citron vs lemon balm

citron

noun
  • The candied rind of the citron fruit. 

  • The fruit of a citron tree. 

  • A greenish yellow colour. 

  • A small citrus tree, Citrus medica. 

adj
  • Of a greenish yellow colour. 

lemon balm

noun
  • A perennial herb, Melissa officinalis, of southern Europe; used as a flavouring in ice cream etc. 

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