hickwall vs old man's pepper

hickwall

noun
  • A European green woodpecker, Picus viridis. 

  • A lesser spotted woodpecker, Dryobates minor. 

old man's pepper

noun
  • Achillea millefolium or common yarrow, a flowering plant native to temperate regions of the Northern Hemisphere. 

How often have the words hickwall and old man's pepper occurred in a corpus of books? (source: Google Ngram Viewer )