Saint John's wort vs St John's wort

Saint John's wort

St John's wort

noun
  • The plant prepared as a medication. 

  • Any of several plants, of the genus Hypericum, especially Hypericum perforatum, that have yellow flowers and traditionally ward off evil. 

How often have the words Saint John's wort and St John's wort occurred in a corpus of books? (source: Google Ngram Viewer )