there an effective way to create a barrier against an established cedar hedge?