Answer: If you trim the foliage back, the trimmed bits do not regrow so you are left with the obviously trimmed tips. Yucca expands from the center outward by generating offsets around the base of the central mother plant. You can try to dig out the offsets to limit its outward spread. Yucca is a pretty big plant with age and unfortunately it is also very difficult to control because it regenerates from any shred of its deep root left in the ground.
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