knotweed vs water couch

knotweed

noun
  • Any of several plants of the genus Polygonum, with jointed stems and inconspicuous flowers 

water couch

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

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