cow parsnip vs knotgrass

cow parsnip

noun
  • Any umbelliferous plant of genus Heracleum. 

knotgrass

noun
  • An annual plant, Polygonum aviculare, found in fields and wasteland 

  • Paspalum distichum, a weedy perennial grass of wet areas 

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