Question
MR. RAY,  MY PROPERTY IS 15 MI. INLAND FROM SAN DIEGO, AND I WANT TO REPLACE MY GRASS YARD WITH STONES & ROCK..IF I REMOVE
GRASS, SPRAY WITH PLANT KILLER, LAY DOWN BLACK PLASTIC, SHOULD THE STONES OR ROCK BE 2 OR 3 OR 4 INCHES DEEP?..I HAVE NOT YET
DETERMINED THE TYPE OF ROCK.  THANKS IN ADVANCE,  FRED  

Answer
Hey, Fred.

If you're planning on putting down black plastic and rocks, I don't think you need to spray with plant killer. That would just be an extra expense. The rocks on top of the black plastic will pretty much keep anything from growing underneath since the black plastic will block any sunlight and prevent photosynthesis.

As far as the depth of the rocks, though, if you're going to have any traffic at all, the deeper the better. Otherwise the rocks get kicked around and you'll have cattle trails everywhere.