The rockwool is just there for selling reasons. It's difficult to sell it otherwise. It can be quite difficult to plant, so, keep a little rockwool on a few stems and plant it that way, it will help to stop it from floating.
Make sure it's in a place where the flow in the tank is good. As it gets quite leggy if left in still water. Once it gets going this plants can be very hardy and very rewarding.
Here's a pic of some glosso in a low tech tank I had running a couple of years back. It's on the left hand side.