knotgrass vs pigweed

knotgrass

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

  • Paspalum distichum, a weedy perennial grass of wet areas 

pigweed

noun
  • Polygonum aviculare (common knotgrass) 

  • Amaranthus species 

  • Portulaca bicolor, of Australia 

  • Heracleum sphondylium 

  • Any of various weedy plants sometimes used as pig fodder 

  • Chenopodium album, white goosefoot 

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