ironhead vs knapweed

ironhead

noun

  • Synonym of knapweed 

  • Synonym of goldeneye 

knapweed

noun

  • Any of various common weeds of the genus Centaurea 

  • especially Centaurea nigra. 

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