I think the latter Owdboggy. I don't think you can take root cuttings of Cephalaria so you should be ok to remove the top. They are propagated by division of the clump so must need the stem/root union points where the bud tissue is to keep growing.

Cheers. I have just finished digging out a clump and there are enough pieces of it to fill 6 wheelbarrows. Heaven knows where I am going to put it. The soilless bits of root can go in the Compost bins, but the rest????
And no, I do not want to plant it somewhere else.

sorry to be a damp squib but i didnt get all of the roots out and whilst not as bad as acanthus or papaver orientalis they have regrown a bit.

Thanks! Not!
I have already dug one out before, but cannot remember if I got rid of all the roots or not. That clump has not shown any signs of regeneration.........yet. But that could be because I managed to remove all but the fibrous roots. We shall see. I ain't digging any deeper.