hawthorn vs hawthorn tree

hawthorn

noun
  • Any of various shrubs and small trees of the genus Crataegus having small, apple-like fruits and thorny branches 

hawthorn tree

noun
  • A tree of the genus Crataegus, having small, apple-like fruits and thorny branches. 

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