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Congestion?I had a big patch of rhubarb in my old garden and would lift and divide the clumps every 3 years to maintain vigour. An 8yr old clump will probably be crying out for nutrients and space and he chance to shed old material and make new roots and shoots.
I think you'll find that rhubarb is like irises or hemerocallis - needs regular lifting and dividing to maintain vigour. The oldest roots in the middle will be woody and not very productive.