knotgrass vs knotweed

knotgrass

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

  • Paspalum distichum, a weedy perennial grass of wet areas 

knotweed

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

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