mullein vs mullen

mullein

noun
  • Any of a few hundred species of European and Asian plants, of the genus Verbascum, especially that majority that have yellow flowers. Some species have been introduced to other continents, where some are now seen as invasive pests. Some species, though far from all, have downy leaves, hence common names such as: "velvet plant". 

mullen

How often have the words mullein and mullen occurred in a corpus of books? (source: Google Ngram Viewer )