knotgrass vs water couch

knotgrass

noun
  • Paspalum distichum, a weedy perennial grass of wet areas 

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

water couch

noun
  • Paspalum distichum, a weedy perennial grass of wet areas. 

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