buckthorn vs wolfberry

buckthorn

noun
  • especially genus Rhamnus, 

  • also genus Frangula, especially Frangula alnus, the alder buckthorn. 

  • especially the common buckthorn or purging buckthorn; Rhamnus cathartica. 

  • Hippophae (family Elaeagnaceae), the sea buckthorn. 

wolfberry

noun
  • Any of flowering species of the genus Lycium. 

  • Lycium barbarum or Lycium chinense 

  • The berry of this plant. 

  • The western snowberry, Symphoricarpos occidentalis, a shrub native to nothern and central North America. 

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