chestnut tree vs horse chestnut

chestnut tree

noun
  • A tree that bears chestnuts; a tree of the genus Castanea 

horse chestnut

noun
  • The large nutlike seed of these trees. 

  • Any of several tree species of genus Aesculus, especially Aesculus hippocastanum, common in temperate northern hemisphere. 

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