water milfoil vs yarrow

water milfoil

noun
  • Any of several submerged aquatic plants of the genus Myriophyllum. 

yarrow

noun
  • Common yarrow, Achillea millefolium, the type species of the genus. 

  • The green woodpecker, Picus viridis. 

  • Any of several pungent Eurasian and North American herbs, of the genus Achillea, used in traditional herbal medicine. 

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