I need help to ID a fast growing "ornamental grass" that was given by a friend. They are lighter green in color, 12" tall, no joint, die back in winter. The roots are fragile, there is a pea sized hard shell ball(?root ball) at the end of the main root. Other roots are swollen at the end. It is very pretty but spreads like crazy. Is it a kind of ornamental grass or weed? How should I get rid of them?

Please help!

Thanks

Connie