Hardiness Zone: 5b
By Lin Morgan from Topeka, KS
I would like to know that myself. I got rid of the main one but have babies coming up now.
I had a number of big ones in this yard of the new house we moved into. First I cut them off as low as I could with a chain saw. Then any babies that pop up, keep cutting them off below the ground level. You need to cut, not the woody part of the plant, but sever the roots from the top. If you feel energized, you can do this with the big ones. Cut, sever the roots from the crown. Without the crown, the roots die.
When it's finally dead, then you can dig a little & cut the existing roots & the rest will die. Or when you dig to put other things in there, you can remove them then.
These are not hard to get rid of. You want to know something that's hard to get rid of? Try oleander. It's a nightmare!
I had almost given up until I had the brilliant idea of driling several holes in the main stem after I cut the plant off at ground level. Then I poured RoundUp concentrate, undiluted, into the drilled holes. Mission accomplished!