Unless your me in which case it never carpets
Don't worry about damaging the roots. Hair Grass is one of the sturdiest plants in the hobby. I often use a blade to scrape it all off.
You'll get quicker growth if you spread the plants out a bit more. I.E., break the clumps up into smaller pieces and spread them around the tank.
Sometimes takes a few weeks for runners to start but once spreading begins, it'll seem like the entire tank is covered in very little time.