ryegrass vs undergrowth

ryegrass

noun
  • A collection of plants of any of the species, as in a lawn or field. 

  • Any of several species of tufted grasses of the genus Lolium. 

undergrowth

noun
  • The plants in a forest which only reach a relatively low height (such as shrubs and bushes). 

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