hawthorn vs mayflower

hawthorn

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

mayflower

noun
  • Any of several plants that flower in May - especially the hawthorn (in Britain) and the trailing arbutus (in the US). 

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