collard greens vs collards

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. 

collards

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