American groundnut vs groundnut

American groundnut

groundnut

noun
  • A climbing vine, Apios americana, of eastern North America, having fragrant brownish flowers and small edible tubers. 

  • The nutlike tuber of such a plant, especially peanuts. 

  • A plant which bears its nutlike seeds underground, such as a peanut (Arachis hypogaea) or a Bambara groundnut (Vigna subterranea). 

  • Any similar plant having underground tubers. 

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