horse chestnut vs horse chestnut tree

horse chestnut

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

  • The large nutlike seed of these trees. 

horse chestnut tree

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