knapweed vs thorn

knapweed

noun
  • Any of various common weeds of the genus Centaurea 

  • especially Centaurea nigra. 

thorn

noun
  • A sharp protective spine of a plant. 

  • That which pricks or annoys; anything troublesome. 

  • A letter of Latin script (capital: Þ, small: þ), borrowed from the futhark; today used only in Icelandic to represent the voiceless dental fricative, but originally used in several early Germanic scripts, including Old English where it represented the dental fricatives that are today written th (Old English did not have phonemic voicing distinctions for fricatives). 

  • Any shrub or small tree that bears thorns, especially a hawthorn. 

verb
  • To pierce with, or as if with, a thorn (sharp pointed object). 

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