blackthorn vs pear hawthorn

blackthorn

noun
  • A large shrub or small tree, Prunus spinosa, that is native to Europe, western Asia, and north Africa. It has a dark bark and bears thorns. 

  • A stick or staff taken from this tree. 

pear hawthorn

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