hopniss vs wapato

hopniss

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

wapato

noun
  • Broadleaf arrowhead (Sagittaria latifolia), or its edible bulbous root. 

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