[Wikipedia] Uncertain Origin, Noxious/Invasive Herb Status: The origin of Lepidium densiflorum is not entirely clear: some believe it was introduced in Europe or Eurasia, some believe it originated in western North America and east of the Mississippi River, and others believe it originated in the east.[4] It is more likely that the species is native to North America and it was first discovered in a rubbish dump in Turku, Finland, in the early 20th century, but its distribution has expanded and it was considered a noxious and invasive herb in many states.[ (link added by Mary Ann Machi)