Salix babylonica vs weeping willow

Salix babylonica

weeping willow

noun
  • A willow of species Salix babylonica, with pendulous branches, native to China. 

  • Any of the numerous decorative cultivars and hybrids of Salix babylonica with white willow (Salix alba) or crack willow (Salix fragilis). 

How often have the words Salix babylonica and weeping willow occurred in a corpus of books? (source: Google Ngram Viewer )