pignut vs pignut hickory

pignut

noun
  • The edible tuber of Conopodium majus, native to western Europe. 

  • Any of various types of hickory or their fruits; a hognut. 

  • Simmondsia chinensis, jojoba. 

pignut hickory

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