Depends on the grass. The stuff that I get in that's collected out west are in clumps with the roots on -no dirt so you plug them in to your base where you want -usually making a hole in the foam or if your case probably go wire mesh style base because its a bigger base. Then its secure with hot glue/epoxy/dextrine -plaster mud mix- depends on the situation and what mood your in I guess. lol.
With some of these Midwest grasses- its a prairie grass and when we collect those we just cut them off at the base of the ground , then when we install into the base sometimes singly other times we clump them up some hot glue at the bottom of the grass and wrapped with string- then plug them in where you want them.
I've been doing lately is dyeing the foam black/brown then dipping the bottom of the grass then set on the base dust with dirt as foam cures and its done, but just have to hold it there a few minutes till the foam cures and locks it in, with 2 guys doing this you can cover some pretty good areas and it doesn't look like plugs. kinda like the difference between bad hair plugs and a toupee.