I have the type of grass that grows long roots that grow under the brick edging I have around my flower beds. I have dug up the whole bed and removed every last root but it is persistent. I want to put in an edging that the roots can't grow under - how deep do I have to go and what type of edging can I use? I have tried mulch and the grass grows right through it. I still use the mulch to control weeds - I can deal with them but this grass is awful!