duck potato vs hopniss

duck potato

noun
  • broadleaf arrowhead (Sagittaria latifolia) 

  • arum-leaved arrowhead (Sagittaria cuneata) 

hopniss

noun
  • Apios americana, a perennial vine that bears edible beans and large edible tubers. 

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