verbena vs vervain

verbena

noun
  • Verbena, a genus of herbaceous plants of which several species are extensively cultivated for the great beauty of their flowers; vervain. 

vervain

noun
  • Any herbaceous plant in the genus Verbena especially if used for medicinal purposes, primarily Verbena officinalis, common in Europe and formerly held to have medicinal properties. 

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