horse chestnut vs sweet chestnut

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. 

sweet chestnut

noun
  • The edible nuts of this tree. 

  • A type of deciduous tree native to Europe, Castanea sativa. 

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