Stone, if you want to kill your grass, plastic isn't enough. You can do it with cardboard & a covering of at least 4" of organic matter. Cutting around the edge with shovel at least 4" deep will greatly increase success & speed of smother, and installing a hard barrier around the edge so nothing jumps back in, like bricks or timbers. Overlap the seams very generously, and remove anything on the ground that would puncture the cardboard. Leave it alone for at least 6 months. Uncover small area to test mortality, use area when confident death of grass has been accomplished. This is how I start all of my beds & there is no grass in any. Torpedograss can try to go under, hopefully you don't have that. A barrier that also goes below ground may be necessary for that.