Barrier for Monkey Grass

Q: I have monkey grass growing next to my lawn and it is invading via underground runners. How deep should I bury a barrier is to control this?

A: I think a barrier buried six inches deep would be fine. I use landscape edging from a home improvement store. There are two kinds of monkey grass found in landscapes. Spreading liriope, Liriope spicata, has spikes of inconspicuous white flowers in July. The more common kind, Liriope muscari, which spreads more slowly, has more obvious flowers, typically some shade of blue.