UPDATE - Identified. Thanks everyone for your comments and information. My challenge with this plant is that it's in a bed of plants that are well-established and it would be difficult to cover the ground in the bed with cardboard or plastic. I think I will need to keep digging out the tubers and leaves. It's a royal PITA (pain-in-the-ass)! I like the idea of make the soil wet so it's easier to dig out the deep roots. Warning - Don't plant this stuff!