Peking willow vs weeping willow

Peking willow

noun

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

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 Peking willow and weeping willow occurred in a corpus of books? (source: Google Ngram Viewer )