collard greens vs kale

collard greens

noun
  • A form of Brassica oleracea, sometimes called Brassica oleracea var. acephala or Brassica oleracea var. viridis, similar to kale, that is a popular food in the rural southern United States. 

kale

noun
  • Any of several cabbage-like food plants that are kinds of Brassica oleracea. 

  • An edible plant, similar to cabbage, with curled leaves that do not form a dense head (Brassica oleracea var. acephala) 

  • Broth containing kale as a chief ingredient. 

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