Question
instead of mulch i want to use lava rocks.  do i treat with weed killer and then put plastic down and what should i plant and how.

Answer
I would put a weed preventative down (Preen)and then put the landscape fabric down, don't use regular old plastic, use landscape fabric. You can plant anything you want, landscape roses, ornamental grasses, perennials, annuals etc...  have fun with the project.

Bud Christmann