poor man's soap vs sweet pepperbush

poor man's soap

noun

  • Synonym of summersweet (“Clethra alnifolia”) 

sweet pepperbush

noun

  • Clethra alnifolia, a deciduous flowering shrub of eastern North America. 

How often have the words poor man's soap and sweet pepperbush occurred in a corpus of books? (source: Google Ngram Viewer )