The number of seeds produced per head varies greatly. Large differences have been found within a site, between sites, in the same year and across years. A two year study in Ontario found that common burdock averaged 11,700 and 13,400 seeds per plant. Seeds are mature by September and are spread until the following spring. There is outcrossing and occasional hybridization in common burdock, as well as great variation in the natural populations.
Taxonomists recognize up to eight species of burdock.
Common burdock is more variable in Europe than it is in North America. Seven subspecies and five varieties were found in Europe while three forms under the subspecies are in North America. A. minus f. purpureum with deep purple corollas 3. This is due to the fact that it is a biennial, so it needs areas that are not severely disturbed on an annual basis. It is found both on moist fertile soils, many with high soil nitrogen content, and on sterile clay soils.