collard greens vs curly 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. 

curly kale

noun
  • The most common form of kale, with crinkly rather than smooth leaves. 

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