I also have sterile hybrid type that has been one spot for years with no sign of spreading.Instead of duging out the roots if you want more plants,what i do is i cut off the foliage down to ground level then cover a thick layer of compost over the top,after a few months when the plant has completely regrown i then cut off the new growth just below the compost layer,you find that the new shoots will have enough roots on to then be grown else where.
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