I think it depends on just how you use it, and how neat and tidy you are. I bury a bunch of chopped Bocking 14 leaves in the planting holes of my tomatoes and the odd bit of that seems to root. I did have a large bed of 100 comfrey plants near a fence and I now have none on this side of the fence, where the sheep grazed it down to nothing, but plenty on the other side, further away than just being from a spreading plant. Either they seeded themselves, which shouldn't be possible, or small bits were flicked by the chomping sheep and have taken root. I also have a lot of Bocking 14 which has magically appeared in my bramble area - I haven't used it as a mulch here and have no idea how it has spread, but it has