It seems that each year I forget, from the year before, what the differences are between Common Burdock (Arctrium minus)and Great Burdock (Arctrium lappa). In the photos below the Common is on the left and the Great is on the right.
In general, Great Burdock is larger. The flowers are larger and on longer stems. The Great Burdock grows 1 to 2.4 metres (3 to 8 feet) tall and the Common grows .6 to 1.2 metres (2 to 4 feet) tall.
The stems of the Great Burdock are solid with deep grooves while the Common Burdock has hollow stems and shallow grooves.
Of course, just to make things more difficult, the two can and do hybridize. I am not sure how to tell the hybrid. Does anyone else know?