lamb's ears vs selfheal

lamb's ears

noun
  • Stachys byzantina, an ornamental low-growing perennial related to betony with soft, silky hairs on the leaves which give it an attractive silvery-gray color. 

  • Used other than figuratively or idiomatically: see lamb, ear. 

selfheal

noun
  • A small, herbaceous European plant with blue-violet flowers from any species of genus Prunella. 

  • Especially, Prunella vulgaris. 

How often have the words lamb's ears and selfheal occurred in a corpus of books? (source: Google Ngram Viewer )