cow's lungwort vs mullein

cow's lungwort

noun
  • mullein (Verbascum thapsus) 

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". 

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