thistles are the wrong object to prove the effectiveness of the
recommended tongue-groove-plier, as they are extremely vulnerable to any
disturbance, the weak root endings are not able to re-establish for
moisture collecting

I would rather recommend you to try your experiments on canadian
goldenrod, momentarily the greatest pest here in central Europe - even
the smallest root piece can re-grow to a new plant, thousandfold - you
just could say "callus differentiation"

cheers kauhl