I've never seen leaves like these before either. I usually keep trying to kill it and every year it comes back bigger! I even took the roots out once, but I must've missed a microscopic root, because that sucker came back bigger and stronger. I think it will outlast me.
That is exactly what we did after I took my photo because I had to search for them almost everywhere and found some we don't even recall from which device they were.
A clever solution of your rhubarb problem. It must be a gigantic rhubarb plant to have such big leaves; from long a go when we had an allotment I don't recall such big leaves.